Relative rates of the termination steps

In practice these are a function of both the chain carrier concentrations and the rate constants for termination. Specific predictions can only be made if all rate constants for propagation and termination are known. It is only when the chain carrier concentrations are comparable that it is possible to say that reactions between the smaller radicals will dominate. 

Question. The following is a schematic mechanism for the bromination of an organic compound RH. 

Using the criteria given in Sections 6.12.1-6.12.3, suggest which is likely to be dominant. 

For this mechanism, both criteria reinforce each other by predicting that step 4 is dominant. 

Note also that the slowest step involves the most complex radicals, and the fastest step involves the simplest radicals. This would be expected from transition state theory predictions (Section 6.12.2). 

---

In radical chain oxidation reactions, the relative rates of the termination steps differentiate oxidation steps and, hence, affect the selectivity. The termination of tertiary peroxides is much faster than that of primary peroxides. The constraints of the zeolite micropore dimensions limit the geometry of the bimolecular encounter of alkane and oxy radical. 

---