Exchange-solvolysis experiments

Table XII. Reactivity-Selectivity Relationships from Exchange-Solvolysis Experiments in AcOH-NaOAc-Et4N82Br at 120 °C (73)...

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... 

Solvolysis was in some cases cairried out in the presence of an excess of methylcyclopentane to trap intermediate ions as fast as possible via hydride transfer. In other experiments the hydride donor was added ten minutes after solvolysis. The results of a series of exchange experiments on the butyl system are shown in Table 5. 

The intermediate which is trapped during benzoyl chloride solvolysis in the experiments of Gold et al. (1954) could be either the acylium ion (equation 38) or the hydrated acylium ion (equation 39). The pathway described by equation (39) or, for that matter, any pathway involving rate-determining bond breaking in the tetrahedral intermediate can be ruled out on the basis of the very small amount of exchange into benzoyl chloride in aqueous solvents containing HgO (Bunton ef al., 1954). 

The relative partitioning of the carbonium ion between hydration (i.e. exchange) and elimination to olefin is obviously closely related to the proportion of olefin formed in the SN1 hydrolysis of related alkyl halides. The value of 2-8% olefin formation in water at 25°C found on the basis of the exchange experiments agrees very satisfactorily with the value of 3 % found in the solvolysis of t-butyl bromide in water (as calculated by extrapolation from aqueous ethanol solutions). 

---