Acylation. Part XXV. Reactions of acyl cyanides. The spontaneous and catalysed benzoylation of butylamine in ether solution

Abstract

The spontaneous benzoylation of n-butylamine by benzoyl cyanide in ether solution at 25° obeys the rate equation –d[BzCN]/dt={16[BuNH₂]+ 8·8 × 10³[BuNH₂]²}[BzCN]. With p-chlorobenzoyl cyanide the overall rate is 3–4-fold faster than for the unsubstituted compound at any given butylamine concentration.

In the presence of the catalyst triethylamine, a base of similar strength to n-butylamine in ether, the term first-order in [BuNH₂] is augmented, becoming with benzoyl cyanide [graphic omitted] [BuNH₂]. There is, however, negligible catalysis of the term second-order in butylamine. The various kinetic effects strongly suggest that the first-order terms reflect an acylation mechanism involving a carbonyl addition intermediate but that the term second-order in butylamine refers to a synchronous substitution occurring via a cyclic, hydrogen-bonded transition state.

Carboxylic acids catalyse the acylation when present in low concentration but retard it at high concentration, the rate passing through a maximum value. This maximum is lower, and lies at a higher concentration, for acetic than for the stronger benzoic acid. The acids catalyse both the first- and second-order terms in butylamine and p-chlorobenzoyl cyanide is more susceptible to such catalysis than is benzoyl cyanide. Mechanisms for the acid-catalysed paths are suggested. The fall in rate at high acid concentration arises from the significant reduction in the free amine concentration because of adduct formation with the acid, the adduct, as expected, being less easily acylated than the free amine.