Electrophilic substitution on the thiophen ring. Part II. Halogenation of substituted thiophens

Abstract

The effect of substituents at the 5- position upon the rate of chlorination and bromination of thiophen at the 2-position, in glacial and 15% aqueous acetic acid respectively, has been studied and found to exhibit a Hammett ρσ relationship, with ρ values of –6·5 and –10. Bromination of deactivated thiophens has been analysed in terms of simultaneous second- and third-order processes and the activation parameters for various substituted thiophens indicate that the rate of reaction is affected more by changes in ΔH^‡ than ΔS^‡.