Deuterium with ethylene

Fig. 17. Deuteroalkane distributions observed in the reactions of deuterium with ethylene at 54°C (0)> propene at 73°C ( ) and but-l-ene at 67°C ( ) over platinum-alumina [103],...

The main difficulty in obtaining the vinyl radical is that the species easily loses the hydrogen atom and is converted into acetylene. Nevertheless, a very low concentration of the radical H2C=CH has been achieved (Shepherd et al., 1988) by vacuum UV photolysis of ethylene frozen in an argon matrix, and a Fourier transform IR study of this intermediate has been carried out. A variety of and deuterium-substituted ethylene parent molecules were used to form various isotopomers of vinyl radical. On the basis of its isotopic behaviour and by comparison with ab initio... 

At least for ethylene hydrogenation, catalysis appears to be simpler over oxides than over metals. Even if we were to assume that Eqs. (1) and (2) told the whole story, this would be true. In these terms over oxides the hydrocarbon surface species in the addition of deuterium to ethylene would be limited to C2H4 and C2H4D, whereas over metals a multiplicity of species of the form CzH D and CsHs-jD, would be expected. Adsorption (18) and IR studies (19) reveal that even with ethylene alone, metals are complex. When a metal surface is exposed to ethylene, selfhydrogenation and dimerization occur. These are surface reactions, not catalysis in other words, the extent of these reactions is determined by the amount of surface available as a reactant. The over-all result is that a metal surface exposed to an olefin forms a variety of carbonaceous species of variable stoichiometry. The presence of this variety of relatively inert species confounds attempts to use physical techniques such as IR to char-... 

Over zinc oxide it is clear that only a limited number of sites are capable of type I hydrogen adsorption. This adsorption on a Zn—O pair site is rapid with a half-time of less than 1 min hence, it is fast enough so that H2-D2 equilibration (half-time 8 min) can readily occur via type I adsorption. If the active sites were clustered, one might expect the reaction of ethylene with H2-D2 mixtures to yield results similar to those obtained for the corresponding reaction with butyne-2 over palladium That is, despite the clean dideutero addition of deuterium to ethylene, the eth-... 

Hydrogen is chemisorbed on a surface partially covered with ethylene and reacts rapidly with the ethylene at room temperature. This is not consistent with observations that hydrogen-deuterium exchange on nickel is strongly inhibited by the presence of chemisorbed ethylene. The latter effect may be due to fragmentation of ethylene which occurs only at higher temperatures. 

In connection with the interpretation of these trends it should be noted that in some reactions (e.g., ethylene hydrogenation) the activation energy remains substantially constant and the frequency factor changes as the metal is varied, while in other reactions (e.g., deuterium-ammonia exchange) the reverse is the case. In the exchange of deuterium with saturated hydrocarbons, a compensation effect (Cremer, 128) has been noted. The significance of these different patterns is not clear. 

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-... 

Similar results were obtained with ethylene but not with ethane. Apparently the catalyst tends to produce deuterium atoms and to open up the double bond so that a deuterium atom can attach to one of the carbon atoms. Either exchange or addition may follow. 

In principle both mfcnnolecular reactions between 8.3 and external water, and intramolecular reactions between coordinated ethylene and coordinated water could lead to the formation of 8.4. On the basis of experiments with deuterium-labeled ethylene followed by stereochemical analysis of the product, it appears that the /nfmnolccular reaction with solvent water molecule is the pathway that is followed. This is shown in Fig. 8.3. Note, however, that the D-labeling results are valid only for a specific set of conditions. These conditions are not the same as in the industrial process. Intramolecular attack by... 

The deuterium isotope effect on the rate of vinylic mercaptan formation was examined with ethylene, using the mercury arc and Cd—R sources. A decrease in the order of 8% appeared in the vm/Res ratio, which hardly exceeds the error of the experiments. 

These experimental secondary deuterium KIEs observed in Diels-Alder reactions have been compared with the respective theoretical KIEs for the stepwise mechanism involving a diradical intermediate (equation 88a) and for concerted synchronous and asynchronous mechanisms (equation 88b) for the Diels-Alder reaction of butadiene with ethylene . ... 

The probability of reductive adsorption of hydrogen at relatively low temperatures is sufficiently problematic that it would be desirable to reinvestigate reactions between deuterium and ethylene on chromias of differing temperatures of activation and with a mass spectrometer of higher sensitivity than that employed in the earlier work. For the moment, the exact mechanism of formation of alkane-do in hydrogenation of olefins with deuterium on chromias activated at lower temperatures must remain uncertain. 

The reactions between one volume of ethylene and two volumes of deuterium were studied with ethylene in the liquid phase at —78° and 400-800 psi over nickel on kieselguhr, palladium on charcoal, and palladium on silica-gel catalysts. The results as shown in Table I indicate an extensive redistribution reaction taking place among the deuteroethanes and no... 

When deuterium and ethylene are bi-ought together in contact with a nickel catalyst, the exchange reaction... 

Wheeler began his catalytic career at an unusual time and place - in the early 1930 s at Princeton (B.S.,M.S. and Ph.D.). Princeton was at that time famous as the exciting U.S. center for catalysis. The concept of active sites had been proposed by Taylor and were discussed vigorously. Deuterium had just been discovered by Urey at Columbia. Wheeler published a remarkable series of papers with Eyring, Pease, and others on the relative rates of combination of H2 and D2 with ethylene and acetylene as well... 

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