An n.m.r. study of keto–enol tautomerism in β-diketones

Abstract

Thermodynamic functions for the enolisation of a series of β-diketones have been obtained from measurements of equilibrium constants and their temperature dependence. The values of –ΔH range from 1·3 to 4·6 kcal. mole^–1 and of –ΔS from 3·4 to 9·5 cal. mole^–1 deg.^–1, and the results are discussed in terms of steric and electronic effects. The effect of solvent on the keto–enol equilibrium has been studied for pentane-2,4-dione and the results in polar solvents are explicable in terms of preferential solvation of the keto tautomer.