The acid-catalysed hydration of sym-dichloroacetone.

Abstract

Part of a continuing study m the kinetics and mechanism of the hydration of carbonyl compounds is reported. The kinetic effect of varying the composition of the aqueous dioxen solvent mixture was investigated for catalysis by water and seven acids, in order to provide a further test of the suggestion of Bell, Millington, and Pink ( Proc.Roy.Soe. A 1968, 303,1) that the water-catalysed reaction passes through a cyclic transition-state made up of the substrate and three water molecules, of which either one or two can be replaced by a molecule of catalyst according to its structure. Solvent deuterium isotope effects were also measured with a view to investigating the configuration of the transition-state. The kinetics and equilibria of the hydration of sym-dlehloroaeotone were measured spectrophotometrically at 25°C in solutions of water mole fraction 0.086-0.320. The isotope effect on the dissociation constant, K, of the ketone hydrate was found to be KD/KH = 0.915 Appreciable concentrations of catalyst led to increased values of K, which were analysed in terms of ground-state hydration of the catalysts. The following kinetic orders with respect to water mole fraction were found for the hydration reaction: CC13COOH 0.91, CH2C1COOH 0.98, C6H5COOH 0.92, CC13COOH 0.98, 0-C6H3C12COOH 2.5, H2O 3.65. From the linear kinetic order plots it is argued that the slopes provide m approximate measure of the number of water molecules taken up in an intimate fashion In the activation equilibrium. Combination of these results with estimates of the degree of hydration of the catalyst suggests on balance, end in company with other evidence, that all the transition-states have similar structures. Solvent isotope effect studies indicated that at least three water molecules are taken up la the water catalysed reaction, that the fractionation factor products for the different transitionstates are very similar, and that there is a primary contribution. For the hydration reaction kH/kD was found to be HC1 1.23, HC104 1.29, CC13COOH 2.46, CH2C1COOH 2.92, C6H5COOH 2.76, CH3COOH 2.94, H2O 3.97. A simple model la proposed for the prediction of transition-state configurations and energies, and gives a reasonable account of the observed catalytic behaviour, isotope effects, and strueture-reactivlty relations. Carbon-oxygen bond formation occurs synchronously with proton transfer from the acid in the transitionstate, but the other proton transfers have been already carried to completion or have not yet started.