The solvolysis of 2-hydroxypropyl p-toluenesulfonate and ethylene di-p-toluenesulfonate

Abstract

Little is known about the neighboring group effects of hydroxyl or sulfonate ester neighboring groups as applied to solvolysis at a primary carbon atom. This research was undertaken to obtain information bearing on these questions.

In the present study, the pseudo-unimolecular rate constants for the solvolysis of 2-hydroxypropyl 2-toluenesulfonate were measured in ethanol-water mixtures ranging from 20% ethanol to 90% ethanol. Measurements were made at four different temperatures, and thermodynamic activation parameters were calculated. Correlations were drawn showing the effect of solvent on the rate of reaction. Using pyri~ dine in equimolar concentration did not affect the rate of the reaction. The rate of solvolysis of 2-hydroxypropyl p-toluenesulfonate was about four times greater than that of ethyl-p-toluenesulfonate under the same conditions. After making the appropriate corrections for steric and inductive effects, the magnitude of the hydroxyl neighboring group effect was estimated. By this estimate it would appear that 2-hydroxy-propyl p-toluenesulfonate solvolyzes 3000 times faster than would be expected in the absence of a neighboring group effect. A mechanism was proposed to account for this effect. The rate constants for the solvolysis of ethylene di-p-toluenesulfonate were measured in ethanol-water mixtures ranging from 50% ethanol to 90% ethanol at two different temperatures. The rate of solvolysis of each p- toluenesulfonate group was estimated to be about 0.7 times that of ethyl p-toluenesulfonate. When considered in the light of steric and inductive effects, this demonstrates that the p-toluenesulfonate group has a significant neighboring effect. However, the magnitude of this effect could not be estimated. It was established that 2-hydroxyethyl p-toluenesulfonate undergoes solvolysis faster than does ethylene de-p-toluenesulfonate, although the former compound could not be isolated for independent study. Mechanisms were proposed for the solvolysis of these compounds.