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

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.