Reactivity of vinyl sulphonic esters. Part IV. Evidence of an SN1-type mechanism involving an ethylenic carbon atom

Abstract

The rate of decomposition of trans-2-phenylthio-1,2-di-p-tolylvinyl 2,4,6-trinitrobenzenesulphonate both in inert and in nucleophilic solvents at 25 °C follows first-order kinetics. The rate depends on the polarity of the reaction medium and not on its nucleophilic properties. Addition of lithium perchlorate enhances the rate by a ‘normal’ salt effect in acetone and methanol and by a ‘special’ salt effect in acetic acid. Common-ion retardation by addition of lithium trinitrobenzenesulphonate (LiTNBS) and radioactive intake into the reacting substrate using ³⁵S labelled lithium trinitrobenzenesulphonate is observed in acetone.

The decomposition of trans-1,2-diphenyl-2-phenylthiovinyl and 1,2-diphenyl-2-p-tolythiovinyl 2,4,6-trinitrobenzenesulphonates in nitromethane yields mainly benzo[b]thiophen and benzil derivatives, whereas in 19:1 (v/v) nitromethane–methanol the methanolysis products are obtained in high yields: the rates in the two media are only slightly different.

The structural effects have been studied by measuring the rates of reaction of several substituted trans-trinitrobenzenesulphonates, Z–C₆H₄(X–C₆H₄S)C:C(TNBS)C₆H₄–Y, in nitromethane at 25 °C. The first-order rate co-efficients are very well correlated by the equation log k_Y,Z,X=–3·38 – 2·85 (σ_Y⁺+ 0·44σ_Z+ 0·52σ_X).

The results are consistent with a mechanism involving a rate-determining heterolysis of the ethylenic carbon–oxygen bond to give a relatively stable unsaturated cationic intermediate. The structural effects indicate a substantial vinyl carbonium ion character for the transition state but also suggest that a significant sulphur participation is involved.