Tracer experiments, deuterium

The solid base catalysed aldol condensation of acetone was performed over a CsOH/Si02 catalyst using a H2 carrier gas. The products observed were diacetone alcohol, mesityl oxide, phorone, iso-phorone and the hydrogenated product, methyl isobutyl ketone. Deuterium tracer experiments were performed to gain an insight into the reaction mechanism. A mechanism is proposed. 

The deuterium tracer experiments of Broida and Gaydon indicated that CH does not come from acetylene itself, a peroxide, or a polymer, so they suggested... 

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-... 

Chromium. The mechanism of hydrogenation at this rarely studied centre has been probed by product analyses and deuterium tracer experiments. The catalysts used were Cr(arene)(CO)3, which selectively catalyse 1,4-addition to diolefins. Key intermediates are Cr(H)2(CO)3 and Cr(H)2(CO)3-(diene). ... 

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. 

Subsequently, hydrogen co-ordinates to the chromium prior to transfer to, and thus reduction of, the diene. Some preliminary observations on photochemical catalytic hydrogenation of conjugated dienes in the presence of Cr(CO)e or of Cr(CO)4(nor) have been published. Deuterium tracer experiments have shown that hydrogen (deuterium) transfer from alkyl-chromium compounds to dienes takes place intermolecularly, from the -position of the alkyl group. ... 

Platinum. Rate laws have been determined for homogeneous hydrogenation of ethylene and acetylene in the presence of platinum(n)-tin(ii) catalysts. This kinetic evidence and ancillary results from deuterium tracer experiments suggest a mechanism in which the molecular hydrogen is split heterolytically to give a Pt—H bond plus H+ ethylene or acetylene then... 

The active sites of D/iM and PHM contain two catalytically essential copper ions. EPR spectra of the oxidized forms of these proteins are typical for Type 2 copper with N, O donor ligands. " The copper centers are not coupled and are at least 5 A apart.Note the stereochemistry of hydroxylation in equations (4) and (5) both reactions proceed with retention. 0 tracer experiments for the enzymes provided direct evidence for the incorporation of molecular oxygen into the substrate, and both exhibit large deuterium kinetic isotope effects. In both cases, ascorbate is the physiological reductant. These biochemical similarities are confirmed by sequence comparisons, which pinpoint a conserved catalytic domain. The copper ligands of both the oxidized or reduced states of D/3M and PHM are conserved the H-X-L (L = His or Met) motif is a common copperbinding sequence. 

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). 

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... 

Several postulated combinations of numbers of electrons and numbers of pathways have been proposed for activated states in which redox centers exchange electrons. Structures of such states are deduced from experimental evidence such as rate laws, product analyses, isotopic tracer experiments, and deuterium isotope effects. Let... 

Solid acids Over Bronsted (protonic) solid acids, the reaction intermediate is a sec-butyl cations formed by the addition of a proton from the solid surface to butene. Hence, when solid acids are deuterated the deuterium of the catalyst is incorporated into both reactant and isomerized butenes. The same intermediate is often involved in the case of Lewis solid acids. In the latter case, protonic acid is induced by the reaction of butene on the Lewis acid site. For example, CH3CH = CH2 + L CH3CH — GHz — L (L Lewis site), where H at CH3 or CH2L acts as acidic proton. The intermediate is illustrated in Fig. 4.1 A. Fig. 4. IB shows the stereochemical reaction scheme for the case in which the deuterium atom (Da) is attached to cw-2-butene from below. If the Hb atom is removed downward from 1 or 2, the intermediate is transformed to (ranf-2-butene. The removal of D can produce tracer experiments based on this model reveals detailed information on the dynamic behavior of the intermediate. ... 

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