Reactivity of 2-halogenothiazoles towards nucleophiles: kinetics and mechanisms of the reactions of 2-halogeno-X-thiazoles with benzenethiolate ion

Abstract

The reactions between 4- and 5-X-substituted 2-chlorothiazoles and the benzenethiolate ion in methanol have been kinetically investigated. The aza-activation in 2-chlorothiazole is much reduced with respect to the commonly observed aza-activation in 6-membered cyclic aromatic derivatives. The substituent effect can be correlated with a Hammett relation when the 2,4- and 2,5-positions are considered as meta- and para-like respectively.

The high ρ value (+5·3) and the high experimental σ^– value for the p-NO₂ group (ca. 1·6) indicate that the thiazole system is very sensitive to the effect of the substituent. This is connected with the small delocalisation of the negative charge in the transition state in the penta-atomic ring.