Acylation

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Aliphatic and aromatic primary and secondary amines undergo acylation through nucleophilic substitution of acid derivatives such as acid halides or anhydrides to form amides.

This reaction generates an acid which may react with the amine to form salt. Thus the amine looses its nucleophilic character and the reaction will not proceed to completion. A base is thus added to remove the acid and facilitate the reaction.

Unlike in the case of alkylation reaction, the amide formed here does not react further with the organic halide because the amide is nonbasic and a poor nucleophile due to its nitrogen unshared pair of electrons being in conjugation with carbonyl group.

Acid chlorides are stronger acylating agents when compared to anhydrides and esters which react very slowly. Carboxylic acid form only salts with amines and do not form amides.

Tertiary amines do not react with acid derivatives because they cannot lose a proton to stabilise the product. So for the acylation of an amine, it should nucleophilic character and also a H-atom on nitrogen is necessary.

Aliphatic and aromatic primary and secondary amines react with acid chlorides of sulphonic acids to form secondary and tertiary sulphonamides. Secondary sulphonamides (pK_a ~ 10) is as acidic as phenol and is freely soluble in aqueous KOH. They can be recovered from the solution by acidification. Tertiary sulphonamide lacking H on nitrogen is insoluble in aqueous KOH.