Nickel hydrogen-deuterium exchange

Recently, other authors when studying the activation of hydrogen by nickel and nickel-copper catalysts in the hydrogen-deuterium exchange reaction concentrated for example only on the role of nickel in these alloys (56) or on a correlation between the true nickel concentration in the surface layer of an alloy, as stated by the Auger electron spectroscopy, and the catalytic activity (57). 

The choice of solvent can also be beneficial in another respect. This possibility was highlighted by the findings of Cioffi on the Raney Nickel catalyzed hydrogen-deuterium exchange of a model carbohydrate [l-0-methyl-/l-D-galactopyranoside] but under ultrasonic irradiation (Tab. 13.2) [43], Extensive deuteration at C-4 position occurred for a series of ethereal solvents, the C-3 position was deuterated by seven solvent systems and the C-2 position deuterated less extensively, also by seven solvent systems. For l,4-dioxane-D20 no labeling at the C-2 position occurred and for l,2-dimethoxyethane-D20 no C-3 labeling was observed. 

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. 

An interesting differentiation between reactions was observed on nickel. The hydrogen-deuterium exchange activity was not affected by a bombardment that altered the rate of ethylene hydrogenation by a factor of 100 (44). Evidently the defects responsible for the latter are... 

Effects of Radiation Quenching, Ion-Bombardment, and Annealing on Catalytic Activity of Pure Nickel and Platinum Surfaces. II. Hydrogenation of Ethylene (continued). Hydrogen-Deuterium Exchange f... 

Cioffi et al., who previously has studied the Raney Nickel catalyzed hydrogen-deuterium exchange of a model carbohydrate (l-O-methyl-yS-n-galactopyranoside) under ultrasonic irradiation, have extended their investigations into the microwave area [58]. Using a D20-tetrahydrofuran (THF) solvent mixture the above compound could rapidly (<10 min) be deuterated with 86% incorporation at one site and an even better incorporation (91%) for sucrose other sites were labeled less extensively. 

Fig 6 3 Plot of H against l/u for hydrogen-deuterium exchange on nickel catalyst (Dr pulse in pure H>)... 

These hydrocarbons are generated from the Raney nickel reduction of the sulfides, and the most abundant member of the sulfides is also the C23 compound. Finally, three deuterium-exchangeable hydrogen atoms were observed for the sulfone derivative of the C23 sulfide, Figure 11. The structure established for the C40 member of the series is ... 

The non-stereospecific process is probably identical with the one which causes the racemization of an optically active saturated hydrocarbon during deuterium exchange experiments over nickel (17). Apparently, the half-hydrogenated state postulated by Horiuti and Polanyi is able to exchange hydrogen atoms rapidly with its neighbors (18) and, in this process, intermediates are formed which allow for the racemization of an asymmetric center (17, 19). The intermediate for racemization must have a plane of symmetry at one of the tertiary carbon atoms. This may be a nonadsorbed double bond or possibly a free radical such as IV (17) ... 

A last case of sonication-sensitive catalytic reactions is the hydrogen-deuterium isotope exchange observed in carbohydrates in the presence of nickel or a nickel-aluminum alloy. A good selectivity is observed, probably due to the lower temperature allowed by sonication. However, a more complex origin can be responsible for these effects, since the sonicated alloy exhibits new features such as distinct catalytic centers, and elemental redistribution within the catalyst bulk, along with more classical consequences (surface cleaning, absence of surface area... 

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