The equilibrium constant for keto-enol tautomerism of 2-phenylacetylfuran K(E) = [enol]/[ketone] has been measured as 1.32 x 10(-6) (pKE = 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 pKa, for ionisation of 2-phenylacetyl furan was measured spectrophotometrically as 14.38. Combining this with pKE gives pKEH = 8.5 for the enol tautomer. Comparison with deoxybenzoin (pKE = 5.15, pKaEH = 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 Bronsted exponent alpha = 0.35 has been measured and is compared with values for related substrates with pKa values in the range 18 to -11. Values of pKa and pKE for acetylfuran are estimated.
Equilibrium Constants for Ionisation and Enolisation of 2-Phenylacetylfuran
FONTANA, Antonella;
1994-01-01
Abstract
The equilibrium constant for keto-enol tautomerism of 2-phenylacetylfuran K(E) = [enol]/[ketone] has been measured as 1.32 x 10(-6) (pKE = 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 pKa, for ionisation of 2-phenylacetyl furan was measured spectrophotometrically as 14.38. Combining this with pKE gives pKEH = 8.5 for the enol tautomer. Comparison with deoxybenzoin (pKE = 5.15, pKaEH = 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 Bronsted exponent alpha = 0.35 has been measured and is compared with values for related substrates with pKa values in the range 18 to -11. Values of pKa and pKE for acetylfuran are estimated.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
