Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

4 ANALYSIS 13Direct decomposition of CHzNOH also occurs at these temperatures, in somecases e ~plosively~~~~~~~.In addition to inhibition. at high concentrations of NOcatalysis also occurs, and with acetaldehyde involves the following initiationprocess2zNO+AcH + Ac*+HNO (30)If R. is Et. or a higher alkyl radical, we also have the possibility of disproportionationwith NO226Et+NO + HNO+C,H, (31)Thus, although the reaction R-+NO is very efficient, NO is not in fact a goodradical trap, because of the complicating reactions due to the instability of the products,except possibly at the lower temperatures of 100-200" C. Thus, Phillipset ~ 1 1 have . ~ used ~ ~ NO in the study of the pressure-dependent decomposition ofthe isopropoxy radical. There are some similarities with the reactions of 02, suchas R e + O2 + RO,. and O2 + RH + HO,. + R-. Here, inhibition takes the specialform of oxidation.Of all the olefinst used as free radical traps, propene is the favourite. It wasfirst used at about the same time as NOzz8. Propene tends to be 1/10 to 1/100 asefficient as NO depending on the conditions. The trapping occurs in two ways.Hydrogen abstraction from the olefin takes place, producing the (hopefully) unreactiveallyl radicalR. + MeCH = CH2 + RH + CH2CH = CH, (32)However, there is some evidence that the allyl radical itself is capable of abstractinghydrogen atoms particularly at high temperatureszz9. Secondly, addition acrossthe double bond occurs. Providing thc resulting radical is stable, we have a usefultrapping process. However, several complications may arise. Should hydrogenatoms add, displacement is almost certain to occur and may well occur with alkylradicals, uiz.H. + MeCH = CH, z? n-PI- or i-Pr + Me. + C,H, (33)EtIEt- + MeCH = CHz z MeCHCH,-+EtCH =CH, + Me. (34)In both these systems and particularly in those where species like the brominet For a list see ref. 218, pp. 174 and 198.References pp. 104-111