Critical comments on the interpretation of heat capacities of activation and on the experimental evidence for other recent mechanistic proposals for solvolyses of t-butyl halides in water and in binary aqueous mixtures

Abstract

Kinetic data for solvolyses of t-butyl bromide in homogeneous solutions in water and in aqueous binary mixtures with acetone, methanol and ethanol at 25 °C are reported, illustrating the development of simple conductometric techniques for studying relatively fast solvolytic reactions of sparingly soluble solutes. Solvolyses of t-butyl chloride in water and in aqueous ethanol are re-examined. Experimental errors due to (a) equilibration of solute between liquid and vapour phases and (b) formation of hydrophobic aggregates which can be disrupted by ultrasonic irradiation are discussed. Four recent mechanistic proposals are critically reassessed. It is shown that: (1) the induction period observed at low solute concentrations for solvolyses of t-butyl chloride in water is due to the buffering action of absorbed carbon dioxide; (2) recent kinetic data on the effects of ultrasonic irradiation are unreliable; (3) deviations from first-order kinetics for solvolyses of t-butyl chloride in 40% ethanol + water mixtures are due to equilibration of solute between liquid and vapour phases. It is also argued that (4) the most recent interpretations of heat capacities of activation, suggesting partially reversible formation of an intermediate in hydrolysis of t-butyl chloride, do not take adequate account of alternative interpretations.