Keto–enol interconversion of 2-phenylacetylthiophene

Abstract

The equilibrium constant for keto–enol tautomerism of 2-phenylacetylthiophene (2PAT) K_T = [enol]/[ketone] has been determined as 3.55 × 10^–7 (pK_T 6.45) by combining rate constants for enolisation and ketonisation. The pK_a^KH for ionisation of 2PAT has been measured spectrophotometrically as 14.60. Combining this with pK_T gives pK_a^EH 8.15 for the enol form of 2PAT. Comparison of these values with those previously reported for 2-phenylacetylfuran and deoxybenzoin shows a significant influence of both five-membered heterocyclic rings (a) in increasing the acidity of the enol and (b) in stabilising the keto relative to the enol tautomer. Micelles from a cationic surfactant (CTABr) strongly decrease the rate of ketonisation of the enolate of 2PAT and increase the apparent acidity of the keto form.