The separation of polar and steric effects. Part VII. The kinetics of the acid-catalysed esterification of substituted phenylacetic acids by methanol
Rate coefficients have been determined for the acid-catalysed esterification by methanol at 40° of phenylacetic acid and various ortho- and meta-substituted phenylacetic acids (substituents: o-F, o- or m-Cl, o-Br, o-l, o- or m-Me, o-Et, o-Pri, or m-NO2). The Hammett ρ-value is –0·31, but the reaction of m-methylphenylacetic acid does not conform with the Hammett equation.
Phenylacetic acid reacts ca. 100 times more rapidly than benzoic acid; the factors underlying this are discussed. Taft Es values are tabulated for ortho-substituents in benzoic and phenylacetic acid. With ortho-alkyl groups the Es values are numerically smaller for the phenylacetic than for the benzoic acids, but the opposite applies for ortho-halogeno-substituents. We suggest that ortho-halogeno-substituents in phenylacetic acid are more heavily solvated than when they are in benzoic acid, and that therefore they behave as if they are larger.