The oxidation chemistry of the xylenes and related aromatic hydrocarbons
The research was concerned with obtaining kinetic data and product information for the oxidation chemistry of the isomeric xylenes and benzyl radicals at 753K. A trace amount (0.01 % for kinetic study, 0.5% for product study) of each xylene was separately added to slowly reacting mixtures of H₂ + O₂ in aged boric-acid-coated Pyrex reaction vessels at 753K and 500 Torr total pressure. From measurements of the relative loss of H₂ and hydrocarbon rate constants for the reactions of the radicals H and H0₂ with each xylene were obtained. H + Xylene → Products k=1.3* 10⁹ 1 mol¯¹ S¯¹ (p-xylene), 1.4*10⁹ I mol¯¹ S¯¹ (m-xylene), 1.8* 10⁹ 1 mol¯¹ S¯¹ (a-xylene). HO₂ + Xylene → Products k=9.4*10⁴ I mol¯¹ S¯¹ (p-xylene), 1.03*10⁵ 1 mol¯¹ S¯¹ (m-xylene), 1.2*10⁵ 1 mol¯¹ S¯¹ (o-xylene). A detailed examination of the reaction products was carried out over a wide range of mixture composition. The major aromatic products from the oxidation of the xylenes were the isomeric tolualdehydes, toluene and a-xylylcne oxide (from a-xylene). These were found to be similar to the products formed from the oxidation of toluene at 753K. From examination of the products available it was also possible to obtain more detailed rate constants for the attack of H radicals at each xylene giving specific products. H + Xylene → Methylbenzyl k=7.3*10⁸ 1 mol¯¹ s¯¹ (p-xylene), 8.3*10⁸ 1 mol¯¹ s¯¹ (m-xylene), 1.2*10⁹ 1 mol¯¹ s¯¹ (o-xylene). H + Xylene → Toluene k=5.6*10⁸ 1 mol¯¹ s¯¹ (for all three xylenes). H + Xylene → CH₃-C₆H₃-CH₃ k=9.1 * 10⁶ I mol¯¹ s¯¹ (p-xylene), 8.4 * 10⁶ I mol¯¹ s¯¹ (m-xylene), 9* 10⁵ I mol¯¹ s¯¹ (a-xylene). In the case of H0₂ + RH, thermochemical considerations lead to the conclusion that both abstraction from and addition to the ring will be considerably less energetically favourable than the equivalent reaction of H atoms. Consequently k(HO₂₊xylene) should be taken to be the rate constant for HO₂ + Xylene → Methylbenzyl. The decomposition of neopentylbenzene in the presence of oxygen and propene at 753K and a total pressure of 60Torr in an aged boric-acid-coated Pyrex reaction vessel was used to determine rate constant" for the reactions between H0₂ and benzyl radicals. The decomposition of neopentylbenzene at 753K produced t-butyl and benzyl radicals. The t-butyl radicals react with O₂ to produce HO₂ radicals. The presence of propene was used to determine the concentration of HO₂ radicals by monitoring the rate of production of propylene oxide. From measurement of the yields of benzaldehyde and toluene yields, and the rate of production of propylene oxide, rate constants for the reactions of HO₂ with benzyl radicals producing benzaldehyde and toluene were determined as 4.36* 10⁹ I mol¯¹ s¯¹ and 8.44* 10⁸ I mol¯¹ s¯¹ respectively at 753K. A detailed examination of the products produced from the decomposition of neopentylbenzene in the presence of oxygen showed that the primary products were benzaldehyde, toluene, i-butene and 2-methyl-l-phenylprop-l-ene. The key features highlighted in this research are the similarity in the oxidation chemistry between the xylenes and toluene and the importance of the reactions of the benzyl radical in the oxidation of methyl-substituted benzenes.