Kinetics of the enolisation reaction of 2- and 3-acetylthiophenes

Abstract

Measurements have been made on the rates of halogenation of 2- and 3-acetylthiophenes in dilute hydrochloric acid, in buffer solutions, in dilute hydroxide solutions and in the presence of several metal ion salts. The reactions are zero-order with respect to halogen concentration, with the rate-determining step being the formation of the enol or enolate ion. The enolisation reactions are general acid-base catalysed and catalytic constants for a number of acids and bases have been determined in water at 25 °C. The results have been compared with those of corresponding reactions of acetophenone. The close similarity between rate accelerations promoted by cadmium and zinc ions on the reactions of the acetylthiophenes and acetophenone suggests that the effect is due to coordination of the carbonyl oxygen in the transition states. For the sulphur atom of 2-acetyl-thiophene no evidence of participation in coordination was found. The results are discussed in relation to those of earlier studies on related systems.