Detection of free radicals and reactions chains

First evidence for the occurrence of free radicals in the thermal decomposition of propionaldehyde was obtained by the para-ortho hydrogen technique . Sworski and Burton investigating the reaction between 850 and 950 °C by the lead-mirror technique, identified methyl and ethyl radicals and also determined their concentration ratio. They showed that the concentration of C2H5 exceeds that of CH3 in the temperature range studied and that the [CH3]/[C2H5] ratio increases with increasing temperature (Table 4). 

The occurrence of chains at temperatures slightly above 100 °C was proved by Volman and Brinton ° . They found that methyl radicals from the pyrolysis of di-/-butyl peroxide induce the chain decomposition of propionaldehyde in the temperature range 122-156 °C. 

Other workers claim that the maximally inhibited reaction is a chain process in which the inhibitor molecule is involved ° . It seems possible to interpret the inhibiting effect as well as the almost identical rate minima, brought about by various inhibitors, without having to postulate a molecular reaction. 

The arguments favouring a molecular mechanism for the fully inhibited reaction do not seem to be sufficiently convincing. On the other hand, further systematic investigations are needed to elucidate certain details of the chain mechanism suggested for the inhibited reaction. Most of the evidence supports the assumption that the contribution of the molecular process is small, compared to that of the chain component, in both the inhibited and the uninhibited decomposition. 

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