Question

Write a short note on ammonolysis.

Answer 1

The carbon-halogen bond in alkyl or benzyl halides can be easily cleaved by a nucleophile. Hence, an alkyl or benzyl halide on reaction with an ethanolic solution of ammonia undergoes a nucleophilic substitution reaction in which the halogen atom is replaced by an amino (–NH₂) group. This process of cleavage of the C–X bond by an ammonia molecule is known as ammonolysis. The reaction is carried out in a sealed tube at 373 K. The primary amine thus obtained behaves as a nucleophile and can further react with alkyl halide to form secondary and tertiary amines and finally quaternary ammonium salt.

\[\ce{\underset{(1^\circ)}{RNH2} ->[RX] \underset{(2^\circ)}{R2NH} ->[RX] \underset{(3^\circ)}{R3N} ->[RX] \underset{Quaternary ammonium salt}{R4\overset{+}{N}\overset{-}{X}}}\]

The free amine can be obtained from the ammonium salt by treatment with a strong base:

\[\ce{R-\overset{+}{N}H3\overset{-}{X} + NaOH -> R-NH2 + H2O + \overset{+}{N}a\overset{-}{X}}\]

Ammonolysis has the disadvantage of yielding a mixture of primary, secondary and tertiary amines and also a quaternary ammonium salt. However, a primary amine is obtained as a major product by taking large excess of ammonia. The order of reactivity of halides with amines is RI > RBr > RCl.