The separation of polar and steric effects. Part VI. The kinetics of the acid-catalysed esterification of substituted benzoic acids by methanol

Abstract

Rate coefficients have been determined for the acid-catalysed esterification of benzoic acid and of o-XC6H4·CO2H (X = F, Cl, Br, I, Me, Et, or Pri) by methanol, with toluene-p-sulphonic acid as catalyst, at several temperatures. The acids m-XC6H4·CO2H (X = Cl, Me, or NO2) have been similarly studied at 60°, and approximate rate coefficients have been determined for the esterification of o-t-butylbenzoic acid at 100 and 120°.Arrhenius parameters and the related quantities of transition-state theory have been evaluated for the reactions of the ortho-substituted acids. The effect of ortho-substitution on the entropy of activation (ΔS‡) in esterification is discussed in terms of a ‘bulk effect’ which tends to decrease ΔS‡, and steric inhibition of solvation of the transition state which tends to increase ΔS‡. All the ortho-substituents studied (except t-butyl) depress the enthalpy of activation (ΔH‡) relative to that for the reaction of the parent acid and two effects are tentatively suggested to account for this, namely, the secondary steric effect of ortho-substituents, and a steric inhibition of ‘ring solvation’ of the initial state, which may be brought about by substituents in any position in the ring.Taft steric substituent constants (Es) have been evaluated for the ortho-substituted acids, and these are compared with values obtained from other systems. The differences found are discussed in terms of the Hammett reaction constant (ρ) and of the nature of the solvent.It is also concluded that results from the kinetics of esterification cannot replace those from acid-catalysed ester hydrolysis for the purpose of accurately satisfying the conditions of the Taft relation.