10. Chemical properties, preparation, and applications of alkanes

There is an enormous number of different alkanes. Their variety is explained both by the possibility of forming carbon chains of various lengths and by isomerism. Because of this, it is not possible to study the chemical properties of each alkane separately. At the same time, the molecules of different alkanes have a similar structure: carbon atoms are joined to one another and to hydrogen atoms by single covalent bonds. Given this, we can expect that the chemical properties of different alkanes will be largely similar.

All alkanes are characterized by low chemical activity. They do not react with solutions of acids, bases, or salts. They are not affected by such a strong oxidizer as KMnO4, nor by such strong reducing agents as alkali metals. You know that alkali metals are very active and react with almost all substances they come into contact with, including being readily oxidized by atmospheric oxygen. To protect alkali metals from oxidation, they are stored under a layer of kerosene — a mixture consisting mainly of saturated hydrocarbons. In this process, the alkanes that make up kerosene do not react with the alkali metals.

Because of the low chemical activity of alkanes, reactions involving them proceed under harsh conditions (upon heating or irradiation with ultraviolet light).

We will study the reactions of alkanes with halogens (Cl2 and Br2) and oxygen (O2), as well as the transformations they undergo upon heating.

1. Halogenation. Reaction with halogens

The reaction of alkanes with halogens — chlorine and bromine — proceeds upon heating or irradiation with ultraviolet light.

If a glass vessel is filled with a mixture of gaseous methane and chlorine and placed in a dark location, no reaction will take place. However, upon heating the mixture or irradiating it with ultraviolet light, a chemical substitution reaction proceeds in which hydrogen atoms in the methane molecule are replaced by chlorine atoms:

In equations for reactions that proceed under irradiation, the letters hv are written above the arrow. This reaction is called a halogenation reaction and belongs to the substitution reactions.

If only one hydrogen atom in the molecule is replaced by a halogen atom, the reaction is called monohalogenation. The reaction shown above is the monochlorination of methane. In excess chlorine, the remaining three hydrogen atoms of the methane molecule can be successively replaced by halogen:

Let us give the equations for all four stages of methane chlorination using molecular formulas:

Let us consider the monochlorination reactions of the homologs of methane.

Monochlorination of ethane

For ethane, the reaction equation is as follows:

Note that in the name «chloroethane» there is no need to indicate the position of the chlorine atom with a number. This is because replacing any hydrogen atom in the ethane molecule with a chlorine atom produces one and the same substance:

Thus, in the monochlorination of ethane, just as in the case of methane, only one organic substance is obtained — chloroethane.

As a result of the monochlorination reaction of propane, two products are formed: 1-chloropropane and 2-chloropropane, which have the same molecular formula C3H7Cl. This is not surprising, since 1-chloropropane and 2-chloropropane — are isomers.

If we write the equation for the monochlorination reaction of propane using molecular formulas, it will look as follows:

A reaction equation written in this form not only fails to indicate which specific product (1-chloropropane or 2-chloropropane) is meant, but also leads to the common misconception that only one organic product is formed in the monochlorination of propane — C3H7Cl, whereas in fact there are two. Therefore, in organic chemistry, structural rather than molecular formulas of substances are usually used when writing reaction equations.

As a result of the chlorination of alkanes, one or more hydrogen atoms in the alkane molecule are replaced by halogen atoms. Therefore, the resulting organic substances are called halogen derivatives of alkanes.

Example. Substitution of two hydrogen atoms by chlorine in the ethane molecule.

The reaction proceeds in two stages.

a) The first stage of ethane chlorination. In the first stage, one hydrogen atom is replaced. This produces only one organic substance — chloroethane:

b) The second stage of ethane chlorination. At this stage, a chlorine molecule reacts with a molecule of chloroethane formed in the first stage. Obviously, two organic substances can be formed as a result:

Indeed, chlorination of ethane can produce a mixture of two dichloro derivatives.

Alkanes also undergo substitution reactions with bromine. For example:

In conclusion, let us note once again that the reactions of alkanes with chlorine and bromine proceed under harsh conditions: under irradiation or heating.

You can learn about the mechanism of the halogenation reaction of alkanes by following the link in the QR code.

In halogenation reactions, the hydrogen atoms in an alkane molecule are replaced by halogen atoms, while the carbon chain of the molecule is preserved. In other reactions of alkanes, their carbon skeleton is altered or completely destroyed. Let us consider such reactions.

2. Pyrolysis

Upon strong heating of alkanes, bonds in their molecules break: and . As a result, alkane molecules can be completely broken down to form carbon and hydrogen. The decomposition of substances at high temperatures is called pyrolysis (from the Greek pyro — fire, heat, and lysis — decomposition, breakdown). For example:

General scheme of the pyrolysis reaction of alkanes (n — the number of carbon atoms in the alkane molecule):

This reaction is used industrially to produce carbon black and hydrogen.

3. Isomerization

Another chemical property of alkanes is isomerization, that is, the conversion of one isomer into another. This property is possible for alkanes starting from butane, since methane, ethane, and propane have no isomers. The isomerization reaction proceeds by passing an alkane through a reactor heated to a high temperature in the presence of a catalyst. In this process, molecules of linear-structure alkanes are converted into molecules of branched structure, for example the isomerization reaction of n-butane:

4. Combustion. Reaction with oxygen

The most important property of alkanes — combustion. Alkanes ignite when exposed to flame. The equation for the combustion reaction of methane:

This reaction is well known to you: it proceeds when gas is lit on a kitchen gas stove, since methane — is the main component of natural gas. Gas cylinders are filled with a mixture of propane and isomeric butanes. The equations for the combustion reactions of these alkanes:

Subsequent members of the homologous series of alkanes also burn when ignited. A general equation for the combustion reaction can be written:

It can be seen that during combustion, the hydrogen atoms from the alkane molecule pass into water molecules, while the carbon atoms — into carbon dioxide molecules. If an alkane burns under conditions of insufficient oxygen, then, along with carbon dioxide (CO2), carbon monoxide (CO) or carbon (C) in the form of soot may be formed: