Ring–chain tautomerism. Part III. Substituted cis-3-benzoylacrylic acids

Abstract

The equilibrium constants for ring-chain tautomerism in a series of substituted cis-3-benzoylacrylic acids have been determined using an i.r. spectroscopic method, in dioxan, and using a ¹H n.m.r. spectroscopic method, in dioxan and 80%(w/w) 2-methoxyethanol–water. The equilibrium constants for the cis-3-(substituted benzoyl)-3-methylacrylic acids have been correlated with the substituent constants σ_I and σ_R° using a modified Hammett equation. The results are related to the factors determining the position of the equilibria. The observed pK_a values, which have been measured in 80%(w/w) 2-methoxyethanol–water, have been corrected for these tautomeric equilibria to give the true pK_a^T. The effects of substitution on the latter values have been assessed by means of the Hammett equation. The reaction constants for the cis-acids indicated comparable transmission of the substituent polar effect to that observed in the related series of trans-acids.