Deuterium tracer studies with

The mechanism for the reaction over alumina supported Co and Ni catalysts based on the deuterium tracer studies is believed to be one in which the adsorbed diene and adsorbed hydrogen are in dynamic equilibrium with a- and a- -bonded half hydrogenated species, resulting in the wide distribution of deuterium in the observed butanes (Fig. 9.2). 

Reaction with epoxides [1, 611, after line 2]. The reaction of epoxides of medium-size rings with lithium diethylamide is complicated. Thus m-cyclooctene oxide (I) gives mainly the bicyclic alcohol (2), and tra/i.s-cyclooctene oxide (5) gives mainly the epimeric bicyclic alcohol (6).38 Using deuterium tracer studies, Cope showed that the bicyclic alcohols (2) and (6) arise by loss of a hydrogen atom from an a-... 

Hydrogen as it occurs in nature is predominantly composed of atoms in which the nucleus is a single proton. In addition, terrestrial hydrogen contains about 0.0156% of deuterium atoms in which the nucleus also contains a neutron, and this is the reason for its variable atomic weight (p. 17). Addition of a second neutron induces instability and tritium is radioactive, emitting low-energy particles with a half-life of 12.33 y. Some characteristic properties of these 3 atoms are given in Table 3.1, and their implications for stable isotope studies, radioactive tracer studies, and nmr spectroscopy are obvious. 

More experimental data where the secondary KIE was larger than the EIE were subsequently published by Subramanian and Saunders (1984). The 2-arylethyl system was employed in these studies because other relevant data, such as the primary deuterium KIE, were available for this reaction. Special techniques were developed to determine the primary and the secondary tritium KIEs for this system. Three isotopically distinct elimination reactions (49-51) are possible for a 2-arylethyl derivative tracer labelled with tritium at the 2-position. 

Catalytic activity measurements and correlations with surface acidity have been obtained by numerous investigators. The reactions studied most frequently are cracking of cumene or normal paraffins and isomerization reactions both types of reactions proceed by carbonium ion mechanisms. Venuto et al. (219) investigated alkylation reactions over rare earth ion-exchanged X zeolite catalysts (REX). On the basis of product distributions, patterns of substrate reactivity, and deuterium tracer experiments, they concluded that zeolite-catalyzed alkylation proceeded via carbonium ion mechanisms. The reactions that occurred over REX catalysts such as alkylation of benzene/phenol with ethylene, isomerization of o-xylene, and isomerization of paraffins, resulted in product distribu-... 

The use of isotopic tracers has demonstrated that the selective oxidation of propylene proceeds via the formation of a symmetrical allyl species. Probably the most convincing evidence is presented by the isotopic tracer studies utilizing, 4C-labeled propylene and deuterated propylene. Adams and Jennings 14, 15) studied the oxidation of propylene at 450°C over bismuth molybdate and cuprous oxide catalysts. The reactant propylene was labeled with deuterium in various positions. They analyzed their results in terms of a kinetic isotope effect, which is defined by the probability of a deuterium atom being abstracted relative to that of a hydrogen atom. Letting z = kD/kH represent this relative discrimination probability, the reaction paths shown in Fig. 1 were found to be applicable to the oxidation of 1—C3He—3d and 1—QH —1 d. 

The unsaturated /i-keto ester 443 is converted to methylenecyclo[3.2.1]octane, 449, with 2 equiv. of Mn(OAC)3 and 1 equiv. of Cu(OAc)2510 (equation 265). Oxidation of 443 with 2 equiv. of Mn(OAc)3 in AcOH without Cu(OAc)2 provides alkene 449 in 14% only and alkanes 450a and 450b in 7% and 17% yields, respectively510. Oxidation of 443 with 2 equiv. of Mn(pic)3 and 1 equiv. of Cu(OAc)2 in AcOH provides none of alkene 449, 15% of alkane 450 and 69% of oligomeric material. To understand the above observations deuterium tracer and KIE studies have been undertaken511. 

Both deuterium and tritium have been used for tracer studies of hydrogen—that is, following the course of hydrogen through a chemical or biochemical reaction. However, the j3 ray given off from tritium is so very soft that its emission is difficult to detect with ordinary radiation counters (for which generally the instrument is placed near the sample) rather, the tritium-containing sample is often put -within the chamber of the instru-... 

The main uses of deuterium are in tracer studies to follow reaction paths and in kinetic studies to determine isotope effects. " A good discussion with appropriate references is in Comprehensive Inorganic Chemistry, Vol. 1, pp. 99-116. The use of deuterated solvents is widespread in proton nmr studies to avoid interference from solvent hydrogen atoms, and deuteriated compounds are also valuable in structural studies involving neutron diffraction techniques. 

A tracer study of the mechanism of the rearrangement of thiolsulphinate with acetic anhydride using 13C and lsO has been carried out by Oae and coworkers194. A deuterium isotope effect study of the mechanism of the reactions of singlet oxygen with allylic and vinylic sulphides,... 

Kaneko and Oki made studies with a deuterium tracer and found v to be between 1 and 2. Kaneko and Oki and Kaneko et al. also conducted experiments with a 0 tracer, finding an apparent v of approximately 2, for measurements both near and far removed from equilibrium. Oki et al. found that the exchange of 0 between CO2 and H2O was very fast (v approaching infinity) but that exchange between CO and CO2 was not as fast (v approximately 2). Mezaki summarized these and other data (Table 9). 

Among the tools used to study reaction mechanisms, the replacement of an atom by one of its isotopes has proven unique in its efficiency. In organic chemistry, which is mostly concerned with carbon and hydrogen containing compounds, replacing protium with deuterium has frequently been used, especially in tracer studies, to follow reaction paths and in kinetic studies and to determine isotope effects on reaction rates. In nature, hydrogen is essentially composed of atoms in which the nucleus is a single proton. However, it contains 0.0156% of deuterium, in which the nucleus also contains a neutron. A major source of deuterium is heavy water, D2O, which is prepared on industrial scale by the electrolytic enrichment of normal water. 

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. 

Molecules labeled with deuterium and/or tritium are useful for fundamental investigations in a variety of chemical fields, particularly spin decoupling in NMR,1 the measurement of coupling constants in ESR,2 mass spectrometry,3-4 and general reaction mechanism studies. Such tagged compounds are also valuable as tracers in biochemical and medical research. 

The biogenesis of nicotine has been studied in the plant with the aid of radioactive tracers. It was first shown that sodium formate is a precursor of the iV-methyl group of nicotine, but that methionine is a more immediate precursor since when these substances labeled with C are fed to the plant, the yield in radioactivity is greater with methionine than with formate (44). It was concluded that the methy-lation by methionine involved a true transmethylation and that the methyl group was not transferred via an intermediate oxidation to formate (44). This view found support in the results of an experiment involving the use of methionine with its methyl group doubly labeled with and deuterium. In this experiment, the ratio of the and... 

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