Volumes of reaction for the formation of some analogues of tetrahedral intermediates

Abstract

Two reactions which comprise equilibrium addition of a protic species to a carbonyl group, the hydroxyketone–hemiacetal interconversion (1a)⇌(1b), and the thiol addition of acetyl cyanide (2a)⇌(2b), have been studied by n.m.r. and their spectra thoroughly analysed. Equilibrium constants have been measured as a function of pressure to obtain volumes of reaction which relate to the formation of the ‘tetrahedral intermediate’ of ester hydrolysis. The intramolecular reaction is shown to have a much less negative volume change than the intermolecular analogue, pointing to the important contribution to ΔV and to ΔV^‡ of molecular association prior to bond formation.