Correlation of ultraviolet absorption frequences of cis and trans substituted cinnamic acids with hammett substituent constants

The ultraviolet absorption spectra of a number of cis and trans-3- and 4-substituted cinnamic acids have been determined. Examination of the obtained spectra shows that there is a fair linear relation between the positions of the absorption bands for the electronic transitions of the carbonyl group and the Hammett substituent constants. The plot of the λ max values of the cinnamic acids versus the rate constants for he corresponding acids with diazodiphenylmethane in ethanol gave a fair correlations. This relationship was discussed in view of the proportionality of the energy of activation for the reaction of these acids with DDM in ethanol and the energy for the electronic transitions in the carbonyl group under the influence of UV radiation.