The separation of polar and steric effects. Part IX. The kinetics of the reactions of ortho-substituted benzoic acids with diazodiphenylmethane in several solvents

Abstract

Rate coefficients are reported for the reactions at 30° of diazodiphenylmethane with benzoic acid and its ortho-substituted derivatives (substituents: Me, Et, Prⁱ, Bu^t, F, Cl, Br, and I) in dioxan, ethyl acetate, 2-methoxyethanol, ethanol, dimethylformamide, and dimethyl sulphoxide.

In dioxan, ethyl acetate, 2-methoxyethanol, and ethanol the rate coefficients for o-alkylbenzoic acids lie in the order H > Me < Et < Prⁱ < Bu^t. The retarding influence of an ortho-methyl group, which varies with the solvent, is probably due to a polar effect, combined with a steric effect on solvation. The increase in rate coefficient through the series of acids is due to a secondary steric effect, which does not depend greatly on the solvent. In dimethylformamide or dimethyl sulphoxide, an ortho-methyl group also retards the reaction, particularly in dimethylformamide, but increase in the size of the alkyl group produces little change. This is attributed to powerful solvation of the carboxylic proton, which greatly reduces the conjugation of the carboxy-group with the ring.

o-Halogeno-substituents markedly increase the reactivity of benzoic acids towards diazodiphenylmethane in the order H ≪ F < Cl < Br ≈ I, with o-Cl ≫p-Cl. With dimethylformamide as solvent, the accelerating influence of ortho-halogens is not so marked as with the other solvents. The influence of ortho-halogens is believed to be due mainly to a large polar effect. The pK_a values for o-halogeno- and o-methyl-benzoic acids are taken to define a rough scale of polar effects, and approximate polar susceptibility constants, ρ_o, are evaluated for the reaction with diazodiphenylmethane. These are less solvent-dependent than the Hammett ρ-values based on the effect of meta-substituents.

Polar substituent constants, σ_o, for ortho-halogen and -methyl based on the pK_a values of benzoic acids, are compared with values calculated on the basis of simple models of the polar effect.