EQUILIBRIUM-CONSTANTS FOR IONIZATION AND ENOLIZATION OF 2-PHENYLACETYLFURAN

The equilibrium constant for keto-enol tautomerism of 2-phenylacetylfu ran K-E = [enol]/[ketone] has been measured as 1.32 x 10(-6) (pK(E) = 5.88) by combining rate constants for enolisation and ketonisation. Ra tes of enolisation were measured by iodination and of ketonisation by quenching the enolate anion into acidic buffers and monitoring relaxat ion of the initially formed enol to its keto tautomer by stopped-flow. The pK, for ionisation of 2-phenylacetyl furan was measured spectroph otometrically as 14.38. Combining this with pK(E) gives pK,EH = 8.5 fo r the enol tautomer. Comparison with deoxybenzoin (pK(E) = 5.15, pK(a) (EH) = 9.6) shows a mild influence of the oxygen of the furan ring (a) in enhancing the acidity of the enol and (b) in stabilising the keto relative to enol tautomer. Phenylacetylfuran shows a high pH-independe nt rate for the tautomerisation reaction which prevents observation of metal-ion catalysis. A Bronsted exponent alpha = 0.35 has been measur ed and is compared with values for related substrates with pK(a)s in t he range 18 to -11. Values of pK(a) and pK(T) for acetylfuran are esti mated.