Alkyl arenesulfonates, solvolysis

The special salt effect is another factor that requires at least two ion-pair intermediates to be adequately explained. Addition of salts typically causes an increase in the rate of solvolysis of secondary alkyl arenesulfonates that is linear with salt concentration. The effect of added lithium perchlorate is anomalous toward certain substrates in producing an initial sharp increase in the solvolysis rate, followed by the expected linear increase at higher lithium perchlorate concentrations. Winstein ascribed this to interaction of lithium perchlorate with the solvent-separated ion pair to form a solvent-separated carbonium ion perchlorate ion pair that does not undergo return to the intimate ion pair or covalent substrate. This new ion pair can go on only to product, and its formation leads to an increase in solvolysis rate more pronounced than for a simple medium effect. 

Nucleophilic substitution reactions that occur under conditions of amine deamination often differ significantly in stereochemistry, compared with that seen in halide or arenesulfonate solvolysis. The results of four key substrates are summarized in Table 5.13. It can be seen (entry 1) that displacement of nitrogen on the 1-butyldiazonium ion is much less stereospecific than the 100% inversion observed on acetolysis of the corresponding brosylate. Similarly, the secondary system (entry 2) affords 2-butyl acetate with only 28% inversion of configuration. Furthermore, a crossover to net retention of configuration is observed as the alkyl group becomes better able to stabilize a carbonium ion. The small net retention (10%) observed in deamination of 1-phenylethylamine increases to 28% retention in the tertiary benzylic system 2-phenyl-2-butylamine. 

The steady-state approximation, and solvolysis of alkyl halides and arenesulfonates... 

Many alkyl halides and arenesulfonates (R—X) undergo first-order solvolysis in ionising solvents (SOH) and the essential experimental features maybe accommodated by the generic mechanism shown in Scheme 4.4. The mechanism includes reversible ionisation of the covalent substrate to give a contact (intimate) ion pair, R+ X-, which undergoes internal return or nucleophilic capture by the solvent (in some cases, there is evidence for the intervention of solvent-separated ion pairs, and even fully dissociated ion pairs, but the minimalist mechanism of Scheme 4.4 serves our present purpose adequately) [13]. 

Aryldiazonium salts are more stable than alkyldiazonium salts, which usually react by substitution or elimination under the conditions of their generation. Marked differences are often observed in the pattern of reactivity exhibited by diazonium salt decomposition compared with that seen in solvolysis reactions of the corresponding alkyl halides or arenesulfonates. ... 

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