Transition from low to high temperature mechanism

The transition from the low to high temperature mechanism is essentially due to a change in the relative rates of reactions (3), (—3), (2) and (14) 

a general expression for the ratio of the rates of product formation by the low and high temperature mechanisms is given by 

Since the rate of isomerization of alkylperoxy radicals depends upon their molecular weight and structure, it can be seen that the temperature at which the transition occurs will be dependent upon the molecular weight and structure of the hydrocarbon. Thus, for example, in the case of propane 1 5 hydrogen transfer is impossible for the prop-2-ylperoxy radical and 1 4 hydrogen transfer involves the cleavage of a primary C—H bond. The expression therefore reduces to 

At a given temperature, this ratio is always higher for n-pentane than for propane (Table 23). It can be seen, therefore, that once again a change in mechanism depends on the restrictions imposed by the molecular weight and structure of the hydrocarbon on the ability of its alkylperoxy radicals to isomerize. In this respect neopentane is again an interesting example. 

The relative rates of the high and low temperature mechanisms for propane and n-pentane 

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