The use of radioisotopes in studies of reaction mechanism. Part III. Thiocyano-exchange and azido-dethiocyanation of 2,4-dinitro-1-thiocyanobenzene

Abstract

The reactions of 2,4-dinitro-1-thiocyanobenzene with azide and thiocyanate ions in methanol and with thiocyanate ions in NN-dimethylformamide (DMF) have been studied using radioactive tracer techniques. The results are compared with those obtained previously for reactions of 2,4-dinitro-1-thiocyanobenzene with methoxide ion and for 1-iodo-2,4-dinitrobenzene with all three nucleophiles. The results show that thiocyano-exchange in methanol is a true S_NAr reaction in which the mobility of the thiocyano-group is similar to that of iodine. Both reactions exhibit a characteristic heavy nucleophile interaction in methanol but not in DMF. A significant increase in ease of displacement of the thiocyano-group in azido-dethiocyanation is speculatively ascribed to a favourable interaction between thiocyano- and azido-groups in the rate-limiting transition state. The much larger increase with methoxide ion in methanol, demonstrated in earlier studies, is linked with alternative attack at the cyano carbon atom. Thiocyano-exchange in DMF is very similar to thocyano-deiodination in DMF. In both, the faster reaction in DMF than in methanol (solvent rate ratio < 100) is a result of less negative ΔS‡ values, with ΔH‡ essentially unchanged.