Gas phase oxidation of benzene: Kinetics, thermochemistry and mechanism of initial steps

A new investigation of the primary steps of the benzene oxidation, involving complementary experimental and theoretical approaches, is presented. The reactions of the OH-adduct (hydroxy-cyclohexadienyl radical c-C6H6-OH) were investigated using laser flash photolysis and producing OH radicals by H2O2 photolysis at 248 nm. It is confirmed that the adduct is in equilibrium with the corresponding peroxy radical RO2, near atmospheric conditions, the measured equilibrium constant being: Kc,2b = (2.62 ± 0.24) × 10−19 cm3 molecule−1 at 295 K, with the temperature dependent expression: ln(Kc,2b/cm3 molecule−1) = −63.29 + 6049/T, obtained by using the calculated entropy of reaction. The rate constant of the association reaction yielding RO2 is: k2b = (1.31 ± 0.12) × 10−15 cm3 molecule−1 s−1. Calculated data are in agreement with those values. In addition, data analysis shows that the reaction c-C6H6-OH + O2 involves an irreversible loss of radical species, yielding phenol and other oxidation products, with the global rate constant: kloss = (2.52 ± 0.40) × 10−16 cm3 molecule−1 s−1. Quoted errors are statistical (2σ), the possible total errors on the above values being estimated at around 40%. By comparison with the kloss value, the rate constant for phenol formation, calculated using a combination of DFT and ab initio CCSD(T) methods, corresponds to a phenol yield of about 55%, in reasonable agreement with experimental observations. Thermochemical and kinetic parameters have been calculated for the formation and for the reactions of the two RO2 stereoisomers, cis and trans. They show that the observed equilibrium must involve the trans isomer, which is more stable and is formed more rapidly than the cis isomer. Calculations show that the only possible reactions of peroxy radicals, under atmospheric conditions, is cyclisation yielding a bicyclic radical. However, cyclisation of the RO2(trans) is calculated to be too slow, compared to the rate of the irreversible radical loss, whereas it is very fast in the case of the cis isomer and can lead readily to oxidation products. On the basis of those results, a reaction mechanism is proposed for the first steps of benzene oxidation, consistent with all experimental and theoretical data, and which accounts for the principal oxidation products observed.