The extension to long-chain alkanes and to high temperatures of the hydroperoxide chain mechanism of autoxidation

Abstract

The liquid-phase oxidation of 2-methylhexadecane has been studied to determine how adequately the hydroperoxide chain mechanism describes the oxidation of high molecular weight alkanes and to elucidate the changes in this mechanism caused by increased temperature. The reaction between 2-methylhexadecane and molecular oxygen has been studied at temperatures of 145 to 230 °C and the oxidation products analysed by gas-chromatographic and chemical methods. Over 160 products including hydroperoxides, alkanes, alcohols, carbonyl compounds and acids have been identified and the dependence of their yields on time, temperature and oxygen concentration measured. The attack on 2-methylhexadecane at temperatures throughout this range is selective, indicating that chain propagation occurs predominantly by the reaction of alkylperoxy radicals with alkane molecules followed by the addition of oxygen to the alkyl radicals so formed. At low temperatures the former reaction is rate-determining. However, an increase in temperature increases the rate of this reaction and reduces the concentration of oxygen dissolved in the alkane; the combined effect of these two factors causes the addition of oxygen to alkyl radicals to become rate-determining at temperatures above ca . 210 °C. As a result of this change, the concentration of alkyl radicals relative to that of alkylperoxy radicals increases with temperature. Consequently, both the yield of alkanes, and the fraction of secondary alkyl radicals that react with 2-methylhexadecane molecules to form more stable tertiary radicals, increase. These results help to predict the necessary properties of a high temperature antioxidant; compounds that react specifically with alkyl radicals rather than with alkylperoxy radicals should function thus.