ENTROPY AND ENTHALPY CONTRIBUTIONS TO SOLVENT EFFECTS ON PHOSPHATE MONOESTER SOLVOLYSIS - THE IMPORTANCE OF ENTROPY EFFECTS IN THE DISSOCIATIVE TRANSITION-STATE

The solvolysis reactions of a series of aryl phosphates in tert-butyl alcohol and in tert-amyl alcohol have been examined. The dianion of p- nitrophenyl phosphate reacts 7500- and 8750-fold faster in these solve nts, respectively, than the corresponding aqueous reactions. The monoa nion reacts 14- and 16-fold slower respectively in tert-butyl alcohol and in tert-amyl alcohol. Analysis of the activation parameters shows that the rate enhancement for the dianion is due solely to entropic fa ctors, while the slower reaction of the monoanion is due to increased enthalpy of activation. The significantly more positive entropy of act ivation for the solvolysis of p-nitrophenyl phosphate dianion in tert- butyl alcohol supports the original proposal that racemization at phos phorus in this reaction is caused by a switch to a D-N + A(N) mechanis m, rather than subsequently proposed mechanisms which avoid the format ion of metaphosphate. Rate enhancements of similar magnitudes are seen for the dianion reactions of all of the aryl phosphates examined; the slope of a plot of the rate constants for solvolysis versus the aqueo us pK(a) of the leaving phenols has a slope of -1.1, within experiment al error of the value for the aqueous reaction. However, in the reacti ons in tert-amyl alcohol, para-substituted and meta-substituted aryl p hosphates fall on separate but parallel lines with para-substituted co mpounds reacting faster than meta-substituted reactants with leaving g roups of similar pK(a). The pK(a) values for a series of para- and met a-substituted phenols in tert-butyl alcohol and in tert-amyl alcohol w ere determined and were found to have a linear relationship with the a queous pK(a) values, with no distinction between para and meta substit ution. Thus the different BrOnsted behavior of para- and meta-substitu ted aryl phosphates in these solvents is not due to differential solve nt-induced perturbations of the pK(a) values of the leaving groups. Th e mechanistic implications of these results and their relevance to enz ymatic phosphoryl transfer are discussed.