Additivity of rate coefficients

From the viewpoint of the combustion chemist, mechanistic and theoretical studies of abstraction reactions serve two purposes. First, they can determine the overall rate coefficient for an abstraction over a range of temperatures, especially when there are limited experimental data. Second, the combustion modeller wishes to know the rate of abstraction at any particular site on a hydrocarbon molecule. For reaction (10) this is trivial as there is only one type of site a primary C—H bond. However, for more complex fuels there will be a variety of different sites which to a first-order approximation can be considered as primary, secondary and tertiary C—H bonds. As mentioned in the introduction to this section, Atkinson et al. [10] and Walker [11] have attempted to describe radical/ alkane kinetics with the following simple model based on equation (2.4) 

Comparison of room temperature rate coefficients for OH abstractions 

The excellent agreement between calculated and experimental determinations indicates that unknown rate coefficients for alkane abstractions can be predicted by this method with a high degree of confidence. Some of the branched chain alkanes may be an exception to this rule. The large deviations between experiment and calculations are attributed by Atkinson et al. [10] to possible steric hindrance. 

Calculated percentages of radicals formed in the overall reaction OH + alkane 

The importance of such calculations not only lies with the prediction of the absolute values, but also in determining the relative site specific rate coefficients, and hence, the fractions of the different isomeric radicals which can be formed (Table 2.4). Experimental determinations of site specific rate coefficients are scarce. As seen in subsequent sections, the final products of combustion depend strongly on the nature of the radical intermediates. 

---