Topic 2. Hydrocarbons

Chapter 1. Alkanes

Compounds of carbon with hydrogen, which have normal (unbranched) and branched chains, are called Alkanes (synonyms - paraffins - obsolete, aliphatic hydrocarbons, saturated hydrocarbons).

The composition of alkanes corresponds to the general empirical formula (CnH2n + 2), where n is the number of carbon atoms in the molecule. Among the homologues of alkanes, each subsequent member of the series differs from the previous one by the structural unit –CH2– (the so-called methylene unit).

2.1.1. Physical properties

Methane, ethane, propane, butane under normal conditions are gases. Pentane, hexane, heptane (liquids), etc.

As the number of carbon atoms increases, the boiling point increases. With the same number of carbon atoms in the molecules of alkane isomers, the boiling point decreases with an increase in the degree of branching of the molecule, and the melting point, on the contrary, increases (due to a more compact packing of "spherical" molecules of branched alkanes in the crystal lattice). The more symmetrically structured the molecule, the easier and stronger its packing into a crystal and the higher the melting point. Factors affecting the boiling point have a smaller effect on the melting point, since during melting the van der Waals forces are only slightly weakened.

Alkanes are insoluble in water, they are limitedly miscible with methanol and ethanol. Pure Alkanes are odorless. Impurities give a specific smell to gasoline, kerosene and diesel fuel. The characteristic smell of natural and liquefied gases is due to the fact that impurities of sulfur compounds of hydrocarbons (thioalcohols and thioethers) are specially added to them in order to facilitate the determination of gas leaks without special devices, i.e. organoleptically. The smell of sulfur compounds is felt even at very small amounts.

2.1.2. Alkane nomenclature

For the naming of alkanes, both trivial names and systematic (IUPAC) and rational (substitutive) nomenclatures are used.

The first four members of the homologous series of alkanes (methane, ethane, propane, and butane) have trivial names. Some other compounds also have trivial names, for example isooctane (2,2,4-trimethylpentane) and neopentane (2,2-dimethylpropane).

The names of linear alkane molecules with the number of carbon atoms of 5 or more are derived from the names of Greek numerals (pentane, hexane, heptane, octane, nonane and decane, respectively, C5, C6, C7, C8, C9 and C10). Alkanes with a number of carbon atoms from 11 to 19 are named with the prefix decane (respectively, undecane C11, dodecane C12, tridecane C13, tetradecane C14, pentadecane C15, hexadecane C16, etc.), and starting from 20 carbon atoms, cozanes (eicosan C20, heneicosan-C21, docosane C22, tricosan C23, tetracosan C24, pentacosane C25, hexacosane, etc.).

Hydrocarbons with the number of carbon atoms in a molecule of 30 - 39 are called triacontans (triacontane C30, gentriacontane C31, dotriacontane C32, tritriacontane C33, tetratriacontane C34, etc.).

2.1.2.1. IUPAC nomenclature rules

1. The longest chain of carbon atoms is found in the molecule, regardless of the direction in which it is located, where it "turns", etc. If there are two such longest chains (paths), choose the one that includes the maximum number of substituents:

2. The chain is numbered from the edge where the deputy is much closer. Since the definition of this condition "by eye" is rather subjective, a simple rule should be used - the correct order should be considered the order of numbering of carbon atoms in the chain, which leads to the minimum sum of substituent numbers. If the chain has two different substituents, each of which is equidistant from its "own" end of the chain, the chain is numbered from the substituent whose name alphabetically begins earlier. In this case, the prefixes iso-, sec- and ter- are not taken into account:

3. If there are several identical substituents, only their numbers are listed, with the addition of the prefixes di-, tri-, tetra-, etc.

4. Substituents are listed in alphabetical order (butyl, methyl, propyl, ethyl, etc.). The name is formed by listing the numbers of the substituents, their names and ends with the name of the hydrocarbon that forms the longest isolated chain.

2.1.2.2. Rational (substitutional) nomenclature

According to this nomenclature, the methane (or ethane) link in the structure of the compound is isolated and the substituents that complement the structure are listed:

It is clear that the use of rational nomenclature is limited to compounds that are relatively simple in structure.

2.1.2.3. Classification of substituents

The names of substituents (radicals) are derived from the name of the corresponding alkane by replacing the –an ending with –yl. Information on the methods of naming radicals is given in the table:

Hydrocarbon	Radical	Naming	Naming of complex radical
СН4 methane	СН3-	methyl	methyl
СН3-СН3 ethane	СН3-СН2-	ethyl	ethyl
СН3-СН2-СН3 propane	СН3-СН2-СН2-	propyl (n-propyl)	prop-1-yl
		iso-propyl	methylethyl prop-2-yl
n-butane	СН3-СН2-СН2-СН2-	butyl (n-butyl)	but-1-yl
		sec-butyl	but-2-yl
Isobutane (2-methylpropane)		iso-butyl	2- methylprop-1-yl
		tert-butyl	2- methylprop-2-yl
n-Pentane	СН3-СН2-СН2-СН2-СН2-	n-pentyl	pent-1-yl
Isopentane (2-methylbutane)		iso-pentyl	3-methylbut-1-yl
		-	2-methylbut-1-yl
		-	1,1-dimethylprop-1-yl
		-	1,2-dimethylprop-1-yl
Neopentane (2,2-dimethylpropane)		neo-pentyl	2,2-dimethylprop-1-yl

If you carefully consider the table, the principle of naming complex radicals becomes clear. The longest chain is found and numbered from an atom that does not have one of the hydrogens (an atom with an unpaired electron, a radical), after which the substituents and their numbers are listed. The name ends in -yl:

1,2-dimethylprop-1-yl

2.1.3. Isomerism of alkanes

For alkanes with an open chain, only one type of isomerism is characteristic - structural. Isomers with the same number of carbon atoms in a molecule differ only in the number, type, and arrangement of substituents in the chain.

The same gross formula of alkanes C5H12 can correspond to several compounds that are different in structure (as well as in physical and chemical properties). Among which are distinguished:

CH3-CH2-CH2-CH2-CH3 n-pentane

(CH3) 2CH-CH2-CH3 iso-pentane (2-methylbutane)

(CH3) 4C neo-pentane (2,2-dimethylpropane)

There are 5 isomers for hydrocarbons with the composition C6H12:

n-hexane, 2-methylpentane (isopentane), 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane

The theoretically possible number of isomers increases exponentially with the number of carbon atoms in a molecule. Thus, a compound of the composition C7H16 can exist in the form of 9 isomers, C8H18-18 isomers, and for pentacosane C25H52, the existence of 36 797 588 isomers is theoretically possible.

2.1.4. The electronic structure of the carbon atom in alkanes

As part of alkanes (as well as cycloalkanes), all carbon atoms have sp3-hybridization, bonds are formed by four equivalent hybridized orbitals, obtained as a result of hybridization of one 2s and three 2p orbitals that are not equivalent:

To ensure minimal steric hindrance and mutual repulsion, these four equivalent molecular orbitals are located in space at equal distances from each other, directed to the vertices of the tetrahedron (the carbon atom nucleus is located in the center of the tetrahedron), and the spatial angles between the orbitals are about 109^o28 ':

In this state, four bonds, as a result of overlapping with the orbitals of other atoms, can be formed without hindrance.

Thus, for example, the ethane molecule looks like (hydrogen atoms are shown as yellow spheres, more precisely, their molecular s-orbitals):

The bonds between the C-C and C-H atoms are formed by overlapping hybridized orbitals. Such bonds are called s-bonds (sigma-bonds). Molecule fragments can rotate around the s-bond, so long hydrocarbon chains can bend freely in space. It should be borne in mind that carbon atoms in long chains are never in a straight line, even if the chain does not bend. The geometry of bonds in alkanes is such that a real molecule in its most "straightened" state may look, for example, like this:

2.1.5. Chemical properties of alkanes

Since all bonds in alkanes are saturated, chemical reactions can occur only as a result of preliminary cleavage of C-C or C-H bonds.

Of the possible options for breaking a bond - heterolytic (when both electrons forming a bond remain in one of the atoms, with the formation of ions):

СН₃-СН₃ à СН₃-СН₂^- + Н⁺ or

СН₃-СН₃ à СН₃-СН₂⁺ + Н^-

and homolytic, when electrons are distributed one at a time for each of the atoms:

СН₃-СН₃ à СН₃-СН₂^.+ Н^.

in alkanes, homolytic bond cleavage occurs. This is due to the close electronegativity of carbon and hydrogen atoms. The formation of electrically charged particles is energetically extremely disadvantageous, since it cannot be compensated for by the energy of solvation (reactions with alkanes often occur either in the gas phase or in the medium of the alkane itself, when solvation is impossible). Formed as a result of homolytic bond cleavage, particles with an unpaired electron are called radicals. Radicals are highly reactive.

The energy of photons is not enough to break the CH bond (98.7 kcal / mol), therefore, such a break is possible only under thermal action, or when reacting with other radicals (detaching hydrogen atoms from the alkane molecule).

1. Combustion

The interaction of alkanes with oxygen is accompanied by such a large release of heat that it is often visualized as the appearance of a flame. The end products with excess oxygen are CO2 and water. Combustion is widely used for energy purposes, but it is not used in chemical practice. Like other reactions, combustion is a radical process initiated with sufficient energy supplied from the outside. However, spontaneous combustion processes are also frequent. Oiled wiping ends, for example, are prone to spontaneous combustion.

2. Halogenation of alkanes

In general, alkanes are characterized by reactions with intermediate formation of radicals, i.e. radical reactions. In reactions involving radicals, three stages are distinguished:

а) initiation: Сl₂ à 2Cl^.

b) propagation (chain transfer): СН₄ + Cl^. à СН₃^. + HCl

СН₃^. + Cl₂ à СН₃Cl + Cl^.

c) inhibition: СН₃^. + Cl^. àСН₃Cl

СН₃^. + СН₃^. à СН₃-СН₃ (recombination)

Cl^. + Cl^. à Cl₂ (recombination)

Reactivity of hydrogen atoms in alkanes.

In the composition of alkanes, primary, secondary and tertiary carbon atoms are distinguished. The separation of carbon atoms is based on the number of carbon-carbon bonds formed by a given carbon atom. The hydrogen atoms attached to the carbon atoms are respectively called primary, secondary and tertiary. Review the figure to understand how atoms are categorized as primary, secondary, and tertiary.

The reactivity of hydrogen atoms in alkanes, and therefore the composition of the products, can be different. The differences are associated with the different stability of the hydrocarbon radicals formed as a result of the abstraction of the hydrogen atom. The most reactive hydrogen atoms are tertiary, then secondary, and finally primary. When carrying out the reaction of alkanes with halogens, tertiary halogen derivatives are formed to a greater extent. Differences in the reactivity of hydrogen atoms decrease only when reacting with highly reactive radicals or at high temperatures.

However, the statistical factor plays a significant role. For example, for the above compound, the probability of collision with one of the 15 primary hydrogen atoms is much higher than the probability of collision with one of the 4 secondary hydrogens or one tertiary. Quaternary carbon atoms do not carry hydrogen atoms and are therefore very reactive.

The real compositions of the products of radical reactions of alkanes are very complex and numerous, depending on many factors (radical activity, reaction temperature, etc.).

3. Nitration of alkanes

Alkane nitration reactions also proceed by a radical mechanism and, depending on the conditions and reagents, lead to various products. Selective nitration (at the most reactive hydrogen atom) proceeds under relatively mild conditions.

So, nitration of alkanes according to Konovalov, which is carried out in dilute nitric acid (10-25%) at a temperature of 110 to 140 C and a slight excess pressure of 1-2 atm (in a sealed glass ampoule) leads exclusively to the substitution of the nitro group of the most reactive hydrogen. From isopentane (2-methylbutane), 2-nitro-2-methylbutane is obtained:

When nitrating alkanes in the vapor phase (at 400-500 C), due to very harsh reaction conditions, the composition of the products is much more diverse and includes nitroalkanes with a lower number of carbon atoms, i.e. products formed as a result of rupture, including carbon-carbon bonds. So, propane at a temperature of 425 C forms a mixture of 1-nitropropane (25%), 2-nitropropane (40%), nitroethane (10%) and nitromethane (25%). Approximately the same products are obtained by vapor-phase nitration of isopentane. Nitrogen dioxide (NO2) is used as a reagent for the vapor-phase nitration of alkanes.

4. Cracking of alkanes

The reaction leading to the formation of a mixture of short-chain alkanes and alkenes from alkanes with a large number of carbon in the molecule is called cracking. Thermal cracking, carried out at temperatures from 400 to 700 C, proceeds, as a rule, by a radical mechanism and is accompanied by the breaking of C-C bonds as well. As a result, a large number of radicals and biradicals are formed. Their interconversions lead to the formation of a mixture of products:

СН₃-СН₂:СН₂-СН₂:СН₂-СН₂-СН₂-СН₃ à СН₃-СН₂^. + ^.СН₂-СН₂^. + ^.СН₂-СН₂-СН₂-СН₃ à

СН₃-СН₃ + СН₂=СН₂ + СН₂=СН-СН₂-СН₃ + СН₂=СН₂ + СН₃-СН₂-СН₂-СН₃ à и т.д.

As a result of cracking, alkanes, methane, hydrogen, and alkenes of various structures are formed.

Heavy alkanes (oil - gas oil fractions) are most easily cracked, resulting in a valuable feedstock for petrochemical industries (alkenes - ethylene and propylene, alkanes - ethane, methane and propane), as well as lighter fractions used as gasoline components. Pyrolysis gasolines, however, have a significant drawback - a tendency to tarnish, due to the presence of a large number of alkenes. Solid waste from hydrocarbon polymers (polyethylene, polypropylene) can also be pyrolyzed. During cracking, liquid fractions and about 30% (by weight) of gaseous products are formed.

5. Sulfochlorination of alkanes

It is used for the production of synthetic detergents on an industrial scale. Into the reaction rather long-chain unbranched alkanes are taken from the heavy fraction of diesel fuel (C13-C17). A mixture of sulfur dioxide and chlorine Cl2 + SO2 is most often used as a reagent, although it is possible to carry out the reaction with sulfuryl chloride SO₂Cl₂.

The reaction takes place in several stages.

Initiation of the reaction leads to the production of halogen radicals:Cl₂ à 2Cl^.

Then, when interacting with a halogen radical, a hydrocarbon radical is formed (usually at the edge of the molecule):

(In general: СnН₂n₊₂ + Cl^. à СnН_2n+1^. + HCl)

A neutral molecule of sulfur dioxide interacts with the hydrocarbon radical, forming a new radical (the sulfur atom now has free valence):

Continuation of the chain consists in the interaction of a free halogen molecule with a sulfonic acid radical, as a result of which the alkanesulfonic acid chloride and chlorine radical are regenerated to continue the chain of reactions (СnН₂n₊₁SO₂^. + Cl₂ à СnН_2n+1SO₂Cl + Cl^. ):

The obtained alkane sulfochlorides are converted into neutral salts of alkane sulphonic acids:

The detergent properties of these compounds are due to the dual nature of the molecule - a long hydrophobic "tail" (alkane chain) and a short polar "head" (ionized sulfonic acid group). The mechanism of the washing action is as follows. Aggregates of pollution molecules (most often of a fatty nature) are enveloped and penetrated into the depth by non-polar hydrophobic "tails" of alkanesulfonic acids (since they penetrate well into them). The resulting micelles, in which the outer surface is covered with ionized groups, perfectly interact with the polar aqueous medium, into which the pollution is extracted (a complex of dirt and ionic detergent).

2.1.6. Methods for obtaining alkanes

2.1.6.1. Industrial methods for producing alkanes

The most important source of alkanes in nature is natural gas, mineral hydrocarbon raw materials - oil and associated petroleum gases. Natural gas is 95 percent methane. The same composition has a swamp gas formed as a result of the processing of carbohydrates by bacteria (decay). Associated petroleum gases are composed mainly of ethane, propane, butane and partly pentane. They are separated from oil in special oil treatment plants. In the absence of gas condensate stations, associated petroleum gases are burned in flares, which is an extremely unwise and ruinous practice in oil production. Simultaneously with gases, oil is cleaned of water, mud and sand, after which it enters the pipe for transportation. From oil during its distillation (distillation, distillation), taking sequentially more and more high-boiling fractions, get:

gasoline - t. kip. from 40 to 180 C, (contains C5-C10 hydrocarbons), consists of more than 100 individual compounds, normal and branched alkanes, cycloalkanes, alkenes and aromatic hydrocarbons;

kerosene 180-230, (С11-С12),

light gas oil (diesel fuel) 230-305 С (С13-С17),

heavy gas oil and light distillate of lubricating oil 305-405 С С18-С25),

lubricating oils (405-515 C) C26-C38).

The residue from the distillation of oil is called asphalt or bitumen.

2.1.6.2. Laboratory methods for obtaining alkanes

From laboratory methods for obtaining alkanes, it should be indicated:

1. Thermal decarboxylation of carboxylic acid salts in the presence of alkalis:

СН₃-СООNa + NaOH à CH₄ + Na₂CO₃,

or in general:

R-COONa + NaOH à R-H + Na₂CO₃

The reaction proceeds more or less successfully only for the salts of lower carboxylic acids - acetic, propionic. For greater convenience, they use not pure caustic alkali, but its mixture with calcium hydroxide, called soda lime. The carboxylic acid salt must be dehydrated.

The resulting hydrocarbons correspond to the carboxylic acid radical (have the same number of carbon atoms in the molecule as they were in the radical).

2. Electrolysis of carboxylic acid salts (Kolbe reaction):

2 СН₃-СОО- - 2е à CH₃-СН₃ + 2 СO₂,

or

2R-COO- -2е à R-R + 2 CO₂

The reaction mechanism is as follows. Oxidation of the carboxylic acid anion at the anode leads to the formation of the radical:

СН₃-СОО^- -е à СН₃-СОО^.

The resulting radical discards the CO2 molecule and forms a hydrocarbon radical:

СН₃-СОО^. à СН₃^. + СО₂

The formed hydrocarbon radicals disproportionate when they meet each other (they are formed in one place - at the anode) and the result is a hydrocarbon:

СН₃^. + СН₃^. à СН₃-СН₃

With such a reaction mechanism, an alkane should be expected as a product, consisting of twice the number of carbon atoms in the molecule compared to the carboxylic acid radical.

So, from the salt of propionic acid (sodium propanoate), butane is formed, and from the salt of isobutyric acid (2-methylpropanoate sodium, or sodium isobutanoate), 2,3-dimethylbutane is formed:

3. Interaction of alkyl halides with metallic sodium (Wurtz reaction):

3 СН₃-СН₂-J + 3 СН₃-J à СН₃-СН₃ + СН₃-СН₂-СН₃ + СН₃-(СН₂)₂-СН₃ + 6 NaJ

(From a mixture of ethyl iodides and methyl, a mixture of hydrocarbons is obtained - ethane, propane and butane).

The Wurtz reaction makes sense only for the production of larger hydrocarbons from one alkyl halide, since otherwise a mixture of alkanes that is difficult to separate under laboratory conditions is obtained.

4. Reduction of halide alkyls with hydrogen iodide:

СН₃-СН₂-СН₂-СН₂-СН₂-J + HJ à СН₃-СН₂-СН₂-СН₂-СН₃ + J₂