Kinetic isotope effects carbon-hydrogen insertions

Some care must be exercised in setting up the crossover equipment to account for kinetic isotope effects associated with abstraction or insertion into a carbon-hydrogen (deuterium) bond. In general, abstraction is expected to exhibit a larger isotope effect than insertion, and this appears to be the case (see below). To accommodate this, and to increase the sensitivity of the experiment, it is often necessary to employ a smaller amount of the hydrocarbon than of its deuteriated analog. 

In some instances, particularly in hydroxyla-tions meta to a halide substituent, the hydrogen on the hydroxylated carbon is quantitatively lost (i.e., there is no NIH shift), and a small deuterium kinetic isotope effect is observed These hydroxylations could result from direct oxygen insertion into the C-H bond, as in a true hydrox-ylation mechanism, but they are more likely to result from oxidation of the aromatic ring without the formation of a discrete epoxide intermediate. Isotope effect studies with deuterated benzenes bearing a variety of substituents have shed some light on this process A small, normal isotope effect is observed for weta-hydroxylation when deuterium is located meta- to the halogen in chlorobenzene = 1.1-1.3), but a small,... 

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