Some experiments on the pyrolysis of toluene

[摘要] A critical survey of aspects of our knowledge of the heat of formation of the benzyl radical is given in view of its relevance to the heat change in the dissociation process:- C₆H₅CH₃ + C₆H₅CH₂- + H- (2) Previous work on the pyrolysis of toluene by flow techniques in summarized and apparatus devised to study the reaction under more defined conditions with independent variation of reaction time and partial pressure. Analytical investigations were extended to continuous observation of hydrogen production, and mass spectrometry was applied to identification of gases, liquids and solids.(3) The existence of the postulated reaction:- H- + C₆H₅CH₃ = C₆H₆ + CH₃- has been checked by decomposing toluene and fluorine together in the flow system. The latter substances provides excess hydrogen atoms, and an increase in decomposition was observed. (4) Decompositions of toluene in the presence of deuterium have shown that products and unchanged reactant all show appreciable deuteration, and it seems that extensive exchange reactions, probably initiated by hydrogen and deuterium atoms, occur at a rate faster than the decomposition process as judged by (hydrogen + methane) formation. (5) The products of the reaction not condensable in liquid air averaged 76.7% hydrogen and 23.3% methane. In addition to methane, ethane and ethylene have been shown to be present in appreciable amounts. The liquid products contained benzene, but not in amount equivalent to the methane and C2-hydrocarbons found to be present. The solid products have been shown, by the application of mass spectrometry to a few milligrams of product, to include the following substances in the molar proportions indicated:- 50-60 mole percent dibenzyl, 1-2 mole percent diphenyl, and the remainder dimethyldiphenyls and monomethyldiphenyls in the ratio 4:1. (6) The kinetic data on the reaction have been obtained within the following range of conditions:- 772-880°c; reaction times 0.565 - 2.076 seconds; toluene partial pressures 0.56 - 2.34 mm.; total gas pressure 3.33- 10.73 mm. The carrier gases have included nitrogen, helium and deuterium. A small number of experiments in a static system extended the temperature range to 750°c. (7) It has been shown that, judged by the (hydrogen + methane) production, the reaction was first order over the range of flow system variables indicated in (6). In order to get consistent data 'seasoning' of the reaction vessel was essential, and this process was followed by application of a thermal conductivity gauge method. (8) The temperature dependence of the first order rate constants was given by log.10k(sec⁻¹) =-84700/2.303RT +15.1.(9) The results of this work have been compared at relevant points with previous data. It is concluded that Szwarc's views on the simple nature of the processes occurring cannot be maintained. Steacie's criticisms of Szwarc's findings are substantiated even within the latter’s temperature range of investigation. It is not felt possible to assert that the experimental activation energy quoted in (8) is the bond dissociation energy for the side chain C-H bond in toluene. It may be the case since it is in agreement with recent independent determinations of the heat of formation of the benzyl radical, but the complexities of the products weaken the case.

[发布机构] University:University of St Andrews;Department:Chemistry (School of)