Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

72 EXPERIMENTAL METHODS FOR SLOW REACTIONSversion has been used to detect radicals in the photolysis of MeI"', H1212,C2H6213, and NH,214. The cis-trans isomerisations of maleic acid, allocinnamicacid and cis-1,2-dichloroethylene are also catalysed by free radicals or atoms2I5.4.2.2 Radical trapsAn important early method (1929) for the detection of gaseous free radicalswas the metallic mirror technique of Paneth2I6. Since then, many compounds havebeen used to determine the presence and extent of free radical reactions in the gasphase decomposition of organic compounds. The subject of inhibition in gas-phasereactions has been reviewed by Ashmore2" and by Gowenlock2".The most extensively used radical trap is nitric oxide, first used by Staveley andHin~helwood~'~. There is no doubt that NO is a very efficient radical trap. However,ensuing reactions make the overall process very complex. The further reactionsthat a nitroso compound (RNO) may undergo, after the combination of NOwith a radical 8, have been discussed in detail by Gowenlock2". Assuming Ris Me., at or close to room temperature, combination is followed by dimerisationof MeNO, uiz. 220Me+NO -+ MeNO -+ *(MeNO), (25)At temperatures above loo" C, dimerisation becomes less important.With excess or NO222 the following reactions occurMeI2Me. + MeNO -+ Me-N-OMe0HNI2N0 + MeNO -+ Me-NONO 2% MeN2N0,MeN,NO, + MeNO,+N, + MeO-+NO,+N,Above 200" C, MeNO isomerises to CH,NOH, homogeneously, heterogeneously,and possibly by a chain mechanism222. In a free radical system, sensitised decompositionmay occurMe.+CH2=NOH -+ CHNOH+CH, + HCN+CH4+*OH (29)replacing methyl by the more reactive .OH, the exothermicity of the process increasingthe overall rate.