Influence of structure on β-diketo–enol equilibria: a kinetic study by the relaxation technique

The effects of structural variation on the position of the tautomeric equilibrium of β-dicarbonyls of the type MeCO·CHR·COMe have been studied by the relaxation technique. The acidities of the enol and keto forms may be determined from the kinetic parameters of the equilibria in basic media without recourse to direct measurement of the enol content. This is not possible by classical techniques. The enol content may itself be calculated from the deprotonation constants of the ketones. Contrary to presently accepted views, it is shown that variation of the enol content as a function of the structure is due to variation in the energy of the ketones and not the enols. The reverse situation is found however in going from pentane-2,4-dione to ethyl acetoacetate. The decrease of the enol content in aqueous media relative to the gaseous state is identical for all members of the series except for R = H.