State of carbon. Lesson summary "Structure of the carbon atom

Carbon (from Latin: carbo "coal") is a chemical element with the symbol C and atomic number 6. Four electrons are available to form covalent chemical bonds. The substance is non-metallic and tetravalent. Three isotopes of carbon occur naturally, 12C and 13C are stable, and 14C is a decaying radioactive isotope with a half-life of about 5,730 years. Carbon is one of the few elements known since ancient times. Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass, after hydrogen, helium and oxygen. The abundance of carbon, the unique diversity of its organic compounds, and its unusual ability to form polymers at temperatures typically found on Earth allow it to serve as a common element to all known life forms. It is the second most abundant element in the human body by mass (about 18.5%) after oxygen. Carbon atoms can bond in different ways, called allotropes of carbon. The most well-known allotropes are graphite, diamond and amorphous carbon. The physical properties of carbon vary widely depending on the allotropic form. For example, graphite is opaque and black, while diamond is very transparent. Graphite is soft enough to form a streak on paper (hence its name, from the Greek verb "γράφειν", meaning "to write"), while diamond is the hardest material known in nature. Graphite is a good electrical conductor, while diamond has low electrical conductivity. Under normal conditions, diamond, carbon nanotubes and graphene have the highest thermal conductivity of any known material. All carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically stable and require high temperatures to react even with oxygen. The most common oxidation state of carbon in inorganic compounds is +4, and +2 in carboxyl complexes of carbon monoxide and a transition metal. The largest sources of inorganic carbon are limestones, dolomites and carbon dioxide, but significant quantities come from organic deposits of coal, peat, petroleum and methane clathrates. Carbon forms a huge number of compounds, more than any other element, with almost ten million compounds described to date, and yet this number is only a fraction of the number of compounds theoretically possible under standard conditions. For this reason, carbon is often referred to as the "king of the elements".

Characteristics

Allotropes of carbon include graphite, one of the softest substances known, and diamond, the hardest natural substance. Carbon readily bonds to other small atoms, including other carbon atoms, and is capable of forming numerous stable covalent bonds with suitable multivalent atoms. Carbon is known to form almost ten million different compounds, the vast majority of all chemical compounds. Carbon also has the highest sublimation point of any element. At atmospheric pressure, it has no melting point since its triple point is 10.8 ± 0.2 MPa and 4600 ± 300 K (~4330 °C or 7820 °F), so it sublimes at about 3900 K. Graphite is much more reactive than diamond under standard conditions, despite being more thermodynamically stable, since its delocalized pi system is much more vulnerable to attack. For example, graphite can be oxidized with hot concentrated nitric acid under standard conditions to mellitic acid C6(CO2H)6, which retains the hexagonal units of graphite while destroying the larger structure. Carbon sublimes in a carbon arc whose temperature is about 5,800 K (5,530 °C, 9,980 °F). Thus, regardless of its allotropic form, carbon remains solid at temperatures higher than the highest melting points such as tungsten or rhenium. Although carbon is thermodynamically prone to oxidation, it is more resistant to oxidation than elements such as iron and copper, which are weaker reducing agents at room temperature. Carbon is the sixth element with a ground state electronic configuration of 1s22s22p2, of which the outer four electrons are valence electrons. Its first four ionization energies are 1086.5, 2352.6, 4620.5 and 6222.7 kJ/mol, much higher than the heavier group 14 elements. Carbon's electronegativity is 2.5, significantly higher than the heavier ones elements of group 14 (1.8-1.9), but is close to most neighboring non-metals, as well as some transition metals of the second and third row. Covalent radii of carbon are generally taken to be 77.2 pm (C-C), 66.7 pm (C=C), and 60.3 pm (C≡C), although these can vary depending on the coordination number and what is bonded to carbon. In general, the covalent radius decreases as the coordination number decreases and the bond order increases. Carbon compounds form the basis of all known life on Earth, and the carbon-nitrogen cycle provides some of the energy released by the Sun and other stars. Although carbon forms an extraordinary variety of compounds, most forms of carbon are relatively unreactive under normal conditions. At standard temperatures and pressures, carbon can withstand all but the strongest oxidizing agents. It does not react with sulfuric acid, hydrochloric acid, chlorine or alkalis. At elevated temperatures, carbon reacts with oxygen to form carbon oxides and removes oxygen from metal oxides, leaving the elemental metal. This exothermic reaction is used in the iron and steel industry to smelt iron and control the carbon content of steel:

Fe3O4 + 4 C (s) → 3 Fe (s) + 4 CO (g)

with sulfur to form carbon disulfide and with steam in the coal-gas reaction:

C(s) + H2O(g) → CO(g) + H2(g)

Carbon combines with certain metals at high temperatures to form metal carbides, such as cementite from iron carbide in steel and tungsten carbide, widely used as an abrasive and to make hard points for cutting tools. The system of carbon allotropes covers a number of extremes:

Some forms of graphite are used for thermal insulation (such as fire barriers and heat shields), but some other forms are good thermal conductors. Diamond is the most famous natural heat conductor. Graphite is opaque. Diamond is very transparent. Graphite crystallizes in the hexagonal system. Diamond crystallizes in the cubic system. Amorphous carbon is completely isotropic. Carbon nanotubes are among the best known anisotropic materials.

Carbon allotropes

Atomic carbon is a very short-lived species and therefore carbon is stabilized in various polyatomic structures with different molecular configurations called allotropes. The three relatively well-known allotropes of carbon are amorphous carbon, graphite, and diamond. Previously considered exotic, fullerenes are now commonly synthesized and used in research; these include buckyballs, carbon nanotubes, carbon nanodots and nanofibers. Several other exotic allotropes have also been discovered, such as lonsaletite, glassy carbon, nanofaum carbon, and linear acetylene carbon (carbyne). As of 2009, graphene is considered the strongest material ever tested. The process of separating it from graphite will require some further technological development before it becomes economical for industrial processes. If successful, graphene could be used in the construction of space elevators. It can also be used to safely store hydrogen for use in hydrogen-based engines in cars. The amorphous form is a collection of carbon atoms in a non-crystalline, irregular, glassy state rather than contained in a crystalline macrostructure. It is present in powder form and is the main component of substances such as charcoal, lamp soot (soot) and activated carbon. At normal pressures, carbon has the form of graphite, in which each atom is trigonally bonded by three other atoms in a plane consisting of fused hexagonal rings, as in aromatic hydrocarbons. The resulting network is two-dimensional, and the resulting flat sheets are folded and loosely connected through weak van der Waals forces. This gives graphite its softness and cleavage properties (sheets easily slide past each other). Due to the delocalization of one of the outer electrons of each atom to form a π cloud, graphite conducts electricity, but only in the plane of each covalently bonded sheet. This results in lower electrical conductivity for carbon than for most metals. Delocalization also explains the energetic stability of graphite over diamond at room temperature. At very high pressures, carbon forms a more compact allotrope, diamond, having almost twice the density of graphite. Here, each atom is tetrahedrally connected to four others, forming a three-dimensional network of wrinkled six-membered rings of atoms. Diamond has the same cubic structure as silicon and germanium, and due to the strength of its carbon-carbon bonds, it is the hardest natural substance, as measured by scratch resistance. Contrary to popular belief that "diamonds are forever", they are thermodynamically unstable under normal conditions and turn into graphite. Due to the high activation energy barrier, the transition to the graphite form is so slow at normal temperatures that it is undetectable. Under certain conditions, carbon crystallizes as lonsalite, a hexagonal crystal lattice with all atoms covalently bonded and properties similar to those of diamond. Fullerenes are a synthetic crystalline formation with a graphite-like structure, but instead of hexagons, fullerenes are composed of pentagons (or even heptagons) of carbon atoms. Missing (or extra) atoms deform the sheets into spheres, ellipses, or cylinders. The properties of fullerenes (divided into buckyballs, bakitubes and nanobads) have not yet been fully analyzed and represent an intensive area of ​​nanomaterials research. The names "fullerene" and "buckyball" are associated with the name of Richard Buckminster Fuller, the popularizer of geodesic domes, which resemble the structure of fullerenes. Buckyballs are fairly large molecules formed entirely from carbon bonds in a trigonal manner, forming spheroids (the most famous and simplest is the football-shaped buckynysterfellerene C60). Carbon nanotubes are structurally similar to buckyballs, except that each atom is trigonally bonded in a curved sheet that forms a hollow cylinder. Nanoballs were first introduced in 2007 and are hybrid materials (buckyballs are covalently bonded to the outer wall of a nanotube) that combine the properties of both in one structure. Of the other allotropes discovered, carbon nanofoam is a ferromagnetic allotrope discovered in 1997. It consists of a cluster assembly of low-density carbon atoms stretched together into a loose three-dimensional network in which the atoms are trigonally linked in six- and seven-membered rings. It is among the lightest solids with a density of about 2 kg/m3. Likewise, glassy carbon contains a high proportion of closed porosity, but unlike regular graphite, the graphite layers are not stacked like pages in a book, but are more randomly arranged. Linear acetylene carbon has the chemical structure - (C:::C)n-. Carbon in this modification is linear with sp orbital hybridization and is a polymer with alternating single and triple bonds. This carbyne is of significant interest for nanotechnology because its Young's modulus is forty times greater than that of the hardest material, diamond. In 2015, a team from the University of North Carolina announced the development of another allotrope, which they called Q-carbon, created by a high-energy, low-duration laser pulse on amorphous carbon dust. Q-carbon is reported to exhibit ferromagnetism, fluorescence, and have a hardness superior to diamonds.

Prevalence

Carbon is the fourth most abundant chemical element in the universe by mass, after hydrogen, helium and oxygen. Carbon is abundant in the Sun, stars, comets and the atmospheres of most planets. Some meteorites contain microscopic diamonds that were formed when the solar system was still a protoplanetary disk. Microscopic diamonds can also form under intense pressure and high temperature where meteorites impact. In 2014, NASA announced an updated database to track polycyclic aromatic hydrocarbons (PAHs) in the Universe. More than 20% of the carbon in the universe can be associated with PAHs, complex compounds of carbon and hydrogen without oxygen. These compounds feature in the global PAH hypothesis, where they are hypothesized to play a role in abiogenesis and the formation of life. PAHs appear to have been formed "a couple of billion years" after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets. The Earth's crust as a whole is estimated to contain 730 ppm of carbon, with 2,000 ppm contained in the core and 120 ppm in the combined mantle and crust. Since the mass of the earth is 5.9 72 × 1024 kg, this would mean 4360 million gigatons of carbon. This is much more than the amount of carbon in the oceans or atmosphere (below). Combined with oxygen in carbon dioxide, carbon is found in the Earth's atmosphere (approximately 810 gigatons of carbon) and dissolved in all bodies of water (approximately 36,000 gigatons of carbon). There are about 1,900 gigatons of carbon in the biosphere. Hydrocarbons (such as coal, oil and natural gas) also contain carbon. Coal "reserves" (not "resources") are about 900 gigatons with perhaps 18,000 Gt of resources. Oil reserves amount to about 150 gigatons. Proven sources of natural gas are about 175,1012 cubic meters (containing about 105 gigatons of carbon), but studies estimate another 900,1012 cubic meters from “unconventional” deposits such as shale gas, amounting to about 540 gigatons of carbon. Carbon has also been found in methane hydrates in polar regions and under the seas. According to various estimates, the amount of this carbon is 500, 2500 Gt, or 3000 Gt. In the past, the amount of hydrocarbons was greater. According to one source, between 1751 and 2008, about 347 gigatons of carbon were released into the atmosphere as carbon dioxide into the atmosphere from burning fossil fuels. Another source adds the amount added to the atmosphere since 1750 to 879 Gt, and the total amount in the atmosphere, sea and land (e.g. peat bogs) is almost 2000 Gt. Carbon is a constituent (12% by mass) of very large masses of carbonate rocks (limestone, dolomite, marble, etc.). Coal contains very high amounts of carbon (anthracite contains 92-98% carbon) and is the largest commercial source of mineral carbon, accounting for 4,000 gigatons or 80% of fossil fuels. In terms of individual carbon allotropes, graphite is found in large quantities in the United States (mainly New York and Texas), Russia, Mexico, Greenland and India. Natural diamonds are found in rock kimberlite contained in ancient volcanic “necks” or “chimneys”. Most diamond deposits are found in Africa, especially in South Africa, Namibia, Botswana, the Republic of Congo and Sierra Leone. Diamond deposits have also been found in Arkansas, Canada, the Russian Arctic, Brazil, and Northern and Western Australia. Diamonds are also now being recovered from the ocean floor off the Cape of Good Hope. Diamonds occur naturally, but now produce about 30% of all industrial diamonds used in the United States. Carbon-14 is formed in the upper troposphere and stratosphere at altitudes of 9-15 km in a reaction that is deposited by cosmic rays. Thermal neutrons are produced and collide with nitrogen-14 nuclei to form carbon-14 and a proton. Thus, 1.2 × 1010% of atmospheric carbon dioxide contains carbon-14. Carbon-rich asteroids are relatively dominant in the outer parts of the asteroid belt in our solar system. These asteroids have not yet been directly examined by scientists. Asteroids could be used in hypothetical space-based coal mining, which may be possible in the future but is currently technologically impossible.

Carbon isotopes

Carbon isotopes are atomic nuclei that contain six protons plus a number of neutrons (2 to 16). Carbon has two stable, naturally occurring isotopes. The isotope carbon-12 (12C) forms 98.93% of carbon on Earth, and carbon-13 (13C) forms the remaining 1.07%. The concentration of 12C increases further in biological materials because biochemical reactions discriminate against 13C. In 1961, the International Union of Pure and Applied Chemistry (IUPAC) adopted the isotope carbon-12 as the basis for atomic weights. Carbon identification in nuclear magnetic resonance (NMR) experiments is carried out with the isotope 13C. Carbon-14 (14C) is a naturally occurring radioisotope created in the upper atmosphere (lower stratosphere and upper troposphere) by the interaction of nitrogen with cosmic rays. It is found in trace amounts on Earth in amounts up to 1 part per trillion (0.0000000001%), mainly in the atmosphere and surface sediments, particularly peat and other organic materials. This isotope decays during β-emission of 0.158 MeV. Due to its relatively short half-life of 5,730 years, 14C is virtually absent from ancient rocks. In the atmosphere and in living organisms, the amount of 14C is almost constant, but decreases in organisms after death. This principle is used in radiocarbon dating, invented in 1949, which has been widely used to date carbonaceous materials up to 40,000 years old. There are 15 known isotopes of carbon and the shortest-lived is 8C, which decays through proton emission and alpha decay and has a half-life of 1.98739 x 10-21 s. Exotic 19C exhibits a nuclear halo, meaning its radius is significantly larger than would be expected if the core were a sphere of constant density.

Education in the Stars

The formation of a carbon atomic nucleus requires a nearly simultaneous triple collision of alpha particles (helium nuclei) within the core of a giant or supergiant star, which is known as the triple alpha process, since the products of further nuclear fusion reactions of helium with hydrogen or another helium nucleus produce lithium-5 and beryllium -8 respectively, both of which are very unstable and decay almost instantly back into smaller nuclei. This occurs under conditions of temperatures greater than 100 megacalvin and helium concentrations, which are unacceptable in the rapid expansion and cooling of the early Universe, and therefore no significant amounts of carbon were created during the Big Bang. According to the modern theory of physical cosmology, carbon is formed inside stars in the horizontal branch through the collision and transformation of three helium nuclei. When these stars die as supernovae, the carbon is dispersed into space as dust. This dust becomes the building material for the formation of second or third generation star systems with accreted planets. The solar system is one such star system with an abundance of carbon, allowing life as we know it to exist. The CNO cycle is an additional fusion mechanism that drives stars where carbon acts as a catalyst. Rotational transitions of various isotopic forms of carbon monoxide (such as 12CO, 13CO and 18CO) are detected in the submillimeter wavelength range and are used in the study of newly forming stars in molecular clouds.

Carbon cycle

Under terrestrial conditions, the conversion of one element to another is a very rare phenomenon. Therefore, the amount of carbon on Earth is effectively constant. Thus, in processes that use carbon, it must be obtained from somewhere and utilized somewhere else. The pathways of carbon in the environment form the carbon cycle. For example, photosynthetic plants extract carbon dioxide from the atmosphere (or seawater) and build it into biomass, as in the Calvin cycle, the process of carbon fixation. Some of this biomass is eaten by animals, while some of the carbon is exhaled by animals as carbon dioxide. The carbon cycle is much more complex than this short cycle; for example, some carbon dioxide dissolves in the oceans; if bacteria don't consume it, dead plant or animal matter can become oil or coal, which releases carbon when burned.

Carbon compounds

Carbon can form very long chains of interlocking carbon-carbon bonds, a property called chain formation. Carbon-carbon bonds are stable. Thanks to catanation (chain formation), carbon forms countless compounds. Evaluation of unique compounds shows that more of them contain carbon. A similar statement can be made for hydrogen because most organic compounds also contain hydrogen. The simplest form of an organic molecule is a hydrocarbon, a large family of organic molecules that consist of hydrogen atoms bonded to a chain of carbon atoms. Chain length, side chains, and functional groups influence the properties of organic molecules. Carbon is found in all forms of known organic life and is the basis of organic chemistry. When combined with hydrogen, carbon forms various hydrocarbons that are important to industry as refrigerants, lubricants, solvents, chemical feedstocks for plastics and petroleum products, and as fossil fuels. When combined with oxygen and hydrogen, carbon can form many groups of important biological compounds, including sugars, lignans, chitins, alcohols, fats and aromatic esters, carotenoids and terpenes. With nitrogen, carbon forms alkaloids, and with the addition of sulfur it also forms antibiotics, amino acids and rubber products. With the addition of phosphorus to these other elements, it forms DNA and RNA, the carriers of the chemical code of life, and adenosine triphosphate (ATP), the most important energy transport molecule in all living cells.

Inorganic compounds

Typically, carbon-containing compounds that are associated with minerals or that do not contain hydrogen or fluorine are treated separately from classical organic compounds; this definition is not strict. Among them are simple carbon oxides. The best known oxide is carbon dioxide (CO2). This substance was once a major component of the paleoatmosphere, but today is a minor component of the Earth's atmosphere. When dissolved in water, this substance forms carbon dioxide (H2CO3), but, like most compounds with several monooxygens on one carbon, it is unstable. However, through this intermediate substance, resonant stabilized carbonate ions are formed. Some important minerals are carbonates, especially calcite. Carbon disulfide (CS2) is similar. Another common oxide is carbon monoxide (CO). It is formed during incomplete combustion and is a colorless, odorless gas. Each molecule contains a triple bond and is quite polar, which causes it to constantly bind to hemoglobin molecules, displacing oxygen, which has a lower binding affinity. Cyanide (CN-) has a similar structure but behaves like halide ions (pseudohalogen). For example, it can form the molecule cyanogen nitride (CN)2), similar to diatom halides. Other uncommon oxides are carbon suboxide (C3O2), unstable carbon monoxide (C2O), carbon trioxide (CO3), cyclopentane peptone (C5O5), cyclohexane hexone (C6O6), and mellitic anhydride (C12O9). With reactive metals such as tungsten, carbon forms either carbides (C4-) or acetylides (C2-2) to form alloys with high melting points. These anions are also associated with methane and acetylene, both of which are very weak acids. With an electronegativity of 2.5, carbon prefers to form covalent bonds. Several carbides are covalent lattices such as carborundum (SiC), which resembles diamond. However, even the most polar and salt-like carbides are not completely ionic compounds.

Organometallic compounds

Organometallic compounds, by definition, contain at least one carbon-metal bond. There is a wide range of such compounds; major classes include simple alkyl-metal compounds (eg, tetraethyl alide), η2-alkene compounds (eg, Zeise salt), and η3-allylic compounds (eg, allylpalladium chloride dimer); metallocenes containing cyclopentadienyl ligands (eg ferrocene); and carbene complexes of transition metals. There are many metal carbonyls (eg tetracarbonylnickel); Some workers believe that the carbon monoxide ligand is a purely inorganic, rather than organometallic, compound. While carbon is thought to exclusively form four bonds, an interesting compound containing an octahedral hexacoordinate carbon atom has been reported. The cation of this compound is 2+. This phenomenon is explained by the aurophilicity of gold ligands. In 2016, hexamethylbenzene was confirmed to contain a carbon atom with six bonds rather than the usual four.

History and etymology

The English name carbon comes from the Latin carbo, meaning "coal" and "charcoal", hence the French word charbon, meaning "charcoal". In German, Dutch and Danish, the names for carbon are Kohlenstoff, koolstof and kulstof respectively, all literally meaning coal substance. Carbon was discovered in prehistoric times and was known in the forms of soot and charcoal in the earliest human civilizations. Diamonds were probably known as early as 2500 BC. in China, and carbon in the form of charcoal was made in Roman times by the same chemistry as today, by heating wood in a pyramid covered with clay to exclude air. In 1722, René Antoine Ferjo de Reamur demonstrated that iron was converted into steel through the absorption of a substance now known as carbon. In 1772, Antoine Lavoisier showed that diamonds are a form of carbon; when he burned samples of charcoal and diamond and discovered that neither produced any water, and that both substances released equal amounts of carbon dioxide per gram. In 1779, Karl Wilhelm Scheele showed that graphite, which was thought to be a form of lead, was instead identical to charcoal but with a small admixture of iron, and that it produced "air acid" (which is carbon dioxide) when oxidized with nitric acid. In 1786, French scientists Claude Louis Berthollet, Gaspard Monge, and C. A. Vandermonde confirmed that graphite was primarily carbon by oxidizing it in oxygen in much the same way as Lavoisier did with diamond. Some iron remained again, which French scientists believed was necessary for the structure of graphite. In their publication, they proposed the name carbone (Latin for carbonum) for the element in graphite that was released as a gas when graphite was burned. Antoine Lavoisier then listed carbon as an element in his 1789 textbook. A new carbon allotrope, fullerene, which was discovered in 1985, includes nanostructured forms such as buckyballs and nanotubes. Their discoverers, Robert Curl, Harold Croteau and Richard Smalley, received the Nobel Prize in Chemistry in 1996. The resulting renewed interest in new forms leads to the discovery of additional exotic allotropes, including glassy carbon, and the realization that "amorphous carbon" is not strictly amorphous.

Production

Graphite

Commercially viable natural graphite deposits occur in many parts of the world, but the most economically important sources are found in China, India, Brazil and North Korea. Graphite deposits are of metamorphic origin, found in association with quartz, mica and feldspars in schists, gneisses and metamorphosed sandstones and limestones as lenses or veins, sometimes several meters or more thick. The supply of graphite at Borrowdale, Cumberland, England, was initially of sufficient size and purity that until the 19th century pencils were made simply by sawing blocks of natural graphite into strips before gluing the strips into wood. Today, smaller deposits of graphite are produced by crushing the parent rock and floating the lighter graphite on water. There are three types of natural graphite - amorphous, flake or crystalline. Amorphous graphite is the lowest quality and is the most common. Unlike in science, in industry "amorphous" refers to a very small crystal size rather than a complete lack of crystalline structure. The word "amorphous" is used to describe products with a low amount of graphite and is the cheapest graphite. Large deposits of amorphous graphite are located in China, Europe, Mexico and the USA. Flat graphite is less common and of higher quality than amorphous; it appears as individual plates that crystallize in metamorphic rocks. The price of granular graphite can be four times higher than the price of amorphous graphite. Good quality flake graphite can be processed into expandable graphite for many applications such as fire retardants. Primary deposits of graphite are found in Austria, Brazil, Canada, China, Germany and Madagascar. Liquid or lump graphite is the rarest, most valuable and highest quality type of natural graphite. It is found in veins along intrusive contacts in hard pieces, and is only mined commercially in Sri Lanka. According to the USGS, global production of natural graphite in 2010 was 1.1 million tons, with China producing 800,000 tons, India 130,000 tons, Brazil 76,000 tons, North Korea 30,000 tons and Canada - 25,000 tons. No natural graphite was mined in the United States, but 118,000 tons of synthetic graphite were mined in 2009 with an estimated value of $998 million.

Diamond

The supply of diamonds is controlled by a limited number of businesses and is highly concentrated in a small number of locations around the world. Only a very small proportion of diamond ore consists of actual diamonds. The ore is crushed, during which care must be taken to prevent large diamonds from being destroyed in the process, and the particles are then sorted by density. Today, diamonds are mined into the diamond-rich fraction using X-ray fluorescence, after which the final sorting steps are carried out manually. Before the widespread use of X-rays, separation was carried out using lubricating belts; It is known that diamonds were discovered only in alluvial deposits in southern India. It is known that diamonds are more likely to stick to the mass than other minerals in the ore. India was a leader in the production of diamonds from their discovery around the 9th century BC until the mid-18th century AD, but the commercial potential of these sources was exhausted by the end of the 18th century, and by then India had been overshadowed by Brazil, where the first diamonds were found in 1725. Diamond production of primary deposits (kimberlites and lamproites) began only in the 1870s, after the discovery of diamond deposits in South Africa. Diamond production has increased over time and a total of 4.5 billion carats have been accumulated since this date. About 20% of this amount has been produced in the last 5 years alone, and within the last ten years 9 new deposits have begun production, with another 4 waiting to be discovered soon. Most of these deposits are located in Canada, Zimbabwe, Angola and one in Russia. In the United States, diamonds have been discovered in Arkansas, Colorado and Montana. In 2004, the astonishing discovery of a microscopic diamond in the United States led to the release in January 2008 of a massive sampling of kimberlite pipes in a remote part of Montana. Today, the majority of commercially viable diamond deposits are found in Russia, Botswana, Australia and the Democratic Republic of Congo. In 2005, Russia produced almost one-fifth of the world's diamond supply, according to the British Geological Survey. Australia's richest diamond pipe reached peak production levels of 42 metric tons (41 t; 46 short tons) per year in the 1990s. There are also commercial deposits, active production of which is carried out in the Northwest Territories of Canada, Siberia (mainly in Yakutia, for example, in the Mir Pipe and in the Udachnaya Pipe), in Brazil, as well as in Northern and Western Australia.

Applications

Carbon is essential for all known living systems. Without it, the existence of life as we know it is impossible. The main economic use of carbon, other than food and wood, is for hydrocarbons, primarily fossil fuels methane gas and crude oil. Crude oil is processed by oil refineries to produce gasoline, kerosene and other products. Cellulose is a natural carbon-containing polymer produced by plants in the form of wood, cotton, flax and hemp. Cellulose is used primarily to maintain the structure of plants. Commercially valuable animal-derived carbon polymers include wool, cashmere, and silk. Plastics are made from synthetic carbon polymers, often with oxygen and nitrogen atoms included at regular intervals in the main polymer chain. The raw materials for many of these synthetics come from crude oil. The uses of carbon and its compounds are extremely diverse. Carbon can form alloys with iron, the most common of which is carbon steel. Graphite combines with clays to form the “lead” used in pencils used for writing and drawing. It is also used as a lubricant and pigment, as a molding material in glass making, in electrodes for dry cell batteries and electroplating and electroforming, in brushes for electric motors, and as a neutron moderator in nuclear reactors. Coal is used as a material for making art, as a barbecue grill, for smelting iron, and has many other uses. Wood, coal and oil are used as fuel for energy production and heating. High quality diamonds are used in jewelry making, and industrial diamonds are used for drilling, cutting, and polishing metal and stone working tools. Plastics are made from fossil hydrocarbons, and carbon fiber, made by pyrolyzing synthetic polyester fibers, is used to reinforce plastics to form advanced, lightweight composite materials. Carbon fiber is made by pyrolyzing extruded and stretched strands of polyacrylonitrile (PAN) and other organic substances. The crystal structure and mechanical properties of the fiber depend on the type of starting material and subsequent processing. Carbon fibers made from PAN have a structure that resembles narrow strands of graphite, but heat treatment can rearrange the structure into a continuous sheet. As a result, the fibers have a higher specific tensile strength than steel. Carbon black is used as a black pigment in printing inks, oil paint and artists' watercolors, carbon paper, automotive trim, inks and laser printers. Carbon black is also used as a filler in rubber products such as tires and in plastic compounds. Activated carbon is used as an absorbent and adsorbent in filter media in applications as varied as gas masks, water purification and kitchen hoods, as well as in medicine to absorb toxins, poisons or gases from the digestive system. Carbon is used in chemical reduction at high temperatures. Coke is used to reduce iron ore into iron (smelting). Hardening of steel is achieved by heating finished steel components in carbon powder. Silicon, tungsten, boron and titanium carbides are among the hardest materials and are used as abrasives for cutting and grinding. Carbon compounds make up the majority of materials used in clothing, such as natural and synthetic textiles and leather, as well as almost all interior surfaces in environments other than glass, stone and metal.

Diamonds

The diamond industry is divided into two categories, one of which is high quality diamonds (gems) and the other is industrial grade diamonds. Although there is a large trade in both types of diamonds, the two markets operate very differently. Unlike precious metals such as gold or platinum, gemstone diamonds are not traded as a commodity: diamonds are sold at a significant premium and the resale market for diamonds is not very active. Industrial diamonds are valued primarily for their hardness and thermal conductivity, with the gemological qualities of clarity and color being largely irrelevant. About 80% of mined diamonds (equal to approximately 100 million carats or 20 tons per year) are unusable and are used in industry (diamond scrap). Synthetic diamonds, invented in the 1950s, found industrial applications almost immediately; 3 billion carats (600 tons) of synthetic diamonds are produced annually. The dominant industrial uses of diamond are cutting, drilling, grinding and polishing. Most of these applications do not require large diamonds; in fact, most gem quality diamonds, with the exception of small size diamonds, can be used industrially. Diamonds are inserted into drill bits or saw blades or ground into powder for use in grinding and polishing. Specialized applications include laboratory use as storage for high pressure experiments, high performance bearings, and limited use in specialty windows. Thanks to advances in synthetic diamond production, new applications are becoming feasible. Much attention has been paid to the possible use of diamond as a semiconductor suitable for microchips and, because of its exceptional thermal conductivity, as a heat sink in electronics.

The idea that chemical bonds could result from the sharing of a pair of electrons between two atoms was put forward by Lewis (1916) and developed by Heitler and London (1927). Subsequently, Linus Pauling introduced the extremely important concepts of directed valence and orbital hybridization.

According to the concept directional valence, the connection of atoms is carried out in the direction that ensures maximum overlap of orbitals. The better the overlap, the stronger the bond must be, and only with maximum overlap is the minimum energy of the system achieved.

The carbon atom in its ground state has the electronic structure 1s22s22p2. Let's look carefully at the distribution of electrons among orbitals in a carbon atom:

Two unpaired electrons can form only two chemical bonds with other atoms, that is, in accordance with this scheme, the carbon atom must be divalent. But in organic chemistry, a carbon atom always has a valence of four.

To form four covalent bonds, a carbon atom must have four unpaired electrons.

How to explain the tetravalency of carbon?

An atom can change its valence state when paired electrons are paired off and transferred to other atomic orbitals. In our case, one electron from the s orbital moves to a free p orbital.

Let's consider the formation of bonds in the molecule of the simplest hydrogen compound of carbon - in the molecule of methane (CH4). Each hydrogen atom has one unpaired electron in the s orbital of the first electron layer (1s1). A carbon atom in an excited state has four unpaired electrons: one in the s and three in the p orbitals of the second layer. One would expect that, due to the different shapes of the s and p orbitals, the bonds between the carbon atom and the hydrogen atoms would be unequal. Research shows that the bonds in the methane molecule are equivalent.

In this case it turns out hybrid atomic orbitals , electrons

on which they have average energy.

So, 1-s electron and 3 p-electrons participate in hybridization, therefore this type of hybridization is called sp 3 -hybridization . This state of the orbitals of the carbon atom is called first valence state. Since four electrons are involved in hybridization, four identical hybrid orbitals are formed. When hybrid orbitals are formed, they move as far as possible from each other. The angle between them turns out to be equal to 109028/, that is, all the hybrid orbitals of the carbon atom in the state of sp3 hybridization are directed towards the vertices of the tetrahedron - a regular triangular pyramid.

A chemical bond is the overlap of atomic orbitals . Since carbon is tetravalent, there will be four chemical bonds. The hydrogen atom has one unpaired electron in the s orbital and is spherical in shape. Therefore, the CH4 methane molecule has the following spatial structure.

The ethane molecule CH3 – CH3 will accordingly have the following spatial structure:

https://pandia.ru/text/80/289/images/image016_17.jpg" align="left" width="147 height=110" height="110">Not all p-orbitals of an atom can take part in hybridization Thus, from one s - and two p-orbitals, three sp2-hybrid orbitals are formed, the angle between which is 1200 (a flat equilateral triangle). The remaining one p-orbital is located perpendicular to the plane in which the non-hybrid orbitals lie. p-electrons will participate in the formation of a π-bond, which is formed when p-clouds overlap laterally and is located above and below the plane of the bonding nuclei.

We encounter sp2 hybridization in compounds with a double bond; the atoms forming a double bond will be in sp2 hybridization.

Let us consider the spatial structure of the ethene molecule CH2 = CH2, in which the carbon atoms are in a state of sp2 hybridization. The wavy line in the figure shows the overlap of non-hybrid p orbitals (π bond).

The third valence state of the carbon atom, sp – hybridization .

When one s and one p orbital of a carbon atom is mixed, sp hybridization occurs. In sp hybridization of atomic orbitals, two p orbitals remain unhybridized. sp-Hybrid orbitals are oriented at an angle of 1800 to each other (linear configuration).

Two p-orbitals not involved in hybridization are located mutually perpendicular and participate in the formation of two π bonds. We encounter sp-hybridization in compounds with a triple bond, the carbon atoms forming a triple bond will be in sp-hybridization.

So , carbon atoms involved in the formation of simple, single σ-bonds are in a state of sp3-hybridization, carbon atoms involved in the formation of double bonds are in a state of sp2-hybridization, carbon atoms involved in the formation of triple bonds are in a state of sp-hybridization. Any multiple bond will always have one σ bond, all others will be π bonds. For example, in a molecule CH2 = CH2, between carbon atoms, one bond is σ-, the other is π-bond. In the CH≡CH molecule between the carbon atoms, there is one σ-bond and two π-bonds.

Test yourself to see how you understand the topic by completing the test task:

1. How many π-bonds does the butene -1 molecule contain (CH3 – CH2 – CH = CH2):

a) 2, b) 4, c) 1, c) 12.

2. How many carbon atoms in the pentin-2 molecule (CH3 – C ≡ C – CH2 – CH3) are in the state of sp3 hybridization:

a) all 5 carbon atoms, b) 2, c) 1, d) 3.

3.What is the expected equilibrium configuration of the CH2 = CH2 molecule:

a) linear, b) angular, c) flat equilateral triangle, d) tetrahedron.

4. Select compounds that are characterized by polar covalent bonds:

a) Cl2; b) C - H;

5. Determine the type of hybridization of atomic orbitals using the following data:

Correct test answers:

1. (c); 2. (d); 3. (c); 4. (c); 5. sp2.

An important area of ​​practical application of the latest discoveries in the field of physics, chemistry and even astronomy is the creation and research of new materials with unusual, sometimes unique properties. We will talk about the directions in which this work is being carried out and what scientists have already managed to achieve in a series of articles created in partnership with the Ural Federal University. Our first text is devoted to unusual materials that can be obtained from the most common substance - carbon.

If you ask a chemist which element is the most important, you can get a lot of different answers. Some will say about hydrogen - the most common element in the Universe, others about oxygen - the most common element in the earth's crust. But most often you will hear the answer “carbon” - it is the basis of all organic substances, from DNA and proteins to alcohols and hydrocarbons.

Our article is devoted to the diverse forms of this element: it turns out that dozens of different materials can be built from its atoms alone - from graphite to diamond, from carbyne to fullerenes and nanotubes. Although they are all composed of exactly the same carbon atoms, their properties are radically different - and the main role in this is played by the arrangement of the atoms in the material.

Graphite

Most often in nature, pure carbon can be found in the form of graphite - a soft black material that exfoliates easily and seems slippery to the touch. Many may remember that pencil leads are made from graphite - but this is not always true. Often the lead is made from a composite of graphite chips and glue, but there are also completely graphite pencils. Interestingly, more than one twentieth of the world's natural graphite production goes into pencils.

What is special about graphite? First of all, it conducts electricity well - although carbon itself is not like other metals. If you take a graphite plate, it turns out that along its plane the conductivity is about a hundred times greater than in the transverse direction. This is directly related to how the carbon atoms in the material are organized.

If we look at the structure of graphite, we will see that it consists of individual layers one atom thick. Each layer is a grid of hexagons, reminiscent of a honeycomb. The carbon atoms inside the layer are linked by covalent chemical bonds. Moreover, some of the electrons that provide a chemical bond are “smeared” over the entire plane. The ease of their movement determines the high conductivity of graphite along the plane of carbon flakes.

The individual layers are connected to each other thanks to van der Waals forces - they are much weaker than a conventional chemical bond, but are sufficient to ensure that the graphite crystal does not delaminate spontaneously. This discrepancy makes it much more difficult for electrons to move perpendicular to the planes - electrical resistance increases 100 times.

Due to its electrical conductivity, as well as the ability to embed atoms of other elements between layers, graphite is used as anodes for lithium-ion batteries and other current sources. Graphite electrodes are necessary for the production of aluminum metal - and even trolleybuses use graphite sliding contacts for current collectors.

In addition, graphite is a diamagnetic material, and has one of the highest susceptibility per unit mass. This means that if you place a piece of graphite in a magnetic field, it will try in every possible way to push this field out of itself - to the point that the graphite can levitate above a sufficiently strong magnet.

And the last important property of graphite is its incredible refractoriness. The most refractory substance today is considered to be one of the hafnium carbides with a melting point of about 4000 degrees Celsius. However, if you try to melt graphite, then at pressures of about one hundred atmospheres it will retain hardness up to 4800 degrees Celsius (at atmospheric pressure, graphite sublimates - evaporates, bypassing the liquid phase). Due to this, graphite-based materials are used, for example, in rocket nozzle housings.

Diamond

Many materials under pressure begin to change their atomic structure - a phase transition occurs. Graphite in this sense is no different from other materials. At pressures of one hundred thousand atmospheres and temperatures of 1–2 thousand degrees Celsius, layers of carbon begin to move closer to each other, chemical bonds arise between them, and once smooth planes become corrugated. Diamond is formed, one of the most beautiful forms of carbon.

The properties of diamond are radically different from those of graphite - it is a hard, transparent material. It is extremely difficult to scratch (ranking 10 on the Mohs hardness scale, this is the maximum hardness). Moreover, the electrical conductivity of diamond and graphite differs by a quintillion times (this is a number with 18 zeros).

Diamond in rock

Wikimedia Commons

This determines the use of diamonds: most of the mined and artificially produced diamonds are used in metalworking and other industries. For example, sharpening discs and cutting tools with diamond powder or coating are widely used. Diamond coatings are even used in surgery - for scalpels. The use of these stones in the jewelry industry is well known to everyone.

The amazing hardness is also used in scientific research - it is with the help of high-quality diamonds that laboratories study materials at pressures of millions of atmospheres. You can read more about this in our material “”.

Graphene

Instead of compressing and heating graphite, we will follow Andrei Geim and Konstantin Novoselov by gluing a piece of tape to the graphite crystal. Then peel it off - a thin layer of graphite will remain on the tape. Let's repeat this operation again - apply the tape to a thin layer and peel it off again. The layer will become even thinner. By repeating the procedure a few more times, we obtain graphene, the material for which the aforementioned British physicists received the Nobel Prize in 2010.

Graphene is a flat monolayer of carbon atoms, completely identical to the atomic layers of graphite. Its popularity is due to the unusual behavior of the electrons in it. They move as if they have no mass at all. In reality, of course, the mass of electrons remains the same as in any substance. The carbon atoms of the graphene frame are to blame for everything, attracting charged particles and forming a special periodic field.

Graphene-based device. In the background of the photo are gold contacts, above them is graphene, above is a thin layer of polymethyl methacrylate

Engineering at Cambridge / flickr.com

The consequence of this behavior is the greater mobility of electrons - they move in graphene much faster than in silicon. For this reason, many scientists hope that graphene will become the basis of the electronics of the future.

Interestingly, graphene has carbon brothers - and. The first of them consists of slightly distorted pentagonal sections and, unlike graphene, does not conduct electricity well. Phagraphene consists of pentagonal, hexagonal and heptagonal sections. If the properties of graphene are the same in all directions, then phagraphene will have a pronounced anisotropy of properties. Both of these materials have been theoretically predicted, but do not yet exist in reality.

A fragment of a silicon single crystal (in the foreground) on a vertical array of carbon nanotubes

Carbon nanotubes

Imagine that you rolled a small piece of graphene sheet into a tube and glued its edges together. The result is a hollow structure consisting of the same hexagons of carbon atoms as graphene and graphite - a carbon nanotube. This material is in many ways related to graphene - it has high mechanical strength (it was once proposed to build an elevator into space from carbon nanotubes), and high electron mobility.

However, there is one unusual feature. The graphene sheet can be rolled parallel to an imaginary edge (the side of one of the hexagons), or at an angle. It turns out that how we twist a carbon nanotube will greatly influence its electronic properties, namely whether it will be more like a semiconductor with a bandgap or more like a metal.

Multi-walled carbon nanotube

Wikimedia commons

It is not known for certain when carbon nanotubes were first observed. In the 1950s to 1980s, various groups of researchers involved in the catalysis of reactions involving hydrocarbons (for example, methane pyrolysis) paid attention to elongated structures in the soot covering the catalyst. Now, in order to synthesize carbon nanotubes of only a specific type (specific chirality), chemists propose using special seeds. These are small molecules in the form of rings, which in turn consist of hexagonal benzene rings. You can read about the work on their synthesis, for example.

Like graphene, carbon nanotubes have many applications in microelectronics. The first transistors based on nanotubes have already been created; their properties are similar to traditional silicon devices. In addition, nanotubes formed the basis of a transistor with.

Carbin

When talking about elongated structures of carbon atoms, one cannot fail to mention carbines. These are linear chains, which, according to theorists, may turn out to be the strongest material possible (we are talking about specific strength). For example, the Young's modulus for carbyne is estimated at 10 giganewtons per kilogram. For steel this figure is 400 times less, for graphene it is at least two times less.

A thin thread stretching towards an iron particle below - carbine

Wikimedia Commons

Carbynes come in two types, depending on how the bonds between the carbon atoms are arranged. If all the bonds in the chain are the same, then we are talking about cumulenes, but if the bonds alternate (single-triple-single-triple and so on), then we are talking about polyines. Physicists have shown that a carbyne thread can be “switched” between these two types by deformation - when stretched, cumulene turns into polyyne. Interestingly, this radically changes the electrical properties of carbyne. If polyyne conducts electric current, then cumulene is an insulator.

The main difficulty in studying carbines is that they are very difficult to synthesize. These are chemically active substances that are also easily oxidized. Today the chains are only six thousand atoms long. To achieve this, chemists had to grow carbyne inside a carbon nanotube. In addition, the synthesis of carbyne will help break the record for the size of the gate in a transistor - it can be reduced to one atom.

Fullerenes

Although the hexagon is one of the most stable configurations that carbon atoms can form, there is a whole class of compact objects where the regular carbon pentagon occurs. These objects are called fullerenes.

In 1985, Harold Kroteau, Robert Curl, and Richard Smalley studied carbon vapor and how carbon atoms clump together when cooled. It turned out that there are two classes of objects in the gas phase. The first is clusters consisting of 2–25 atoms: chains, rings and other simple structures. The second is clusters consisting of 40–150 atoms, which have not been observed before. Over the next five years, chemists were able to prove that this second class consists of hollow frameworks of carbon atoms, the most stable of which consists of 60 atoms and is shaped like a soccer ball. C 60, or buckminsterfullerene, consisted of twenty hexagonal sections and 12 pentagonal sections, fastened together into a sphere.

The discovery of fullerenes aroused great interest among chemists. Subsequently, an unusual class of endofullerenes was synthesized - fullerenes in the cavity of which there was some foreign atom or small molecule. For example, just a year ago, a molecule of hydrofluoric acid was first incorporated into a fullerene, which made it possible to very accurately determine its electronic properties.

Fullerites - fullerene crystals

Wikimedia Commons

In 1991, it turned out that fullerides - crystals of fullerenes in which part of the cavities between neighboring polyhedra are occupied by metals - are molecular superconductors with a record high transition temperature for this class, namely 18 kelvin (for K 3 C 60). Later, fullerides were found with an even higher transition temperature - 33 kelvin, Cs 2 RbC 60. Such properties turned out to be directly related to the electronic structure of the substance.

Q-carbon

Among the recently discovered forms of carbon is the so-called Q-carbon. It was first introduced by American materials scientists from the University of North Carolina in 2015. Scientists irradiated amorphous carbon using a powerful laser, locally heating the material to 4000 degrees Celsius. As a result, approximately a quarter of all carbon atoms in the substance adopted sp 2 hybridization, that is, the same electronic state as in graphite. The remaining Q-carbon atoms retained the hybridization characteristic of diamond.

Q-carbon

Unlike diamond, graphite and other forms of carbon, Q-carbon is ferromagnetic, such as magnetite or iron. At the same time, its Curie temperature was about 220 degrees Celsius - only with such heating did the material lose its magnetic properties. And by doping Q-carbon with boron, physicists obtained another carbon superconductor, with a transition temperature of about 58 kelvin.

***

The following are not all known forms of carbon. Moreover, right now, theorists and experimenters are creating and studying new carbon materials. In particular, such work is being carried out at the Ural Federal University. We turned to Anatoly Fedorovich Zatsepin, associate professor and chief researcher at the UrFU Institute of Physics and Technology, to find out how we can predict the properties of materials that have not yet been synthesized and create new forms of carbon.

Anatoly Zatsepin is working on one of the six breakthrough scientific projects at UrFU “Development of the fundamental principles of new functional materials based on low-dimensional modifications of carbon.” The work is carried out with academic and industrial partners in Russia and the world.

The project is being implemented by the UrFU Institute of Physics and Technology, a strategic academic unit (SAU) of the university. The university’s position in Russian and international rankings, primarily in subject rankings, depends on the success of researchers.

N+1: The properties of carbon nanomaterials are highly structure dependent and vary widely. Is it possible to somehow predict the properties of a material based on its structure?

Anatoly Zatsepin: It is possible to predict, and we are doing this. There are computer modeling methods that allow calculations to be made from first principles ( ab initio) - we lay down a certain structure, model and take all the fundamental characteristics of the atoms that make up this structure. The result is the properties that the material or new substance that we are modeling may have. In particular, with regard to carbon, we were able to simulate new modifications unknown to nature. They can be created artificially.

In particular, our laboratory at the Physics and Technology Institute of UrFU is currently developing, synthesizing and researching the properties of a new type of carbon. It can be called this: two-dimensionally ordered linear chain carbon. Such a long name is due to the fact that this material is a so-called 2D structure. These are films composed of individual carbon chains, and within each chain the carbon atoms are in the same “chemical form” - sp 1 hybridization. This gives completely unusual properties to the material; in sp 1 carbon chains, the strength exceeds the strength of diamond and other carbon modifications.

When we form films from these chains, a new material is obtained that has the properties inherent in carbon chains, plus the combination of these ordered chains forms a two-dimensional structure or superlattice on a special substrate. This material has great prospects not only due to its mechanical properties. The most important thing is that carbon chains in a certain configuration can be closed into a ring, which gives rise to very interesting properties, such as superconductivity, and the magnetic properties of such materials can be better than those of existing ferromagnets.

The challenge remains to actually create them. Our modeling shows the path where to go.

There is always an error, but the point is that calculations and modeling from first principles use the fundamental characteristics of individual atoms - quantum properties. And when structures are formed from these quantum atoms at such a micro- and nanoscale, the errors are associated with the existing limitation of the theory and those models that exist. For example, it is known that the Schrödinger equation can be solved exactly only for the hydrogen atom, and for heavier atoms certain approximations must be used if we are talking about solids or more complex systems.

On the other hand, errors can occur due to computer calculations. In all this, gross errors are excluded, and the accuracy is quite sufficient to predict one or another property or effect that will be inherent in a given material.

When it comes to carbon materials, there is a lot of variation, and I'm sure there is a lot that has yet to be explored and discovered. UrFU has everything to research new carbon materials, and there is a lot of work ahead.

We also work on other objects, for example, silicon materials for microelectronics. Silicon and carbon are, by the way, analogues; they are in the same group in the periodic table.

Vladimir Korolev

In this book, the word “carbon” appears quite often: in stories about green leaves and iron, about plastics and crystals, and in many others. Carbon - “giving birth coal” - is one of the most amazing chemical elements. Its history is the history of the emergence and development of life on Earth, because it is part of all living things on Earth.

What does carbon look like?

Let's do some experiments. Let's take sugar and heat it without air. It will first melt, turn brown, and then turn black and turn into coal, releasing water. If you now heat this coal in the presence of , it will burn without a residue and turn into . Therefore, sugar consisted of coal and water (sugar, by the way, is called a carbohydrate), and “sugar” coal is, apparently, pure carbon, because carbon dioxide is a compound of carbon with oxygen. This means carbon is a black, soft powder.

Let's take a gray soft graphite stone, well known to you thanks to pencils. If you heat it in oxygen, it will also burn without a residue, although a little slower than coal, and carbon dioxide will remain in the device where it burned. Does this mean that graphite is also pure carbon? Of course, but that's not all.

If a diamond, a transparent sparkling gemstone and the hardest of all minerals, is heated in oxygen in the same device, it too will burn, turning into carbon dioxide. If you heat a diamond without access to oxygen, it will turn into graphite, and at very high pressures and temperatures you can get a diamond from graphite.

So, coal, graphite and diamond are different forms of existence of the same element - carbon.

Even more amazing is the ability of carbon to “participate” in a huge number of different compounds (which is why the word “carbon” appears so often in this book).

The 104 elements of the periodic table form more than forty thousand studied compounds. And over a million compounds are already known, the basis of which is carbon!

The reason for this diversity is that carbon atoms can be connected to each other and to other atoms by strong bonds, forming complex ones in the form of chains, rings and other shapes. No element in the table except carbon is capable of this.

There is an infinite number of shapes that can be built from carbon atoms, and therefore an infinite number of possible compounds. These can be very simple substances, for example, the illuminating gas methane, in a molecule of which four atoms are bonded to one carbon atom, and so complex that the structure of their molecules has not yet been established. Such substances include

Carbon in the periodic table of elements is located in the second period in group IVA. Electronic configuration of carbon atom ls 2 2s 2 2p 2 . When it is excited, an electronic state is easily achieved in which there are four unpaired electrons in the four outer atomic orbitals:

This explains why carbon in compounds is usually tetravalent. The equality of the number of valence electrons in the carbon atom to the number of valence orbitals, as well as the unique ratio of the charge of the nucleus and the radius of the atom, gives it the ability to equally easily attach and give up electrons, depending on the properties of the partner (Section 9.3.1). As a result, carbon is characterized by various oxidation states from -4 to +4 and the ease of hybridization of its atomic orbitals according to the type sp 3, sp 2 And sp 1 during the formation of chemical bonds (section 2.1.3):

All this gives carbon the opportunity to form single, double and triple bonds not only with each other, but also with atoms of other organogenic elements. The molecules formed in this case can have a linear, branched or cyclic structure.

Due to the mobility of common electrons -MOs formed with the participation of carbon atoms, they are shifted towards the atom of a more electronegative element (inductive effect), which leads to the polarity of not only this bond, but also the molecule as a whole. However, carbon, due to the average electronegativity value (0E0 = 2.5), forms weakly polar bonds with atoms of other organogenic elements (Table 12.1). If there are systems of conjugated bonds in molecules (Section 2.1.3), delocalization of mobile electrons (MO) and lone electron pairs occurs with equalization of the electron density and bond lengths in these systems.

From the point of view of the reactivity of compounds, the polarizability of bonds plays an important role (Section 2.1.3). The greater the polarizability of a bond, the higher its reactivity. The dependence of the polarizability of carbon-containing bonds on their nature is reflected in the following series:

All the considered data on the properties of carbon-containing bonds indicate that carbon in compounds forms, on the one hand, fairly strong covalent bonds with each other and with other organogens, and on the other hand, the common electron pairs of these bonds are quite labile. As a result, both an increase in the reactivity of these bonds and stabilization can occur. It is these features of carbon-containing compounds that make carbon the number one organogen.

Acid-base properties of carbon compounds. Carbon monoxide (4) is an acidic oxide, and its corresponding hydroxide - carbonic acid H2CO3 - is a weak acid. The carbon monoxide(4) molecule is non-polar, and therefore it is poorly soluble in water (0.03 mol/l at 298 K). In this case, first, the hydrate CO2 H2O is formed in the solution, in which CO2 is located in the cavity of the associate of water molecules, and then this hydrate slowly and reversibly turns into H2CO3. Most of the carbon monoxide (4) dissolved in water is in the form of hydrate.

In the body, in red blood cells, under the action of the enzyme carboanhydrase, the equilibrium between CO2 hydrate H2O and H2CO3 is established very quickly. This allows us to neglect the presence of CO2 in the form of hydrate in the erythrocyte, but not in the blood plasma, where there is no carbonic anhydrase. The resulting H2CO3 dissociates under physiological conditions to a hydrocarbonate anion, and in a more alkaline environment to a carbonate anion:

Carbonic acid exists only in solution. It forms two series of salts - hydrocarbonates (NaHCO3, Ca(HC0 3)2) and carbonates (Na2CO3, CaCO3). Hydrocarbonates are more soluble in water than carbonates. In aqueous solutions, carbonic acid salts, especially carbonates, easily hydrolyze at the anion, creating an alkaline environment:

Substances such as baking soda NaHC03; chalk CaCO3, white magnesia 4MgC03 * Mg(OH)2 * H2O, hydrolyzed to form an alkaline environment, are used as antacids (acid neutralizers) to reduce the increased acidity of gastric juice:

The combination of carbonic acid and bicarbonate ion (H2CO3, HCO3(-)) forms a bicarbonate buffer system (section 8.5) - a nice buffer system of the blood plasma, which ensures a constant blood pH at pH = 7.40 ± 0.05.

The formation of Mg(OH)2 is caused by complete hydrolysis of the magnesium cation, which occurs under these conditions due to the lower solubility of Mg(0H)2 compared to MgC03.

In medical and biological practice, in addition to carbonic acid, one has to deal with other carbon-containing acids. This is primarily a large variety of different organic acids, as well as hydrocyanic acid HCN. From the standpoint of acidic properties, the strength of these acids is different:

These differences are due to the mutual influence of the atoms in the molecule, the nature of the dissociating bond, and the stability of the anion, i.e., its ability to delocalize the charge.

Hydrocyanic acid, or hydrogen cyanide, HCN - colorless, highly volatile liquid (T kip = 26 °C) with the smell of bitter almonds, miscible with water in any ratio. In aqueous solutions it behaves as a very weak acid, the salts of which are called cyanides. Alkali and alkaline earth metal cyanides are soluble in water, but they hydrolyze at the anion, which is why their aqueous solutions smell like hydrocyanic acid (the smell of bitter almonds) and have a pH > 12:

As a result of this reaction, potassium cyanide (potassium cyanide) and its solutions lose their toxicity during long-term storage. Cyanide anion is one of the most powerful inorganic poisons, since it is an active ligand and easily forms stable complex compounds with enzymes containing Fe 3+ and Cu2(+) as complexing ions (Sect. 10.4).

Redox properties. Since carbon in compounds can exhibit any oxidation state from -4 to +4, during the reaction free carbon can both donate and gain electrons, acting as a reducing agent or an oxidizing agent, respectively, depending on the properties of the second reagent:

Under conditions of anaerobic oxidation with a lack or absence of oxygen, carbon atoms of an organic compound, depending on the content of oxygen atoms in these compounds and external conditions, can turn into C0 2, CO, C and even CH 4, and other organogens turn into H2O, NH3 and H2S .

In the body, the complete oxidation of organic compounds with oxygen in the presence of oxidase enzymes (aerobic oxidation) is described by the equation:

From the given equations of oxidation reactions it is clear that in organic compounds only carbon atoms change the oxidation state, while the atoms of other organogens retain their oxidation state.

During hydrogenation reactions, i.e., the addition of hydrogen (a reducing agent) to a multiple bond, the carbon atoms that form it reduce their oxidation state (act as oxidizing agents):

Organic substitution reactions with the emergence of a new intercarbon bond, for example in the Wurtz reaction, are also redox reactions in which carbon atoms act as oxidizing agents and metal atoms act as reducing agents:

A similar thing is observed in the reactions of the formation of organometallic compounds:

As a result of the reactions of addition of a polar reagent to the substrate via a multiple intercarbon bond, one of the carbon atoms lowers the oxidation state, exhibiting the properties of an oxidizing agent, and the other increases the oxidation degree, acting as a reducing agent:

In these cases, an intramolecular oxidation-reduction reaction of carbon atoms of the substrate takes place, i.e., the process dismutation, under the influence of a reagent that does not exhibit redox properties.

Typical reactions of intramolecular dismutation of organic compounds due to their carbon atoms are the decarboxylation reactions of amino acids or keto acids, as well as the rearrangement and isomerization reactions of organic compounds, which were discussed in section. 9.3. The given examples of organic reactions, as well as reactions from Sect. 9.3 convincingly indicate that carbon atoms in organic compounds can be both oxidizing agents and reducing agents.

Carbon atom in a compound- an oxidizing agent, if as a result of the reaction the number of its bonds with atoms of less electronegative elements (hydrogen, metals) increases, because by attracting the common electrons of these bonds to itself, the carbon atom in question lowers its oxidation state.

Carbon atom in a compound- a reducing agent, if as a result of the reaction the number of its bonds with atoms of more electronegative elements increases(C, O, N, S), because by pushing away the shared electrons of these bonds, the carbon atom in question increases its oxidation state.

Thus, many reactions in organic chemistry, due to the redox duality of carbon atoms, are redox. However, unlike similar reactions in inorganic chemistry, the redistribution of electrons between the oxidizing agent and the reducing agent in organic compounds can only be accompanied by a displacement of the common electron pair of the chemical bond to the atom acting as the oxidizing agent. In this case, this connection can be preserved, but in cases of strong polarization it can be broken.

Complexing properties of carbon compounds. The carbon atom in compounds does not have lone electron pairs, and therefore only carbon compounds containing multiple bonds with its participation can act as ligands. Particularly active in complex formation processes are the electrons of the polar triple bond of carbon monoxide (2) and the hydrocyanic acid anion.

In the carbon monoxide molecule (2), the carbon and oxygen atoms form one and one -bond due to the mutual overlap of their two 2p-atomic orbitals according to the exchange mechanism. The third bond, i.e., another -bond, is formed according to the donor-acceptor mechanism. The acceptor is the free 2p-atomic orbital of the carbon atom, and the donor is the oxygen atom, which provides a lone pair of electrons from the 2p-orbital:

The increased bond ratio provides this molecule with high stability and inertness under normal conditions in terms of acid-base (CO is a non-salt-forming oxide) and redox properties (CO is a reducing agent at T > 1000 K). At the same time, it makes it an active ligand in complexation reactions with d-metal atoms and cations, primarily with iron, with which it forms iron pentacarbonyl, a volatile toxic liquid:

These equilibria are shifted towards the formation of carboxyhemoglobin ННbСО, the stability of which is 210 times greater than that of oxyhemoglobin ННbО2. This leads to the accumulation of carboxyhemoglobin in the blood and therefore reduces its ability to carry oxygen.

The hydrocyanic acid anion CN- also contains easily polarizable electrons, which is why it effectively forms complexes with d-metals, including life metals that are part of enzymes. Therefore, cyanides are highly toxic compounds (Section 10.4).

Carbon cycle in nature. The carbon cycle in nature is mainly based on the reactions of oxidation and reduction of carbon (Fig. 12.3).

Plants assimilate (1) carbon monoxide (4) from the atmosphere and hydrosphere. Part of the plant mass is consumed (2) by humans and animals. The respiration of animals and the decay of their remains (3), as well as the respiration of plants, the rotting of dead plants and the combustion of wood (4) return CO2 to the atmosphere and hydrosphere. The process of mineralization of the remains of plants (5) and animals (6) with the formation of peat, fossil coals, oil, gas leads to the transition of carbon into natural resources. Acid-base reactions (7) operate in the same direction, occurring between CO2 and various rocks with the formation of carbonates (medium, acidic and basic):

This inorganic part of the cycle leads to loss of CO2 in the atmosphere and hydrosphere. Human activity in the combustion and processing of coal, oil, gas (8), firewood (4), on the contrary, abundantly enriches the environment with carbon monoxide (4). For a long time there was confidence that thanks to photosynthesis, the concentration of CO2 in the atmosphere remains constant. However, at present, the increase in CO2 content in the atmosphere due to human activity is not compensated by its natural decrease. The total release of CO2 into the atmosphere is growing exponentially by 4-5% per year. According to calculations, in 2000 the CO2 content in the atmosphere will reach approximately 0.04% instead of 0.03% (1990).

After considering the properties and characteristics of carbon-containing compounds, the leading role of carbon should once again be emphasized

Rice. 12.3. Carbon cycle in nature

Organogen No. 1: firstly, carbon atoms form the skeleton of molecules of organic compounds; secondly, carbon atoms play a key role in redox processes, since among the atoms of all organogens, it is carbon that is most characterized by redox duality. For more information about the properties of organic compounds, see module IV "Fundamentals of Bioorganic Chemistry".

General characteristics and biological role of p-elements of group IVA. Electronic analogues of carbon are elements of group IVA: silicon Si, germanium Ge, tin Sn and lead Pb (see Table 1.2). The radii of the atoms of these elements naturally increase with increasing atomic number, and their ionization energy and electronegativity naturally decrease (Section 1.3). Therefore, the first two elements of the group: carbon and silicon are typical non-metals, and germanium, tin, and lead are metals, since they are most characterized by the loss of electrons. In the series Ge - Sn - Pb, metallic properties increase.

From the point of view of redox properties, the elements C, Si, Ge, Sn and Pb under normal conditions are quite stable with respect to air and water (the metals Sn and Pb - due to the formation of an oxide film on the surface). At the same time, lead compounds (4) are strong oxidizing agents:

Complexing properties are most characteristic of lead, since its Pb 2+ cations are strong complexing agents compared to the cations of other p-elements of group IVA. Lead cations form strong complexes with bioligands.

Elements of group IVA differ sharply both in their content in the body and in their biological role. Carbon plays a fundamental role in the life of the body, where its content is about 20%. The content of other group IVA elements in the body is within 10 -6 -10 -3%. At the same time, if silicon and germanium undoubtedly play an important role in the life of the body, then tin and especially lead are toxic. Thus, with increasing atomic mass of group IVA elements, the toxicity of their compounds increases.

Dust consisting of particles of coal or silicon dioxide SiO2, when systematically exposed to the lungs, causes diseases - pneumoconiosis. In the case of coal dust, this is anthracosis, an occupational disease of miners. When dust containing Si02 is inhaled, silicosis occurs. The mechanism of development of pneumoconiosis has not yet been established. It is assumed that with prolonged contact of silicate sand grains with biological fluids, polysilicic acid Si02 yH2O is formed in a gel-like state, the deposition of which in cells leads to their death.

The toxic effect of lead has been known to mankind for a very long time. The use of lead to make dishes and water pipes led to massive poisoning of people. Currently, lead continues to be one of the main environmental pollutants, since the release of lead compounds into the atmosphere amounts to over 400,000 tons annually. Lead accumulates mainly in the skeleton in the form of poorly soluble phosphate Pb3(PO4)2, and when bones are demineralized, it has a regular toxic effect on the body. Therefore, lead is classified as a cumulative poison. The toxicity of lead compounds is associated primarily with its complexing properties and high affinity for bioligands, especially those containing sulfhydryl groups (-SH):

The formation of complex compounds of lead ions with proteins, phospholipids and nucleotides leads to their denaturation. Often lead ions inhibit EM 2+ metalloenzymes, displacing life metal cations from them:

Lead and its compounds are poisons that act primarily on the nervous system, blood vessels and blood. At the same time, lead compounds affect protein synthesis, the energy balance of cells and their genetic apparatus.

In medicine, the following external antiseptics are used as astringents: lead acetate Pb(CH3COO)2 ZH2O (lead lotions) and lead(2) oxide PbO (lead plaster). The lead ions of these compounds react with proteins (albumin) in the cytoplasm of microbial cells and tissues, forming gel-like albuminates. The formation of gels kills microbes and, in addition, makes it difficult for them to penetrate into tissue cells, which reduces the local inflammatory response.