THE INFLUENCE OF BORATE BUFFERS ON THE HYDRATION RATE OF CYANOHYDRINS - EVIDENCE FOR AN INTRAMOLECULAR MECHANISM

The effects of borate buffers on the pseudo first-order rate constants for the hydration of hydroxyacetonitrile (1) have been examined in the pH range 8.6-10.3 at 80 °C. Kinetic results are compatible with a reaction of the neutral cyanohydrin with borate anion or any other kinetically equivalent mechanism. Cyanohydrin structural effects are consistent with a pathway involving the pre-equilibrium formation of a borate-substrate adduct, followed by a rate-determining intramolecular nucleophilic attack on the nitrile group. Phenylboronate ion has also been shown to act as an efficient catalyst, but it was not possible to detect any influence due to disubstituted borate ions. Thus, it is suggested that a trigonal borate anion actually acts as a nucleophile.