Preparation of Alkanes
Preparation of Alkanes from Unsaturated Hydrocarbons
Unsaturated hydrocarbons (alkenes and alkynes) react with H2 in the presence of finely divided catalysts such as platinum, palladium or nickel to form alkanes. This process is called hydrogenation.
Preparation of Alkanes from Alkyl Halides
The following methods are used to prepare alkanes from alkyl halides (R−X) :
- Alkyl halides (except alkyl fluorides) on reduction with zinc and dilute hydrochloric acid produce alkanes.
- Haloalkanes (Alkyl halides) react with metallic sodium in dry ether to form symmetrical alkanes with double the number of carbon atoms present in
the alkyl halide. This reaction is known as Wurtz reaction. Limitations of Wurtz reaction are discussed in
reactions of haloalkanes.
Preparation of Alkanes from Carboxylic Acids
-
Decarboxylation : Sodium salts of carboxylic acids (RCOONa) on heating with soda lime (NaOH + CaO) form alkanes containing one carbon atom less than the salt.
CaO is used to keep NaOH dry as NaOH tends to absorb moisture from air. - Kolbe's electrolytic method : A concentrated aqueous solution of sodium or potassium salt of carboxylic acid on
electrolysis forms alkane containing even number of carbon atoms at the anode.
The reaction is supposed to occur through the following ways :
i)
ii) At anode :
iii) At cathode :
