Keto-enol interconversion of 2-Phenylacetylthiophene

The equilibrium constant for keto–enol tautomerism of 2-phenylacetylthiophene (2PAT) KT = [enol]/[ketone] has been determined as 3.55 × 10-7 (pKT 6.45) by combining rate constants for enolisation and ketonisation. The pKaKH for ionisation of 2PAT has been measured spectrophotometrically as 14.60. Combining this with pKT gives pKa 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.