Equilibrium constants for ionisation and enolisation of 2-phenylacetylfuran

Abstract

The equilibrium constant for keto–enol tautomerism of 2-phenylacetyIfuran K_E=[enol]/[ketone] has been measured as 1.32 × 10^–6(pK_E= 5.88) by combining rate constants for enolisation and ketonisation. Rates of enolisation were measured by iodination and of ketonisation by quenching the enolate anion into acidic buffers and monitoring relaxation of the initially formed enol to its keto tautomer by stopped-flow. The pK_a for ionisation of 2-phenylacetyl furan was measured spectrophotometrically as 14.38. Combining this with pK_E gives pK_a^EH= 8.5 for 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-independent rate for the tautomerisation reaction which prevents observation of metal-ion catalysis. A Brønsted exponent α= 0.35 has been measured and is compared with values for related substrates with pK_as in the range 18 to –11. Values of pK_a and pK_T for acetylfuran are estimated.