The separation of polar and steric effects. Part XI. Kinetics of the reactions of ortho-substituted phenylacetic acids with diazodiphenyl-methane in various alcohols

Abstract

Rate coefficients have been determined for the reactions at 30° of diazodiphenylmethane with various ortho-substituted phenylacetic acids (substituents: Me, Et, Prⁱ, Bu^t, F, Cl, Br, I, and NO₂) in methanol, ethanol, propan-2-ol, 2-methylpropan-1-ol, 2-methylpropan-2-ol, and 2-methylbutan-2-ol.

All four ortho-alkylphenylacetic acids are less reactive than the parent acid. The rate coefficients are interpreted in terms of the electron-releasing character of the groups, and of a primary steric effect which is most marked for the Bu^t group, and for all group in the less polar solvents.

The ortho-halogenophenylacetic acids react more quickly than the parent acid in all six alcohols, in the order H < F < Cl < Br < I. Only in the more polar alcohols are the ortho-substituted acids more reactive than the corresponding para-compounds. For the nitrophenylacetic acids the rate coefficients lie in the order H p-NO₂ < o-NO₂ in the more polar alcohols, but in the less polar alcohols the order is p-NO₂ > o-NO₂.

The results for the o-alkyl groups form the basis of a separation of polar and steric effects for o-Hal and o-NO₂. The essence of the treatment lies in considering the differential action of the solvent on the positive and the negative pole of the substituents in the various positions of the ring. The treatment is reasonably successful in accounting for the substituent effects in alcohols of a wide range of polarity.

The multiple correlation of log k_o with polar and steric parameters is briefly discussed but is regarded as inferior to the more detailed analysis already mentioned.