Eliminations promoted by weak bases. Part 8. Kinetics and mechanisms of reactions of cyclohexyl toluene-p-sulphonate with thiourea in various solvents

Abstract

Cyclohexyl tosylate decomposes in the presence of thiourea to cyclohexene and cyclohexylisothiuronium tosylate, the product proportions depending on the solvent. In acetone and dimethylformamide, the substitution route is dominant and first-order kinetics are observed. The rate coefficients are increased relatively mildly on increasing the thiourea concentration. In acetone, the rate-determining step appears to be ionization, with thiourea consequently partitioning an intermediate between elimination and substitution pathways. In dimethylformamide, the rate determining step is the same, but thiourea appears not to be kinetically active in producing cyclohexene. In the protic solvents ethanol and t-butyl alcohol, thiourea acts as if it is a weak anionic base, and the substitution : elimination ratio is almost 1 : 1, with second-order kinetics prevailing. Reasons for the anion-like behaviour of thiourea in these solvents are given, with reference to the E2C elimination mechanism.