Carbocations, continued solvolysis

The difficulties encountered in using the analysis of substituent effects in solvolyses as a mechanistic probe mostly arise from the mechanistic involvement of the solvent (Shorter, 1978, 1982 Tsuno and Fujio, 1996). Consequently, the behaviour of benzylic carbocations in the gas phase should be the best model for the behaviour of the solvolysis intermediate in solution (Tsuno and Fujio, 1996). The intrinsic substituent effects on the benzylic cation stabilities in the gas phase have also been analysed by equation (2), and they will be compared here with the substituent effects on the benzylic solvolysis reaction. In our opinion, this provides convincing evidence for the concept of varying resonance demand in solvolysis. Finally, we shall analyse the mechanisms of a series of benzylic solvolysis reactions by using the concept of a continuous spectrum of varying resonance demand. 

The formation of ion pairs in solvolysis reactions is now well documented75. The separation of the charged species during heteroly tic bond cleavage is not a continuous process but proceeds through a series of progressively more dissociated intermediates an intimate or internal ion pair (51), a solvent separated or external ion pair (52), and the dissociated carbocation and counterion (53). Attack by a nucleo-... 

The reaction continues as the carbocation is captured by the solvent, often water, as in Figure 7.53, in what is called a solvolysis reaction. Solvolysis simply means that the solvent, here water, plays the role of nucleophile in the reaction. Next, any base present, here likely to be bromide or water, can remove a proton (deprotonate the protonated alcohol) to give the alcohol itself (Fig. 7.53). 

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