The separation of polar and steric effects. Part X. Kinetics of the reactions of phenylacetic acid and meta- or para-substituted phenylacetic acids with diazodiphenylmethane in various alcohols

Abstract

Rate coefficients have been determined for the reactions at 30° of diazodiphenylmethane with phenylacetic acid and various meta- or para-substituted phenylacetic acids (substituents : m- or p-Me, p-F, m- or p-Cl, p-Br, p-l, or m- or p-NO₂) in methanol, ethanol, propan-2-ol, 2-methylpropan-1-ol, 2-methylpropan-2-ol, and 2-methylbutan-2-ol. For phenylacetic acid and the meta-substituted derivatives rate coefficients have also been determined for the reactions in propan-1-ol, benzyl alcohol, and in 2-methoxyethanol. Log k₀ and Hammett ρ-values have been calculated for the reactions in the nine solvents. The reactions of phenylacetic acid in butan-1-ol, butan-2-ol, dimethyl sulphoxide, and ethyl acetate have also been studied, and those of p-nitrophenylacetic acid in propan-1-ol and in benzyl alcohol.

The reactions of phenylacetic acid with diazodiphenylmethane in ten of the alcohols show a good linear free-energy relationship with the corresponding reactions of benzoic acid. The solvent effect can be interpreted in terms of the effect of the dielectric constant, and the influence of hydrogen bonding. Multiple linear correlation of log k with groups of three suitable solvent parameters is shown to be quite successful.

The Hammett ρ-values for the reactions of phenylacetic acids (ρ₂) with diazodiphenylmethane in eight alcohols are related to the ρ-values for the corresponding reactions of benzoic acids (ρ₁) by the equation ρ₂= 0·68ρ₁– 0·235. This expression is interpreted in terms of Bowden, Chapman, and Shorter's treatment of the transmission of polar effects through side-chains. The correlation of ρ with pairs of solvent parameters is also examined.

The calculated Hammett σ-constants for the substituents in para-substituted phenylacetic acids, when the acids react with diazodiphenylmethane, show considerable variation with the polarity of the alcoholic solvent. Two possible interpretations of this are discussed.

‘Internal’ linear free-energy relationships, i.e., those involving only log k for reactions of phenylacetic acid or substituted phenylacetic acids with diazodiphenylmethane, are examined. Also a careful examination of the kinetic order for certain reactions is presented; rate coefficients at 37·0° and 48·0° and Arrhenius parameters are also recorded for the reactions of selected phenylacetic acids with diazodiphenylmethane in some of the alcohols.