Solvent reorganization solvolysis

For both tertiary cations and secondary benzylic carbocation, the ratio of substitution to elimination is quite high. For example, for l-(/i-tolyl)ethyl cation in 50 50 TFE-water, the ratio is 1400."° For the fert-butyl cation, the ratio is about 30 in water" and 60 in 50 50 TFE-water. " These ratios are on the order of 10 if account is taken of the need for solvent reorganization in the substitution process." The generalization is that under solvolysis conditions, tert-alkyl and sec-benzyl carbocations prefer substitution to elimination. 

A Global scheme for solvolysis 2 Clocks for reactions of ion pairs 3 Addition of solvent to carbocation-anion pairs i Protonation of a carbocation-anion pair 11 Isomerization of ion pair reaction intermediates Reorganization of ion pairs in water 13 Internal return of isotopically labeled ion pairs Racemization of ion pairs 22 Concluding remarks 24 Acknowledgements 24 References 24... 

Much is known about the lifetimes of carbocation intermediates of solvolysis, and these data have proven critical in the design of experiments to estimate absolute rate constants for reorganization of ion pairs. Consider reorganization of an ion-pair reaction intermediate that exchanges the positions of the nucleophilic atoms of the leaving group (, Scheme 9) and that occurs in competition with diffusional separation to free ions (k-d) which is much faster than addition of solvent to the ion pair. Ion-pair separation is irreversible and will result in formation of solvolysis reaction products s ). Reorganization of the ion pair will result in formation of isomerization reaction product and the yield of this reaction product will provide a measure of the relative rate constant... 

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