Oxidation chemistry of CH3 radicals

CH3 is a unique alkyl radical, first because it is present in virtually all alkane and alkene oxidations, particularly at high temperatures, and secondly because its range of reactions is very limited. Above 700 K, the main source of CH3 radicals is through homolysis of alkyl radicals (3), for example (3p). 

At low temperatures, the CH3 -I- O2 CH3O2 equilibrium favours CH3O2 and this radical is consumed mainly in radical-radical processes such as 

Lesclaux etal. [86] used flash-photolysis u.v. absorption to follow reactions (47) and (48) between 373 and 573 K. In particular, the presence of polar gases did not affect the derived rate constants and no pressure effect was apparent in contrast to behaviour observed for the HO2 -H HO2 reaction at these temperatures. They reported (/c 47a + 4 b) = 1-3 x 10 exp(365/T) cm molecule s with = 45 exp(-1465/T), so that /c47a dominates above about 500 K. They also reported /c48 = 4.3 X 10 exp(780/T) 

The unique behaviour of CH3 radicals is brought out starkly by comparison of the products of the separate oxidations of propanal and ethanal at 813 K using a mixture containing 2, 30, and 28Torr of aldehyde, O2 and N2, respectively [41,46]. Mechanisms for the initial stages of reaction may be written as follows. 

The high temperature CH3 + O2 reaction has constantly attracted controversy [16,89], Reaction (52) has in recent years been considered the major process above 1000 K. However, Imamura et al. [8, 89] reported that (53) dominates below 2500 K beeause of its low activation energy 

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