Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

76 EXPERIMENTAL METHODS FOR SLOW REACTIONSsensitisation from radical trapping, since in many cases the two phenomena occurtogether. The catalytic action of NO in the decomposition of organic compoundsis well established. Its catalytic effect in the H, and C1, system is quite spectacular.Addition of 0.5 torr NO to 50 torr of an equimolar mixture of H, and C1, lowersthe explosion limit from 400” to 270” C242. An increase in the C1 atom concentrationoccurs via the replacement ofClz+M *2C1*+M by NO+C1, Z? NOCl+CIthe latter process having a much lower activation energy. A thermal explosion isdue to the exothermicity of the process, but autocatalysis may also occur via chainbranching, which may give, therefore, a “chemically sensitised” explosion, for examplein the H2/02 system243.However, sensitisation can have an independent role. For example, the methylradical-sensitiseddecomposition of acetaldehyde allows the Arrhenius parametersfor the processMe- + MeCHO -+ CH, + Acto be determined244, confirming that this is the major route to product formationin the pyrolysis. The use of HCl as a catalyst for the decomposition of dimethylether245showed that the slow step in the uncatalysed reaction was reaction (43)Me. + Me,O + CH, + *CH,OMe (43 1Various halogen compounds have a pronounced effect on the oxidation of hydrocarbon~~~~,~~~.Since they are usually converted to halogen acids these are themost useful catalysts, particularly HBr247. The addition of HBr sensitises the oxidationof most hydrocarbons. The mechanism probably involves the reactionsHBr + O2 z? H02. + Br. (surface)Br. + RH 4 HBr + R.R*+O, --f R02*RO,. + HBr 4 RO,H + Br.It is interesting to note that with HCl the last process would play a very minor rolesince it would be endothermic by some 25 kcal. mole-’.Perhaps the most striking use of chemical sensitisation has been by Benson andhis co-workers for the determination of various thermodynamic quantities248.Chemical sensitisation may also be used to generate a particular species that maybe capable of further decomposition. For example, 1,in and Laidler249 have producedthe sec-butyl radical by the azomethane-sensitised decomposition of n-bu-