Elimination–addition. Part XV. Rates of reactions of ω-bromoalkyl p-tolyl sulphides with alkoxides: the role of neighbouring group participation by sulphur and the involvement of ion-pair equilibria

Abstract

Determination of rates of reactions of a series of sulphides, p-Me·C₆H₄·S·[CH₂]_n·Br (n= 3–6), with sodium ethoxide in ethanol and with potassium t-butoxide in t-butyl alcohol, together with analysis of the products of elimination and substitution that are obtained, allow the effect of the distant p-tolylthio-group on these two types of reaction to be assessed. The results for t-butoxide reactions show that elimination and substitution are both accelerated by a remote p-tolylthio-group, but that the effect is much less than that found earlier for a toluene-p-sulphonyl group.

Behaviour of 4-bromobutyl and 5-bromopentyl p-tolyl sulphides in ethanol and t-butyl alcohol, in the presence and absence of bases, has been studied. Reactions involve neighbouring group participation by the sulphur atom. Under the conditions used, formation of cyclic sulphonium salt from the 4-bromobutyl compound is faster than the reactions of bases with covalent sulphide, and release of bromide in the reactions is zero order in base. For the 5-bromopentyl compound, formation of sulphonium salt is slower, and in reactions with alkoxides, competition occurs between attack on sulphonium salt and on sulphide.

Rate and equilibrium constants for ionisation to cyclic sulphonium salt of the 4-bromobutyl and 5-bromopentyl sulphides have been measured. These are very dependent on solvent and upon added non-common-ion salts (e.g., LiClO₄). The insensitivity of equilibrium constants, in concentrated (0·1M) solutions, to common ion suggests considerable involvement of ion-pairing. Activation parameters for ion-recombination in the 4-bromobutyl system in t-butyl alcohol have been obtained.

The results are discussed in relation to earlier work.