Deuterium tracer studies

More recently, Finlayson et al. (66) performed deuterium tracer studies using 2,2,4,4-tetramethylpentane as a model reactant, and proved that the bond-shift isomerization to 2,2,5-trimethylhexane may go via a monoad-sorbed intermediate on Pd (as well as on Pt, Ir, and Rh). The authors show that in the mechanisms of nondestructive reactions such as cyclization and bond-shift isomerization of alkanes, only one metal atom is required as the catalytic center. 

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-... 

In support of the 6-membered transition state mechanism (28) for MPV reductions depicted in Scheme III, deuterium tracer studies by von Doering and Ash-ner (33,34), and by Williams and coworkers (35) show that the source of hydride transferred to the carbonyl substrate is indeed the carbinol carbon. Later data from Moulton, et al. (34), in 1961 did not preclude the existence of a non-... 

Detailed kinetic studies, product studies, NMR spectroscopic analyses, LC-MS (liquid chromatography-mass spectrometry) chromatography, and Cl and deuterium tracer studies have shown that the homogeneous hydrolysis of 2-chloroethyl ethyl sulfide in water and in 1 1 acetone/water at concentrations above 10 M occurs by the mechanism in Scheme 10. °... 

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. 

It was detected by Urey, Brickwedde and Murphy in 1932. It occurs in all natural compounds of hydrogen including water, as well as in free hydrogen molecules at the ratio of about one part per 6,000 parts hydrogen. The principal application of deuterium is in tracer studies for measuring rates and kinetics of chemical reactions. It also is used in thermonuclear reactions and as a projectile in cyclotrons for bombardment of atomic nuclei to synthesize isotopes of several transuranium elements. Deuterium oxide, D2O, or heavy water is used as a neutron moderator in nuclear reactors. 

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. 

Deuterium tracer and isotope effect study of Mn(lll)-based oxidative free-radical cyclizations... 

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-... 

Deuterium isotope effect and tracer study of the mechanism of decomposition of the cationic ruthenium complexes... 

In the following text, we will present the data on catalytic hypophosphite oxidation on nickel using deuterium tracer and online electrochemical mass spectrometry studies of partial reactions and their mutual interaction as a function of electrode potential, the modeling of the catalyst surface state upon the oxidation of hypophosphite, isotopic gas composition during electroless Ni-P alloy... 

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). 

A more elegant and convincing demonstration of the ylid mechanism is accomplished by tracer studies. In the ylid mechanism, the trimethylamine produced contains one deuterium atom per molecule if the beta hydrogen atom is labelled in the substrate (47), whereas the E2 mechanism yields unlabelled material. Analysis by mass spectroscopy or nuclear magnetic resonance affords a convenient diagnosis of mechanism. Tracer studies confirm the absence of the ylid mechanism for the Hofmann reactions of a series of quaternary ammonium salts (51) , (52) . 

In the rigid 2,3-dihalogenonorbornane system, the only possible eliminating conformations in the substrate are planar syn and anti-clinal. Rate studies on the dibromo and dichloro substrates , the use of deuterium tracers "... 

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. 

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