14.2 GENERAL METHODS OF PREPARATION OF ALKANES

1. By the Hydrogenation of Alkenes and Alkynes

Addition of hydrogen molecule across carbon-carbon multiple-bond is called hydrogenation. Hydrogenation takes place in presence of finely divided nickel at 200-300°C and high pressure. Hydrogenation can also be done in presence of Pt or Pd at room temperature. Alkynes add two molecules of hydrogen. Why?

CH≡CH + H₂ → Ethyne
CH₂=CH₂ + H₂ → Ethene
CH₂=CH₂ (NL 200-300°C)
CH₃-CH₃ (NL 200-300°C)

CONCEPT ASSESSMENT EXERCISE 14.1

Complete the following reactions:

CH₃-CH=CH₂ + H₂ → CH₃-CH₂-CH₃
CH≡CH + 2H₂ → CH₃-CH₃ (NL 200-300°C)

2. By the Reduction of Alkyl Halides

When an alkyl halide is treated with Zn in presence of an aqueous acid, an alkane is produced. Usually aqueous solution of HCl or CH₃COOH is used. Zn reacts with aqueous acid to liberate atomic hydrogen called nascent hydrogen. Nascent hydrogen reduces alkyl halide. Addition of nascent hydrogen is called reduction.

CONCEPT ASSESSMENT EXERCISE 14.2

Complete the following reactions:

(a) CH₂CH₂Cl + 2[H] → CH₃-CH₃
Zn/HCl(a)
(b) CH₃-CHCl-CH₃ + 2[H] → CH₃-CH₂-CH₃

3. By the Cracking of Larger Hydrocarbons

A large hydrocarbon (alkane) molecule breaks into smaller hydrocarbons when heated at high temperatures such as 450-750°C and high pressure. This process is called thermal cracking. This process produces a mixture of alkanes and alkenes. For example, when decane is heated at high temperature and high pressure, it breaks down into octane and ethene.

C₁₀H₂₂ (Decane) → C₈H₁₈ (Octane) + C₂H₄ (Ethene)