SEMI-EMPIRICAL MODEL FOR PREDICTING PHENOL CARBOXYLIC ACIDS ANTIRADICAL ACTIVITY

Abstract

Using the spectral method, the rate constants of natural phenolcarboxylic acids reaction with the radical 2,2'-diphenyl-1-picrylhydrazyl in benzene at a temperature of 293±2 K were determined. It is established that the reaction corresponds to the second-order kinetic equation and proceeds by the mechanism of the hydrogen atom transfer. This confirmed by the presence of the deuterium isotope effect. The parameter, affecting the proceed of this mechanism in a non-polar medium, is the energy of the homolytic rupture of the weakest phenolic O–H bond in the phenolcarboxylic acid molecule, calculated using the quantum-chemical methods. Changes in the strength of phenolic O–H bonds in an acid molecule lead to corresponding changes in their reactivity with respect to the hydrazyl radical. It is seen that the compounds with low bond strengths of functional groups – 3 pyrogallolcarboxylic and gallic acids, methyl- and ethyl- gallate – showed the most antiradical activity. According to the calculated and experimental data, a semiempirical linear single-factor equation is proposed. This equation describes the relationship between the antiradical activity of phenolic acids and the descriptor of their structure and allows to predict the reactivity of the antioxidant in lipid-like media. The applicability of the proposed model was proved by studying the control group of hydroxyacetophenones which belong to plant phenol compounds. According to the forecast, 3,4- and 2,5-hydroxyacetophenones can be recommended as potential effective antioxidants in non-polar environments. The unit relative deviations of the predicted rate constants from their experimental values vary from 2 to 9% with an average approximation error equals to 7.9%, which indicates a good selection of the linear model.