Anchimeric assistance in the specific acid-catalysed hydration of benzonitriles

Abstract

2-(Acylamido)benzonitriles react to give exclusively the corresponding 2-(acylamido)benzamides under relatively mild aqueous acid conditions. The kinetics for hydration at 50 °C in aqueous HCI with ionic strength kept at 1 mol dm^–3 with KCl obey the equation rate =k_H×[H⁺][nitrile] at low acid concentrations: at higher concentrations of acid the pseudo first order rate constants show a positive deviation from linearity. The rate constants (k_H) for the substituted benzamido-benzonitriles fit a Hammett relationship. log k_H=–0.36 ± 0.04σ– 3.49 ± 0.01 (r= 0.963)

The reactivity due to the ortho amido group is more than 25 000 that of the unsubstituted benzonitrile under the same conditions. The mechanism is considered to involve protonation of the nitrile followed by rate limiting intramolecular attack of the amido oxygen to form an intermediate which rapidly breaks down to the acylamidobenzamide.