Intramolecular reactions. Part XI. Cyclisation of ω-halogenoalkyl- sulphides: the significance of bromide:chloride ratios in displacement reactions

Abstract

Solvolysis of ω-halogenoalkyl sulphides has been investigated. Under appropriate conditions, the rate-limiting step is formation of a cyclic sulphonium salt. Three-membered cyclic sulphonium salts are formed more rapidly than their five-membered analogues and the enthalpy of activation for three-membered ring formation is lower than that for five-membered ring formation. Attachment of a phenyl group adjacent to the leaving group accelerates formation of rings of both sizes but the three-membered ring is favoured to a considerably greater extent by an entropic and not an enthalpic change. The results are discussed in terms of special conjugative effects of three-membered rings. Bromide:chloride ratios have been measured for reactions giving three- and five-membered ring sulphonium salts. The relevance of bromide:chloride ratios in assessment of bond extension in transition states is considered.