International nomenclature of alkanes. Alkanes: structure, properties

Alkanes or aliphatic saturated hydrocarbons are compounds with an open (non-cyclic) chain, in the molecules of which the carbon atoms are connected to each other by a σ bond. The carbon atom in alkanes is in a state of sp 3 hybridization.

Alkanes form a homologous series in which each member differs by a constant structural unit -CH 2 -, which is called a homological difference. The simplest representative is methane CH4.

• General formula of alkanes: C n H 2n+2

For alkanes, in addition to structural isomerism, there is conformational isomerism and, starting with heptane, enantiomerism:

IUPAC nomenclature Prefixes are used in the names of alkanes n-, second-, iso, tert-, neo:

• n- means normal (uncorroded) structure of the hydrocarbon chain;
• second- applies only to recycled butyl;
• tert- means alkyl of tertiary structure;
• iso branches at the end of the chain;
• neo used for alkyl with a quaternary carbon atom.

The nomenclature of branched alkanes is based on the following basic rules:

• To construct a name, a long chain of carbon atoms is selected and numbered with Arabic numerals (locants), starting from the end closer to which the substituent is located, for example:

• If the same alkyl group occurs more than once, then multiplying prefixes are placed in front of it in the name di-(before a vowel di-), three-, tetra- etc. and designate each alkyl separately with a number, for example:

• If various alkyl substituents are placed in the side branches of the main chain, then they are rearranged alphabetically (with multiplying prefixes di-, tetra- etc., as well as prefixes n-, second-, tert- are not taken into account), for example:

• If two or more options for the longest chain are possible, then choose the one that has the maximum number of side branches.
• The names of complex alkyl groups are constructed according to the same principles as the names of alkanes, but the numbering of the alkyl chain is always autonomous and begins with the carbon atom having free valence, for example:

• When used in the name of such a group, it is put in brackets and the first letter of the entire name is taken into account in alphabetical order:

Industrial extraction methods 1. Extraction of alkanes gas. Natural gas consists mainly of methane and small admixtures of ethane, propane, and butane. Gas under pressure at low temperatures is divided into appropriate fractions.

2. Extraction of alkanes from oil. Crude oil is purified and processed (distillation, fractionation, cracking). Mixtures or individual compounds are obtained from processed products.

3. Hydrogenation of coal (method of F. Bergius, 1925). Hard or brown coal in autoclaves at 30 MPa in the presence of catalysts (oxides and sulfides of Fe, Mo, W, Ni) in a hydrocarbon environment is hydrogenated and converted into alkanes, the so-called motor fuel:

nC + (n+1)H 2 = C n H 2n+2

4. Oxosynthesis of alkanes (method of F. Fischer - G. Tropsch, 1922). Using the Fischer-Tropsch method, alkanes are obtained from synthesis gas. Synthesis gas is a mixture of CO and H 2 with different ratios. It is obtained from methane by one of the reactions that occur at 800-900°C in the presence of nickel oxide NiO supported on Al 2 O 3:

CH 4 + H 2 O ⇄ CO + 3H 2

CH 4 + CO 2 ⇄ 2CO + 2H 2

2CH 4 + O 2 ⇄ 2CO + 4H 2

Alkanes are obtained by the reaction (temperature about 300°C, Fe-Co catalyst):

nCO + (2n+1)H 2 → C n H 2n+2 + nH 2 O

The resulting mixture of hydrocarbons, consisting mainly of alkanes of the structure (n = 12-18), is called “syntin”.

5. Dry distillation. Alkanes are obtained in relatively small quantities by dry distillation or heating of coal, shale, wood, and peat without access to air. The approximate composition of the resulting mixture is 60% hydrogen, 25% methane and 3-5% ethylene.

Laboratory extraction methods 1. Preparation from haloalkyls

1.1. Reaction with metallic sodium (Wurz, 1855). The reaction consists of the interaction of an alkali metal with a haloalkyl and is used for the synthesis of higher symmetrical alkanes:

2CH 3 -I + 2Na ⇄ CH 3 -CH 3 + 2NaI

If two different haloalkyls participate in the reaction, a mixture of alkanes is formed:

3CH 3 -I + 3CH 3 CH 2 -I + 6Na → CH 3 -CH 3 + CH 3 CH 2 CH 3 + CH 3 CH 2 CH 2 CH 3 + 6NaI

1.2 Interaction with lithium dialkyl cuprates. The method (sometimes called the E. Core - H. House reaction) consists of the interaction of reactive lithium dialkyl cuprates R 2 CuLi with haloalkyls. First, lithium metal reacts with a haloalkane in an ether environment. Next, the corresponding alkyl lithium reacts with copper(I) halide to form a soluble lithium dialkyl cuprate:

CH 3 Cl + 2Li → CH 3 Li + LiCl

2CH 3 Li + CuI → (CH 3 ) 2 CuLi + LiI

When such a lithium dialkyl cuprate reacts with the corresponding haloalkyl, the final compound is formed:

(CH 3 ) 2 CuLi + 2CH 3 (CH 2 ) 6 CH 2 -I → 2CH 3 (CH 2 ) 6 CH 2 -CH 3 + LiI + CuI

The method makes it possible to achieve a yield of alkanes of almost 100% when using primary haloalkyls. With their secondary or tertiary structure, the yield is 30-55%. The nature of the alkyl component in lithium dialkyl cuprate has little effect on the yield of the alkane.

CH 3 I + H 2 → CH 4 + HI (Pd catalyst)

CH 3 CH 2 I + 2H → CH 3 CH 3 + HI

CH 3 I + HI → CH 4 + I 2

The method has no preparative value; a strong reducing agent, iodine water, is often used.

2. Preparation from salts of carboxylic acids.
2.1 Electrolysis of salts (Kolbe, 1849). The Kolbe reaction involves the electrolysis of aqueous solutions of carboxylic acid salts:

R-COONa ⇄ R-COO - + Na +

At the anode, the carboxylic acid anion is oxidized, forming a free radical, and is easily decarboxylated or eliminated by CO 2 . Alkyl radicals are further converted into alkanes due to recombination:

R-COO - → R-COO . + e -

R-COO. →R. +CO2

R. +R. → R-R

2.2 Fusion of salts of carboxylic acids with alkali. Alkali metal salts of carboxylic acids, when combined with alkali, form alkanes:

CH 3 CH 2 COONa + NaOH → Na 2 CO 3 + CH 3 CH 3

Alkanes are much lighter than water, non-polar and difficult to polarize, but they are soluble in most non-polar solvents, due to which they themselves can be a solvent for many organic compounds.

Alkanes :

Alkanes are saturated hydrocarbons, in the molecules of which all atoms are connected by single bonds. Formula -

Physical properties :

• Melting and boiling points increase with molecular weight and length of the carbon backbone
• Under normal conditions, unbranched alkanes from CH 4 to C 4 H 10 are gases; from C 5 H 12 to C 13 H 28 - liquids; after C 14 H 30 - solids.
• Melting and boiling points decrease from less branched to more branched. So, for example, at 20 °C n-pentane is a liquid, and neopentane is a gas.

Chemical properties:

· Halogenation

this is one of the substitution reactions. The least hydrogenated carbon atom is halogenated first (tertiary atom, then secondary, primary atoms are halogenated last). The halogenation of alkanes occurs in stages - no more than one hydrogen atom is replaced in one stage:

1. CH 4 + Cl 2 → CH 3 Cl + HCl (chloromethane)
2. CH 3 Cl + Cl 2 → CH 2 Cl 2 + HCl (dichloromethane)
3. CH 2 Cl 2 + Cl 2 → CHCl 3 + HCl (trichloromethane)
4. CHCl 3 + Cl 2 → CCl 4 + HCl (carbon tetrachloride).

Under the influence of light, a chlorine molecule breaks down into radicals, then they attack alkane molecules, taking away a hydrogen atom from them, as a result of which methyl radicals CH 3 are formed, which collide with chlorine molecules, destroying them and forming new radicals.

· Combustion

The main chemical property of saturated hydrocarbons, which determines their use as fuel, is the combustion reaction. Example:

CH 4 + 2O 2 → CO 2 + 2H 2 O + Q

In case of lack of oxygen, carbon monoxide or coal is produced instead of carbon dioxide (depending on the oxygen concentration).

In general, the combustion reaction of alkanes can be written as follows:

WITH n H 2 n +2 +(1,5n+0.5)O 2 = n CO 2 + ( n+1)H 2 O

· Decomposition

Decomposition reactions occur only under the influence of high temperatures. An increase in temperature leads to the rupture of carbon bonds and the formation of free radicals.

Examples:

CH 4 → C + 2H 2 (t > 1000 °C)

C 2 H 6 → 2C + 3H 2

Alkenes :

Alkenes are unsaturated hydrocarbons containing in the molecule, in addition to single bonds, one carbon-carbon double bond. Formula - C n H 2n

The belonging of a hydrocarbon to the class of alkenes is reflected by the generic suffix –ene in its name.

Physical properties :

• The melting and boiling points of alkenes (simplified) increase with molecular weight and length of the carbon backbone.
• Under normal conditions, alkenes from C 2 H 4 to C 4 H 8 are gases; from C 5 H 10 to C 17 H 34 - liquids, after C 18 H 36 - solids. Alkenes are insoluble in water, but are highly soluble in organic solvents.

Chemical properties :

· Dehydration is the process of splitting off a water molecule from a molecule of an organic compound.

· Polymerization is a chemical process of combining many initial molecules of a low molecular weight substance into large polymer molecules.

Polymer is a high-molecular compound whose molecules consist of many identical structural units.

Alcadienes :

Alkadienes are unsaturated hydrocarbons containing in the molecule, in addition to single bonds, double carbon-carbon bonds. Formula -

Physical properties :

Butadiene is a gas (boiling point −4.5 °C), isoprene is a liquid boiling at 34 °C, dimethylbutadiene is a liquid boiling at 70 °C. Isoprene and other diene hydrocarbons are capable of polymerizing into rubber. Natural rubber in its purified state is a polymer with the general formula (C5H8)n and is obtained from the milky sap of some tropical plants.

Rubber is highly soluble in benzene, gasoline, and carbon disulfide. At low temperatures it becomes brittle and sticky when heated. To improve the mechanical and chemical properties of rubber, it is converted into rubber by vulcanization. To obtain rubber products, they are first molded from a mixture of rubber with sulfur, as well as fillers: soot, chalk, clay and some organic compounds that serve to accelerate vulcanization. Then the products are heated - hot vulcanization. During vulcanization, sulfur chemically bonds with the rubber. In addition, vulcanized rubber contains sulfur in a free state in the form of tiny particles.

Diene hydrocarbons polymerize easily. The polymerization reaction of diene hydrocarbons underlies the synthesis of rubber. They undergo addition reactions (hydrogenation, halogenation, hydrohalogenation):

H 2 C=CH-CH=CH 2 + H 2 -> H 3 C-CH=CH-CH 3

Alkynes :

Alkynes are unsaturated hydrocarbons whose molecules contain, in addition to single bonds, one triple carbon-carbon bond. Formula-C n H 2n-2

Physical properties :

Alkynes resemble the corresponding alkenes in their physical properties. Lower (up to C 4) are colorless and odorless gases that have higher boiling points than their analogues in alkenes.

Alkynes are poorly soluble in water, but better in organic solvents.

Chemical properties :

Halogenation reactions

Alkynes are capable of adding one or two halogen molecules to form the corresponding halogen derivatives:

Hydration

In the presence of mercury salts, alkynes add water to form acetaldehyde (for acetylene) or ketone (for other alkynes)

Alkanes are compounds of the homologous series of methane. These are saturated non-cyclic hydrocarbons. The chemical properties of alkanes depend on the structure of the molecule and the physical state of the substances.

Structure of alkanes

An alkane molecule consists of carbon and hydrogen atoms, which form methylene (-CH 2 -) and methyl (-CH 3) groups. Carbon can form four covalent nonpolar bonds with neighboring atoms. It is the presence of strong σ-bonds -C-C- and -C-H that determines the inertness of the homologous series of alkanes.

Rice. 1. The structure of an alkane molecule.

The compounds react when exposed to light or heat. Reactions proceed by a chain (free radical) mechanism. Thus, bonds can only be broken down by free radicals. As a result of hydrogen substitution, haloalkanes, salts, and cycloalkanes are formed.

Alkanes are classified as saturated or saturated carbons. This means that the molecules contain the maximum number of hydrogen atoms. Due to the absence of free bonds, addition reactions are not typical for alkanes.

Chemical properties

General properties of alkanes are given in the table.

To understand how the reaction proceeds and which radicals are replaced, it is recommended to write down the structural formulas.

Rice. 2. Structural formulas.

Application

Alkanes are widely used in industrial chemistry, cosmetology, and construction. The compounds are made from:

• fuel (gasoline, kerosene);
• asphalt;
• lubricating oils;
• petrolatum;
• paraffin;
• soap;
• varnishes;
• paints;
• enamels;
• alcohols;
• synthetic fabrics;
• rubber;
• addehydes;
• plastics;
• detergents;
• acids;
• propellants;
• cosmetics.

Rice. 3. Products obtained from alkanes.

What have we learned?

Learned about the chemical properties and uses of alkanes. Due to the strong covalent bonds between carbon atoms, as well as between carbon and hydrogen atoms, alkanes are inert. Substitution and decomposition reactions are possible in the presence of a catalyst at high temperatures. Alkanes are saturated hydrocarbons, so addition reactions are impossible. Alkanes are used to produce materials, detergents, and organic compounds.

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Rules for systematic nomenclature:

• Main chain rule.

The main circuit is selected based on the following criteria:

• Maximum number of functional substituents.
• Maximum number of multiple connections.
• Maximum length.
• Maximum number of side hydrocarbon groups.
• Rule of smallest numbers (locants).

The main circuit is numbered from one end to the other in Arabic numerals. Each substituent is assigned the number of the main chain carbon atom to which it is attached. The numbering sequence is chosen in such a way that the sum of the numbers of the substituents (locants) is the smallest. This rule also applies when numbering monocyclic compounds.

• Radical rule.

All hydrocarbon side groups are considered to be monovalent (simply connected) radicals. If the side radical itself contains side chains, then according to the above rules, an additional main chain is selected, which is numbered starting from the carbon atom attached to the main chain.

• Alphabetical order rule.

The name of the compound begins with a list of substituents, indicating their names in alphabetical order. The name of each substituent is preceded by its number in the main chain. The presence of several substituents is indicated by numerator prefixes: di-, tri-, tetra-, etc. After this, the hydrocarbon corresponding to the main chain is named.

In table Table 12.1 shows the names of the first five hydrocarbons, their radicals, possible isomers and their corresponding formulas. The names of radicals end with the suffix -yl.

Example. Name all isomers of hexane.

Example. Name the alkane with the following structure

In this example, from two twelve-atom chains, the one in which the sum of the numbers is the smallest is selected (rule 2).

Using the names of branched radicals given in table. 12.2,

The name of this alkane is somewhat simplified:

10-tert-butyl-2,2-(dimethyl)-7-propyl-4-isopropyl-3-ethyl-dodecane.

When a hydrocarbon chain closes into a cycle with the loss of two hydrogen atoms, monocycloalkanes are formed with the general formula C n H 2 n. Cyclization starts with C 3, names are formed from C n with the cyclo prefix:

Polycyclic alkanes. Their names are formed by the prefix bicyclo-, tricyclo-, etc. Bicyclic and tricyclic compounds contain, respectively, two and three rings in the molecule; to describe their structure, the number of carbon atoms in each of the chains connecting the node atoms is indicated in decreasing order in square brackets ; under the formula is the name of the atom:

This tricyclic hydrocarbon is commonly called adamantane (from the Czech adamant, diamond) because it is a combination of three fused cyclohexane rings in a form that results in the arrangement of carbon atoms in the crystal lattice that is characteristic of diamond.

Cyclic hydrocarbons with one common carbon atom are called spiranes, for example, spiro-5,5-undecane:

Planar cyclic molecules are unstable, so various conformational isomers are formed. Unlike configurational isomers (the spatial arrangement of atoms in a molecule without taking into account orientation), conformational isomers differ from each other only by the rotation of atoms or radicals around formally simple bonds while maintaining the configuration of the molecules. The energy of formation of a stable conformer is called conformational.

Conformers are in dynamic equilibrium and transform into each other through unstable forms. The instability of planar cycles is caused by significant deformation of bond angles. While maintaining the tetrahedral bond angles for cyclohexane C 6H 12, two stable conformations are possible: chair-shaped (a) and bath-shaped (b):

Hydrocarbons in whose molecules the atoms are connected by single bonds and which correspond to the general formula C n H 2 n +2. In alkane molecules, all carbon atoms are in a state of sp 3 hybridization. This means that all four hybrid orbitals of the carbon atom are identical in shape, energy and are directed to the corners of an equilateral triangular pyramid - a tetrahedron. The angles between the orbitals are 109° 28′. Almost free rotation is possible around a single carbon-carbon bond, and alkane molecules can take on a wide variety of shapes with angles at the carbon atoms close to tetrahedral (109° 28′), for example, in the molecule n-pentane. It is especially worth recalling the bonds in alkane molecules. All bonds in the molecules of saturated hydrocarbons are single. The overlap occurs along the axis, connecting the nuclei of atoms, i.e. these are σ-bonds. Carbon-carbon bonds are nonpolar and poorly polarizable. The length of the C-C bond in alkanes is 0.154 nm (1.54 10 - 10 m). C-H bonds are somewhat shorter. The electron density is slightly shifted towards the more electronegative carbon atom, i.e. the C-H bond is weakly polar. The absence of polar bonds in the molecules of saturated hydrocarbons leads to the fact that they are poorly soluble in water and do not interact with charged particles (ions). The most characteristic reactions for alkanes are those involving free radicals. Homologous series of methane Homologs- substances that are similar in structure and properties and differ by one or more CH 2 groups. Isomerism and nomenclature Alkanes are characterized by so-called structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkane, which is characterized by structural isomers, is butane. Nomenclature Basics 1. Selection of the main circuit. The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in the molecule, which is, as it were, its basis. 2. Numbering of atoms of the main chain. The atoms of the main chain are assigned numbers. The numbering of the atoms of the main chain begins from the end to which the substituent is closest (structures A, B). If the substituents are located at an equal distance from the end of the chain, then numbering starts from the end at which there are more of them (structure B). If different substituents are located at equal distances from the ends of the chain, then numbering begins from the end to which the senior one is closest (structure D). The seniority of hydrocarbon substituents is determined by the order in which the letter with which their name begins appears in the alphabet: methyl (-CH 3), then ethyl (-CH 2 -CH 3), propyl (-CH 2 -CH 2 -CH 3 ), etc. Please note that the name of the substituent is formed by replacing the suffix -an with the suffix - silt in the name of the corresponding alkane. 3. Formation of the name. At the beginning of the name, numbers are indicated - the numbers of the carbon atoms at which the substituents are located. If there are several substituents at a given atom, then the corresponding number in the name is repeated twice separated by a comma (2,2-). After the number, the number of substituents is indicated with a hyphen ( di- two, three- three, tetra- four, penta- five) and the name of the substituent (methyl, ethyl, propyl). Then, without spaces or hyphens, the name of the main chain. The main chain is called a hydrocarbon - a member of the homologous series of methane ( methane CH 4, ethane C 2 H 6, propane C 3 H 8, C 4 H 10, pentane C 5 H 12, hexane C 6 H 14, heptane C 7 H 16, octane C 8 H 18, nonan S 9 H 20, dean C 10 H 22). Physical properties of alkanes The first four representatives of the homologous series of methane are gases. The simplest of them is methane - a colorless, tasteless and odorless gas (the smell of “gas”, upon sensing which you need to call 04, is determined by the smell of mercaptans - sulfur-containing compounds specially added to methane used in household and industrial gas appliances so that people , located next to them, could detect the leak by smell). Hydrocarbons of composition from C 4 H 12 to C 15 H 32 are liquids; heavier hydrocarbons are solids. The boiling and melting points of alkanes gradually increase with increasing carbon chain length. All hydrocarbons are poorly soluble in water; liquid hydrocarbons are common organic solvents. Chemical properties of alkanes Substitution reactions. The most characteristic reactions for alkanes are free radical substitution reactions, during which a hydrogen atom is replaced by a halogen atom or some group. Let us present the equations of characteristic reactions halogenation: In case of excess halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms with chlorine: The resulting substances are widely used as solvents and starting materials in organic syntheses. Dehydrogenation reaction(hydrogen abstraction). When alkanes are passed over a catalyst (Pt, Ni, Al 2 0 3, Cr 2 0 3) at high temperatures (400-600 ° C), a hydrogen molecule is eliminated and an alkene is formed: Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons burn to form carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode. 1. Combustion of saturated hydrocarbons is a free radical exothermic reaction, which is very important when using alkanes as fuel: In general, the combustion reaction of alkanes can be written as follows: 2. Thermal splitting of hydrocarbons. The process occurs via a free radical mechanism. An increase in temperature leads to homolytic cleavage of the carbon-carbon bond and the formation of free radicals. These radicals interact with each other, exchanging a hydrogen atom, to form an alkane molecule and an alkene molecule: Thermal decomposition reactions underlie the industrial process of hydrocarbon cracking. This process is the most important stage of oil refining. 3. Pyrolysis. When methane is heated to a temperature of 1000 °C, methane pyrolysis begins - decomposition into simple substances: When heated to a temperature of 1500 °C, the formation of acetylene is possible: 4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with a branched carbon skeleton are formed: 5. Aromatization. Alkanes with six or more carbon atoms in the chain cyclize in the presence of a catalyst to form benzene and its derivatives: Alkanes enter into reactions that proceed through the free radical mechanism, since all carbon atoms in alkane molecules are in a state of sp 3 hybridization. The molecules of these substances are built using covalent nonpolar C-C (carbon-carbon) bonds and weakly polar C-H (carbon-hydrogen) bonds. They do not contain areas with increased or decreased electron density, easily polarizable bonds, i.e., such bonds in which the electron density can shift under the influence of external factors (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, since the bonds in alkane molecules are not broken by the heterolytic mechanism. Did you like the article? 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