Nucleophilic substitution reactions of aralphyl halides

Abstract

The investigation is concerned with the effect of electrolytes on the rates of reaction of p-methoxybenzyl chloride and benzbydryl chloride in 70% acetone, where both compounds hydrolyse entirely by mechanism S(_N)1. The work was designed to test the possibility that better nucleophilic reagents than water could react with p-mothoxybenzyl chloride by the concurrent operation of mechanisms S(_N)1 and S(_N)2. Since electrolytes had been previously demonstrated to have specific effects on the rates of ionlsation of organic compounds(^29,86,87,) it was also necessary to measure their effect on an Ionization process before the quantitative treatment of the first problem was possible. Benzhydryl chloride, which is not susceptible to bimolecular attack(^61b), was chosen for this purpose because its rate of ionisation was known to have the same sensitivity as that of p-methosybenzyl chloride to changes in the solvent composition and to additions of sodium perchlorate(^29). The studies have shorn that the specific effects of electrolytes on the rate of reaction of benzhydryl chloride in the present solvent are consistent with the operation of two effects 1) a non-specific acceleration of the rate of reaction duo to ion-atmosphere stabilisation of the transition state for ionisation(^80), ii) a specific change in the “effective” solvent composition due to solvent of the electrolytes(^87). The effect is greater for p-methoxybenzyl chloride than for benzhydryl chloride by a constant small amount. The application of these principles to the effects of electrolytes on the rate of reaction of p-methoxybenzyl chloride, has confirmed that azlde ions and chloride ions react with this compound by the simultaneous operation of the S(_N)1 and S(_N)2 processes. This is probably also the case for the substitutions by bromide, nitrate and fluoride ions. The non-electrolyte, pyridine, has also been shown to react with p-methoxybenzyl chloride by concurrent operation of the two S(_N) mechanisms, but no allowance for the medium effect could be made in tills case, because other inert non-electrolytes were found to affect the rates of ionisation of beazhydryl chloride and p-methoxybenzyl chloride in different ways. Additional evidence is provided for the validity of the criterion(^61)of mechanism on which it was concluded(^29) that p-methoxybenzyl chloride hydrolyses by mechanism S(_N)1 70% aqueous acetone. This mechanistic criterion requires that the value of the ratio ΔC*/∆S* should be independent of the nature of the substrate for S(_N)1 reactions. The value of this ratio for the hydrolysis of p-nitrobenzhydryl chloride in 50% aqueous acetone is shown to be the same as the corresponding values for the structurally different compounds tert-butyl chloride, benzylidene chloride, p-methylbenzylidene chloride and benzotrichloride in the same solvent(^62), where the reactions all follow the S(_N)1 path.