Nucleophilic reactions of haloalkanes

Nucleophilic reactions of haloalkanes

Replacement by alkoxy group

(Formation of ethers) - Williamson Synthesis

This reaction is called 'Williamson's synthesis' and is quite useful for preparing ethers. Haloalkanes can also be converted into ethers by heating with dry silver oxide.

Substitution by Cyano Group

Haloalkanes react with alcoholic solution of potassium cyanide (KCN) to give alkane nitriles or alkyl cyanides as the major products along with a small amount of alkyl isocyanides.

Alkyl cyanides can further be converted into acid amides, carboxylic acids and primary amines under different conditions. Therefore, they are useful starting materials for these compounds.

The reaction of haloalkanes with alcoholic KCN is important because the product formed has one more carbon atom than the alkyl halide. Therefore, the reaction is a good method for ascending the homologous series (increasing the length of the carbon chain by one carbon atom).

Substitution by Isocyanide Group

Isocyanides are obtained when haloalkane is treated with alcoholic silver cyanide (AgCN). These are also called carbylamines and have extremely unpleasant smell.

(Alkane isonitrile)

On reduction with sodium or alcohol, alkyl isocyanides give secondary amines.

Substitution by Amino Group (Formation of Amines)

A primary amine is formed when haloalkane is heated with alcoholic ammonia solution in a sealed tube at 383 K; the halogen is substituted by -NH2 group.

However, when haloalkane is in excess, a mixture of primary, secondary and tertiary amines is formed, as one or both the hydrogen atoms of the amino alkane (primary amine) are replaced by alkyl groups.

 The tertiary amines which result, can also form quaternary ammonium salt by combining with another molecule of alkyl halide.

This reaction is called Hoffmann ammonolysis reaction.

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