Isotope effects carbon atom reactivity

It is a basic assumption in tracer work that labelled and unlabelled molecules have identical reactivities. If the labelling involves one of the atoms attached directly to the valency bond concerned in the reaction, this assumption is not valid. The differences between the reactivities is referred to as an isotope effect. When the isotopes involved are those of hydrogen, the isotope effect is quite large for heavier elements, the effects are much smaller but in the case of carbon they can be detected in careful work. To some extent, isotope effects impose a limitation upon the accuracy of results obtained from tracer studies, but they can also lead to a fuller understanding of the mechanisms of reactions of certain types. In routine tracer work, it is advisable whenever possible to label molecules at sites remote from the points of reactivity. 

The carcinogenicity of dimethylnitrosamine and 4-nitrosomorpholine was reduced by deuterium substitution for hydrogen on carbon atoms alpha to the amino nitrogen.81 82 Consistent with the hypothesis that alpha-carbon oxidation is required for reactive metabolite formation from nitrosamines, there is a substantial primary deuterium isotope effect (ku/kg = 3.8) on the rate of dimethylnitrosamine N-demethylation. 3 Specific deuteration of 3-methylcholanthrene, a potent polycyclic hydrocarbon carcinogen, showed that oxidation of the 1-carbon atom is critical in the tumor-initiating process in mouse skin. ... 

Matsuoka and Szwarc (1961) photolyzed diazomethane in the presence of isooctane and styrene in one case, and in the presence of isooctane and styrene-a,j8,j8-d3 in the other case, and thus determined the methyl affinities of styrene and of trideuteriostyrene with respect to isooctane by the method of competing reactions. The isotope effect ( d/ h) was 1-07-1 ll. Matsuoka and Szwarc used these data to justify the conclusion that in radical addition reactions the initial structure of a reactive center is preserved in the transition state. In styrene and in trideuteriostyrene the reactive centers are both terminal carbon atoms doubly bonded to another carhon. In the product the reactive carbon atom becomes tetrahedral, and thus the transition state could conceivably resemble either reactant (trigonal) or product (tetrahedral). It can be calculated, however, that a change of configuration from trigonal to tetrahedral in the transition state should exhibit an isotope effect kj)jk ) of about 1-8 in the reactions studied by Matsuoka and Szwarc the tetrahedral configuration was therefore excluded by these authors. 

It has to be mentioned here that similar effects as for bombardment of carbon surfaces with hydrogen isotopes are observed if a carbon surface is bombarded by any kind of energetic species in the presence of atomic hydrogen. In such a situation, the energetic species provides the kinetic energy required to break bonds and the chemical reactivity is provided by the atomic hydrogen species (Jacob and Roth 2007). 

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