Reaction between various benzaldehydes and phenylhydroxylamine: special behaviour compared with other amines

Abstract

The kinetics and mechanism of the reaction between phenylhydroxylamine and benzaldehydes to give nitrones has been studied. Phenylhydroxylamine exhibits special behaviour compared with other amines. We have determined the following. (i) The pH–rate profile of the reaction of phenylhydroxylamine with benzaldehydes does not correspond to any of those previously encountered for this kind of reaction. Unlike amines of similar pK_a values, the pH–rate profile for phenylhydroxylamine does not show a negative break, because the rate-determining step is the dehydration of the dihydroxy intermediate over the entire pH range (pH 1–11). (ii) The point for the reaction of phenylhydroxylamine shows a positive deviation from the Brønsted relation between the hydronium ion catalytic rate constant and the pK_a of the amines. It is evident that this behaviour is caused by a very fast first step for the reaction of phenylhydroxylamine and it is suggested that this is due to a pre-association mechanism of attack of phenylhydroxylamine on carbonyl compounds.