Ring-chain tautomerism. Part 8. Substituted 2-(2-oxopropyl) and 2-(2-oxo-2-phenylethyl)benzoic and 2-(2-acetyl and 2-benzoylphenyl)acetic acids

Abstract

The observed pK_a values for 2-[2-(substituted phenyl)-2-oxoethyl]benzoic, 2-[2-(substituted benzoyl)phenyl]acetic, 2-(2-oxopropyl) and 2-(1,1-dimethyl-2-oxopropyl)benzoic, 2-(2acetylphenyl)acetic and 2-(2-acetylphenyl)-2,2-dimethylacetic acids in 80%(m/m) 2-methoxyethanol–water have been measured. The occurrence of ring–chain tautomerism has been examined in these acids by IR and NMR spectroscopic methods. The substituted benzoyl acids are predominantly in the chain (normal) form and the Hammett reaction constants have been used to study the transmission of polar effects in these systems. The equilibrium constants for ring–chain tautomerism for the acetyl acids have been determined directly by IR and NMR spectroscopy and indirectly from their observed pK_a values. The results for ring-chain tautomerism in acyl carboxylic acids employing a comprehensive series of linking groups are discussed in terms of stereochemistry and steric ‘bulk’ interactions.