Mechanism of bimolecular substitution reactions of hydroximoyl halides with amines in actonitrile solution

The reaction in acetonitrile solution of (Z)-O-methylbenzohydroximol chlorides [ArC (Cl)NOCH3] with morpholine, pyrrolidine and azetidine gives the corresponding (Z)-amidoximes [ArC(NR1R2)NOCH3] in acetonitrile. The rates of these reactions were measured under pseudo-first-order conditions (excess amine). The reactions were found to follow overall second-order kinetics (first order in amine). The Hammett ρ-value (with σ) for the reaction with pyrrolidine is +0·92. The reaction of the p-nitro compound with pyrrolidine gives a significant element effect [k(p-nitrobenzohydroximoyl bromide)/k(p)-nitrobenzohydroximoyl chloride = 10]. A Brϕonsted β-value of 0·38 was estimated from the reactions of morpholine and piperidine with the p-nitrohydroximoyl chloride. The slower reaction of (E)-O-methyl-p-nitrobenzohydroximoyl chloride with azetidine gives primarily the (E)-amidoxime (E:Z ≈ 98:2) This reaction also follows second-order kinetics. The kinetic observations made in this study are compared with the corresponding results obtained in earlier work in benzene solution. It is suggested that in acetonitrile solution the reactions proceed by an addition–elimination mechanism with rate-determining loss of chloride ion (AN + DN#). It is further suggested that acetonitrile is assisting in the breakdown of the tetrahedral intermediates formed in these reactions.