Some of the general methods of preparation of alkanes are:
FROM ALKYL HALIDES (By Wurtz Reaction)
Alkyl halides are halogen derivatives of alkanes with general formula R-X. They can be used to prepare alkanes by Wurtz reaction:
It involves the chemical reaction between alkyl halides (usually bromides and iodides) and metallic sodium in the presence of dry ether. The product is the symmetrical alkane containing twice the number of carbon atoms present in the alkyl halide.
FROM UNSATURATED HYDROCARBONS
Unsaturated hydrocarbon can be converted into alkanes by catalytic hydrogenation. The catalyst used may be nickel, platinum or palladium. These metals absorb dihydrogen gas on their surface and weaken the hydrogen-hydrogen bond. Platinum and palladium catalyse the reaction at room temperature while nickel requires higher temperature and pressure. The reaction is called Sebatier and Senderen’s reaction.
FROM CARBOXYLIC ACIDS
(a) Soda-lime Decarboxylation. It involves the treatment of sodium salt of carboxylic acids with soda lime (NaOH and CaO in the ratio 3 : 1).
The alkane fanned has one carbon less than the sodium salt of carboxylic acid.
In this reaction CaO does not participate in the reaction but helps in fusion of the reaction mixture.
(b) Electrolytic Decarboxylation. This reaction is also called Kolbe’s reaction. In this reaction an aqueous solution of sodium or potassium salt of a carboxylic acid is electrolysed. Alkane is formed at anode.
In general, the electrolytic decarboxylation can be represented as