Kinetics of Radical Elementary Steps

The first direct measurements of radical reactions were made by kinetic ESR, in continuous flow arrangements of the Hartridge-Roughton type, with the time 

Most radicals in solution react with each other at about the diffusion limit, but because of a quantum mechanical restriction only one of the four possible electron spin states of radical pair ( f t, t J, J, f and J, J.) is productive. For most solvents, the recombination rate at room temperature is 5 x 10 M s ,  

Direct measurement of the reaction of interest is sometimes possible using rapid reaction techniques. In laser flash photolysis, an intense, short-lived pulse of light irradiates the sample and the products are monitored by a variety of techniques, from basic UV/Vis spectroscopy to techniques - such as laser-excited fluorescence -which require a second, analytical pulse of radiation. In pulse radiolysis, a short (1-10 ns) pulse of high-energy (1-10 MeV) electrons irradiates the sample and the decay of the fragments can be analysed in the same way as the fragments from flash photolysis. The equipment for pulse radiolysis is even more complex and costly than that for flash photolysis, and tends to be concentrated in national facilities. 

In these circumstances, where routine kinetic measurements are uninformative and direct measurements of the product-forming steps difficult, comparative methods, involving competition between a calibrated and a non-calibrated reaction, come into their own. Experimentally, ratios of products from reaction cascades involving a key competition between a first-order and a second-order processes are measured as a function of trapping agent concentration. Relative rates are converted to absolute rates from the rate of the known reaction. The principle is much the same as the Jencks clock for carbenium ion lifetimes (see Section 3.2.1). However, in radical chemistry Newcomb prefers to restrict the term clock to a calibrated unimolecular reaction of a radical, but such restriction obscures the parallel with the Jencks clock, where the calibrated reaction is a bimolecular diffusional combination with and the unknown reaction a pseudounimolecular reaction of carbenium ion with solvent. Whatever the terminology, the practical usefulness of the method stems from the possibility of applying the same absolute rate data to all reactions of the same chemical type, as discussed in Section 7.1. 

If the conversion of R to R is irreversible (k i = 0) and PCY] is present in large enough excess to ensure first-order conditions, the ratio of the products is simply related to the ratio ki and Ic2 by eqn. (7.5)  

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