Kinetics of the decomposition of t-butyl nitrate in acetonitrile and a comparison with solvolytic studies

Abstract

The decomposition of t-butyl nitrate in acetonitrile has been studied and the activation parameters have been determined. At 45·0°, the rate of decomposition is 2890-fold greater than that fort-butyl chloride, a factor which is considerably higher than the corresponding values of 28–51 in four aqueous–organic solvents. An analysis of the results for each of the four aqueous–organic solvents in terms of activation parameters shows that t-butyl nitrate reacts faster than t-butyl chloride owing to a more favourable entropy of activation (the enthalpies of activation are virtually identical), and that it reacts at almost the same speed as t-butyl bromide owing to compensating factors of entropy and enthalpy of activation.

Evidence is presented that a tendency towards a reduced reaction rate, which accompanies transfer of a unimolecular heterolysis of a neutral substrate from a protic to a dipolar aprotic solvent, is primarily due to a considerable decrease (frequently in excess of 10 e.u.) in the entropy of activation. This large entropy (rather than enthalpy) effect is contrary to the Hughes–Ingold charge–solvent interaction theory. It is possible that the effect is largely associated with a considerable increase in ion-pair return.