Reactions of Alkanes

 Reactions of Alkanes

1. Halogenation of Alkanes

Halogenation is a substitution reaction where hydrogen atoms in alkanes are replaced by halogens (Cl, Br). The reaction occurs via a free-radical mechanism involving initiation, propagation, and termination steps.

Mechanism:

Initiation: Formation of chlorine radicals.

Propagation: Hydrogen abstraction and substitution.

Termination: Combination of radicals.

2. Nitration

Nitration involves the replacement of a hydrogen atom with a nitro group (−NO2) using nitric acid.

 

3. Sulfonation

Formation of alkane sulfonic acid by reacting alkanes with sulfuric acid or oleum. The ease of replacement of hydrogen atom with methyl sulphonic acid will be in the order of: Tertiary>secondary>primary

4. Combustion

Although alkanes as a class are considered to be unreactive, they readily burn if ignited in the presence of air or oxygen and are oxidized to carbon dioxide and water, with the evolution of a large amount of heat. For example, combustion (burning) of methane is accompanied by the evolution of heat equal to 891 kJ/mol, which is called the heat of combustion of methane.

The amount of heat evolved when one mole of a hydrocarbon is burnt to carbon dioxide and water is called the heat of combustion. Although the reaction is highly exothermic, it requires a very high temperature to initiate the reaction, and once the reaction has started by a flame or a spark, it produces sufficient heat to continue itself.

5. Pyrolysis

The thermal decomposition (cleavage by heat) of organic compounds is known as pyrolysis. The name derived from the Greek word 'pyro' meaning ‘fire’ and ‘lysis’ meaning ‘loosing’. When applied to alkanes, the name is called cracking.

When large alkane molecules are passed through a chamber heated to a high temperature (400-700°C), they undergo decomposition, through a free-radical mechanism, to a variety of smaller molecules of alkanes, alkenes, and hydrogen.

6. Hydrolytic Reforming

When alkanes containing six or more carbon atoms are heated at 450-550°C in the presence of a catalyst such as chromic oxide supported on alumina, they undergo dehydrogenation (elimination of hydrogen) cyclization and isomerization to form aromatic compounds. The process is called hydroforming or catalytic reforming.

7. Isomerization

Alkanes on treatment with aluminum chloride at 300°C in the presence of a small amount of hydrogen chloride or trace water (to form HCl from the alkane) are isomerized to branched-chain alkanes. For example, n-butane isomerizes to isobutane.

The reaction is reversible, and an equilibrium mixture of n-butane and isobutane is obtained. Isomerization is an important industrial process.