0023-1584/03/4401- $25.00 © 2003 MAIK “Nauka /Interperiodica” 0091 Kinetics and Catalysis, Vol. 44, No. 1, 2003, pp. 91–100. Translated from Kinetika i Kataliz, Vol. 44, No. 1, 2003, pp. 100–110. Original Russian Text Copyright © 2003 by Tavadyan, Martoyan, Minasyan. INTRODUCTION The kinetic analysis of models for the inhibition of complex (multistep) chemical reactions is a topical problem. It is closely associated with solving an impor- tant problem—the nonempirical choice of an effective inhibitor for given conditions of a chemical reaction. The mechanisms of action of inhibitor antioxidants on the oxidation of organic compounds in a condensed phase were studied in considerable detail, and reactions with the participation of intermediates resulting from inhibitor conversion were examined [1–13]. However, it should be noted that complicated kinetic models of antioxidant action are very difficult to analyze. The main reason consists in the occurrence of great num- bers of steps and components in these models. In the analysis of various mechanisms of inhibited reactions, it is of paramount importance to obtain infor- mation on the roles of elementary steps regardless of the complexity of the initial kinetic model. This is also a prerequisite for the characterization of the reactivity of an inhibitor in a multistep chemical transformation. For example, this information is required for determin- ing the contributions of reactions with the participation of an inhibitor and its reaction products in the inhibition of the process. It is also of interest to relate the molec- ular structure of an inhibitor with reaction characteris- tics (rate, induction period, etc.). In this work, we performed the numerical “value” analysis [14–18] of a kinetic model for the inhibition of the autoxidation reaction of ethylbenzene by para-sub- stituted phenols and determined the kinetic “signifi- cance” of each individual step of the model. We consid- ered the kinetic model of the given reaction as an illus- tration in some sense; the aim of this study was to demonstrate the capabilities of the numerical analysis of kinetic models of inhibited oxidation by the “value” method. On the other hand, the value analysis of kinetic mod- els of inhibition provides an opportunity to optimize reaction conditions. This is associated with solving the following kinetic problems: (a) the determination of the molecular structure of an effective inhibitor by model- ing in terms of a given kinetic model and under chosen reaction conditions and (b) the determination of an opti- mum initial concentration of the inhibitor. THEORETICAL GROUNDS Calculation of the Time Profiles of Value Quantities It important to reveal the effects of elementary steps on the overall reaction rate (w) for inhibition pro- cesses. In accordance with this, the kinetic significance of an elementary step is determined as the “value” (G) [14–18]—the ratio of a response of the overall reaction rate at the point t in time to a variation in the rate of the Determination of the Kinetic Significance of Elementary Steps in the Reaction of Ethylbenzene Oxidation Inhibited by para-Substituted Phenols: Choice of an Effective Antioxidant L. A. Tavadyan, G. A. Martoyan, and S. H. Minasyan Institute of Chemical Physics, Academy of Sciences of Armenia, Yerevan, Armenia Received April 19, 2002 Abstract—A conceivable method (“value” method) for analyzing the reaction kinetics models of inhibited liq- uid-phase oxidation of organic compounds was considered. A procedure for the numerical calculations of the molecular structure parameters of an inhibitor and an inhibitor concentration that results in a maximum inhibition of the reaction was presented. In the kinetic model of a chain reaction of liquid-phase ethylbenzene oxidation in the presence of a para-methylphenol inhibitor, the dynamics of contributions from individual steps was calcu- lated. This allowed us to simplify the reaction mechanism. The inhibition of the oxidation reaction due to reac- tions with the participation of a phenoxyl radical was found to occur under conditions of the quasi-equilibrium The molecular structures and initial concentrations of para-substituted phenols, which are most effective for the given reaction conditions, were determined. HO 'R O 'R . . RO 2 + RO 2 H + .