Formation of D and H Atoms in the Pyrolysis of Benzene-d6 and Chlorobenzene behind Shock Waves

Article abstract of DOI:10.1021/j150668a049

Dilute mixtures (3-20 ppm) of C6D6 (benzene-d₆) were pyrolyzed behind reflected shock waves at temperatures of 1630-1940 K and total pressures of 2-3 atm.Progress of the reaction was followed by analysis for D atoms using resonance absorption spectroscopy.Appearance of D atoms was a first-order process with respect to benzene concentration, and with respect to time during the first part of each experiment.An Arrhenius equation for the formation of D atoms, based on 34 experiments, is k_D = 9.7E12 exp(-87100 cal/RT) s^-1 with an estimated uncertainty of a factor of 1.5.From measurements of H atoms during pyrolysis of chlorobenzene under similar conditions at 1570-1790 K, the first-order rate constant for the dissociation of chlorobenzene to chlorine atoms and phenyl radicals was found to be k₆ = 1.2E14 exp(-90000 cal/RT) s^-1, and that for the dissotiation of phenyl radicals to H atoms and other products k_3a = 1.2E15 exp(-82000 cal/RT) s^-1.With this information, the rate constant for dissociation of benzene-d₆ to phenyl-d₅ and D atoms was found to be k_1D = 4.6E13 exp(-95000 cal/RT) s^-1.The rate constant for the exchange reaction H + C6D6 -> C6D5H + D was found to be k_4a = 3.2E13 exp(-4200 cal/RT) mol^-1 cm³ s^-1 over the range 300-1400 K by combining our results with others at lower temperatures.A very simple kinetic model based on a reaction chain with H as carrier can relate our data to other shock-tube work at higher benzene concentrations.