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 σI and σR° using a modified Hammett equation. The results are related to the factors determining the position of the equilibria. The observed pKa values, which have been measured in 80%(w/w) 2-methoxyethanol–water, have been corrected for these tautomeric equilibria to give the true pKaT. 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.