Influence of solvent on the ortho substituent effect in the alkaline hydrolysis of phenyl esters of substituted benzoic acids

Prediction of ortho substituent effect in alkaline hydrolysis of phenyl esters of substituted benzoic acids in aqueous acetonitrile

The second-order rate constants k for the alkaline hydrolysis of phenyl esters of meta-, para- and ortho-substituted benzoic acids, X-C6H4CO2C6H5, in aqueous 50.9% acetonitrile have been measured spectrophotometrically at 25°C. The log k values for meta and para derivatives correlated well with the Hammett σm,p substituent constants. The log k values for ortho-substituted phenyl benzoates showed good correlations with the Charton equation, containing the inductive, σI, resonance, σ○ R, and steric, E s B, and Charton υ substituent constants. For ortho derivatives the predicted (log k X)calc values were calculated with equation (log k ortho)calc = (log k H AN)exp + 0.059 + 2.19σI + 0.304σ○ R + 2.79E s B − 0.0164ΔEσI — 0.0854ΔEσ○ R, where DE is the solvent electrophilicity, ΔE = E AN — E H20 = −5.84 for aqueous 50.9% acetonitrile. The predicted (log k X)calc values for phenyl ortho-, meta- and para-substituted benzoates in aqueous 50.9% acetonitrile at 25°C precisely coincided with the experimental log k values determined in the present work.

The substituent effects from the benzoyl moiety and aryl moiety were compared by correlating the log k values for the alkaline hydrolysis of phenyl esters of substituted benzoic acids, X-C6H4CO2C6H5, in various media with the corresponding log k values for substituted phenyl benzoates, C6H5CO2C6H4-X.