Isothiazole chemistry. III. Kinetics and mechanism of cyclization of cis-3-Thiocyanoacrylamide

Abstract

The cyclization of cis-3-thiocyanoacrylamide to isothiazol-3-one has been studied over the pH range 0-5.5 and it is shown that two cyclization mechanisms exist. Within the pH range 5.5-3.5, the only important mechanism is one involving a rapid, unimolecular cyclization of the thiocyanoacrylamide anion, while at pH less than 3.00, the process is essentially a slow cyclization of the neutral molecule, followed by rapid proton loss. The large rate difference between the two mechanisms is ascribed to a lowering of ΔF* in the anion cyclization mechanism by a more favourable TΔS* term, and possible reasons for this are discussed.