Techniques to Study Radicals Clocks and Traps

Radical clocks are one experimental technique that has received considerable use in the analysis of radical reactions. Most radical clocks involve an intramolecular free radical rearrangement that proceeds with a well-defined rate constant. The prototype is the rearrangement of 5-hexenyl radical to cyclopentylmethyl radical, which occurs with a unimo-lecular rate constant of 1.0 X 10 s at 25 °C (Eq. 8.75). The clock strategy is to embed a 5-hexenyl unit into the reactive system of interest. If a radical forms, and if its lifetime is comparable to or greater than 10 s, cyclopentylmethyl-derived products should form. 

Various Radical Clocks and Their Rate Constants for Rearrangements  

In order to study the lifetimes of various radicals in new reactions, one requires several radical clocks with varying lifetimes. Incorporation of these clocks into the molecules under study is used both to show that radical intermediates do or do not exist, and if they do, their lifetimes relative to the clock. Several free radical clocks with their rate constants for rearrangement are shown in Table 8.7. Such a collection has been termed an horlogerie, after a French term for a small shop that sells clocks. Seven orders of magnitude can be spanned by choosing the correct clocks. 

Another tool for studying radicals is the use of a spin trap, in an experiment called spin trapping. The addition of a free radical to a nitroso or nitrone group (the spin trap see Eqs. 8.76 and 8.77) creates a spin adduct. The spin adduct is another radical, but it is typically a long lived radical that can often be studied using EPR spectroscopy. The EPR spectrum can be informative about the structure of the radical that added. 

Determination of 1,4-Biradical Lifetimes Using a Radical Oock 

---