Chemical Applications of Tracers

The use of radiotracers is an excellent technique for measuring the solubility product constant of sparingly soluble salts or for making other studies of substances present in low concentrations. Another very important and classic example of the use of radiotracers is that of studying the occurrence and properties of isotopic exchange reactions—reactions of the type 

Perhaps the most significant of the numerous applications of radiotracers in chemistry has been the study of chemical reaction mechanisms. In fact, most of the proposed reaction mechanisms have been verified by means of a radiotracer study. One of the simplest mechanistic experiments using radiotracers is to test the equivalence of various atoms in molecules in chemical reactions. An example of this type of study is the work of Volpin et al. (1959) on the equivalence of the seven carbon atoms in the tropylium ring. Volpin et al. reacted labeled diazomethane with benzene and brominated the cyclohepatriene product to form a labeled tropylium bromide, as shown below  

Then the tropylium bromide was subjected to a Grignard reaction and the product oxidized to give a labeled benzoic acid. Thus, one had 

The specific activity of the labeled benzoic acid was found to be one-seventh that of the initial labeled diazomethane, thus showing the equivalence of the seven carbon atoms of the tropylium ring. 

Another popular use of radiotracers in studying chemical reaction mechanisms is the study of molecular rearrangments. An example of this class of reactions that illustrates the use of radiotracers is the cyclization of w-phenoxyaceto-phenone (I) to 2-phenylbenzofuran (II). Two possible mechanisms are shown below  

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