Kinetics and mechanism of the formation of Girard T hydrazones from naphthaldehydes

Abstract

In aqueous solution, Girard T hydrazone formation from naphthaldehydes does not proceed to completion under the conditions used. The corresponding equilibrium constants have been determined and employed to calculate the rate of the reaction at completion. For all the reactions studies, the pH–rate profiles, extrapolated to zero buffer concentration, show one break at pH 4–5, characteristic of a change in the rate-determining step from aminomethanol dehydration to aminomethanol formation on going from pH 7 to pH 1. The formation of the aminomethanol is subject to catalysis by hydronium ion and by carboxylic acids present in the buffers used to maintain pH. Brönsted α values for this catalysis vary from 0.24 to 0.26. We suggest a stepwise preassociation mechanism for the reaction catalysed by carboxylic acids.