ELEMENTARY STEPS IN THE PYROLYSIS OF TOLUENE AND BENZYL RADICALS

The thermal dissociation of toluene was studied in shock waves under high concentration conditions in the range 1200-1500 where the mechanism of secondary reactions simplifies. A high pressure rate constant of the primary C -H bond split of 1015.0 exp(-360 kJ mol-l/RT)s-1 was derived. Using the pyrolysis of ethyl iodide as a hydrogen atom source, a rate constant of the reaction H + toluene → benzyl + H2 was determined over the range 1300-1800 K. The combination with low temperature results gave an expression of 1014.1 exp(-35kj mol-1 s-1) cm3 mol-1s-1 over the range 600-1800 K. Strongly curved Arrhenius plots of the apparent rate constants of benzyl dissociation, such as derived from earlier studies, now could be interpreted by means of secondary reactions of H atoms formed during the pyrolysis of benzyl, in particular by the rapid combination of benzyl with H which leads to toluene. In addition it is suggested that benzyl pyrolysis can proceed via a bimolecular pathway involving the thermal dissociation of dibenzyl to stilbene + 2H and a unimolecular dissociation also producing one H atom per dissociation benzyl. A rate constant for benzyl fragmentation near the high pressure limit of 1015.3 exp(-350 kJ mol-l/RT)s-1 was derived. © by R. Oldenbourg Verlag