Isotope tracer experiments using deuterium

The calculated values are those obtained by assuming the ratedetermining step to be abstraction of an allylic hydrogen with an isotope effect equal to the discrimination effect obtained from the retention of deuterium in the products. The observed kinetic isotope effects show clearly that this first abstraction is the slow step. The good agreement with the discrimination values gives excellent confirmation for the stepwise mechanistic scheme proposed. In the tracer experiments using deuterium it was found that appreciable propylene isomerization occurred unless ammonia was present. The oxidation rate was unaffected by the ammonia. This was taken to indicate that the initial oxidative abstraction reaction had no carbonium ion characteristic. However, this conclusion does not apply in the case of cuprous oxide, where ammonia very severely inhibited the oxidation reaction. 

A quite different use of isotopes in mechanistic studies involves their use as labels for ascertaining the location of a given atom involved in a reaction. As in kinetic experiments, the isotopic substitution will not qualitatively affect the course of the reaction. The nuclei most commonly used for isotopic tracer experiments in organic chemistry are deuterium, tritium, and the and isotopes of carbon. There are several means of locating isotopic labels. Deuterium can frequently be located by analysis of NMR spectra. In contrast to the normal isotope, deuterium... 

Solvent effects on, and products from, reaction of styrene with ethylene in the presence of di-)ti-chloro-dichlorobis(styrene)dipalladium(n), [Pd-(Ph CH—CH2)Cl2]2, indicate a mechanism similar to (i)->(iv) above, with the addition of a preliminary equilibrium between the dimer and solvated monomers. The mechanism of reaction of styrene with vinyl compounds, catalysed by the same chloride-bridged dipalladium complex, has been studied using isotopic tracer (H, D) experiments. Palladium-acetate-catalysed reaction of styrene with benzene, also investigated using deuterium tracer experiments, involves no hydride shift, in contrast to the rather closely related Wacker process. The importance of intermediates with palladium-carbon n-bonds in palladium(ii)-catalysed alkylation and arylation of alkenes has been demonstrated. 

The differential method has also been used to determine deuterium isotope effects in the formation of the 2,4-dinitrophenylhydrazones of acetophenone-mef l-dg and other deuterated ketones (Baaen et ah, 1964), using carbon-14 as the tracer. The procedure requires that the isotope effects caused by the carbon-14 itself be known, and these were determined in separate experiments. Known mixtures of deuterated and undeuterated species were then prepared—one of which was always labeled with carbon-14— and the rate of change of carbon-14 content as a function of fraction of reaction was determined by removing small aliquots of reaction product at selected known intervals (or fractions f) of reaction. The data were then fitted to equations (35) or (36) by means of linear or non-linear least-squares codes, respectively, with an IBM 7090 computer. Some typical experimental results are given in Table 7. 

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