Zinc-hydrogen exchange reactions

Alcohols also undergo a timilar hydrogen-exchange reaction with ethylene as the acceptor to give aldehydes or ketones in yields of 40-75 per cent. The reaction takes place at atmospheric (nessure, at a temperature of about 280°C, while using a copfter-zinc-mickel-barium chromate as the catalyst. 

This enzyme [EC 1.2.99.2], also known as acetyl-CoA synthase, catalyzes the reaction of carbon monoxide with water and an acceptor to produce carbon dioxide and the reduced acceptor. The cofactors of this enzyme include nickel and zinc ions as well as non-heme iron. Methyl viologen can act as the acceptor substrate. The enzyme is isolated from Clostridium sp. Interestingly, it also catalyzes an exchange reaction of carbon between Cl of acetyl-CoA and carbon monoxide. The protein participates in the synthesis of acetyl-CoA from carbon dioxide and hydrogen in the organisms. 

Because zinc oxide is a relatively well-understood oxide semiconductor, we shall first review its properties as a hydrogenation catalyst in the catalytic hydrogen-deuterium exchange reaction. Since the latter essentially measures the rate of reversible chemisorption of hydrogen at equilibrium, data on the hydrogen chemisorption will be included in this survey. Any theory of hydrogen chemisorption on zinc oxide must explain all the following well-established facts. 

Fig. 3. Relation between frequency factors and heats of activation for the hydrogen-deuterium exchange reaction on zinc oxide catalysts (ref. 28a).

In this connection, the dependence of the rate of the hydrogen-deuterium exchange reaction catalyzed by zinc oxide, upon the concentration of a foreign oxide in ZnO is interesting. This system was investigated by Molinari and Parravano (94). As shown in Fig. 16, the rate of... 

An example for the use of the boron-zinc exchange reaction for copper-mediated SN2 -substitutions of allylic electrophiles is the hydroboration of nitroolefin 130 with diethylborane, followed by successive transmetallation of the borane 131 with diethylzinc and CuCN-2LiCl, and final trapping with allyl bromide to give the product 133 with 83% yield over four steps (Scheme 34).34,34a This transformation again demonstrates the tolerance of the method towards functional groups and acidic hydrogen atoms. 

In considering the properties of the solid surface and its influence on the chemistry of the reactants, I should like to recall to your attention papers by Harrison and McDowell (9) which merit, I believe, a measure of careful consideration. The authors were principally concerned with a detailed and quantitative examination of the phenomenon published in 1941 by Turkevich and Selwood. These authors had found that a mixture of zinc oxide and a,a-diphenyl- 3-picrylhydrazyl was much more powerfully a converter of para- to orthohydrogen than would be concluded on the basis of the mixture law and their separate activities in the conversion process. This phenomenon can be rationalized on the basis of concepts developed by Wigner. The more recent paper of Harrison and McDowell demonstrates, however, that, whereas neither the hydrazyl nor zinc oxide has any marked ability to produce the hydrogen-deuterium exchange reaction at 77° K, the reaction proceeds on the mixture at a rapid and reproducible rate, 2.4 times faster than the parahydrogen conversion on the mixture at the same temperature ( ) and 81 times faster than it would have occurred on the zinc oxide constituent. 

The great advantage of the study of exchange reactions of isotopic molecules on catalysts is that only one molecular species is involved both as reactants and products. One is freed from the restrictions imposed with two reactants where the displacement of one reactant by another or by a reaction product must steadily be taken into consideration. The catalysts which reveal heterogeneity by desorption-readsorption studies should show the same variation in activation energy of reaction with temperature that Taylor and Smith found with zinc oxide in the hydrogen-deuterium reaction. Research in this direction is under way. 

Platinum is an inert metal and the rate of hydrogen evolution is orders of magnitude greater than that on metallic zinc (exchange cmrent density for the hydrogen evolution reaction in IN HCl is 10 Acm on Pt and 10 Acm on Zn in H2SO4). 

Substitution or replacement reactions occur when a component of a chemical compound is replaced by something else. For example, zinc displaces hydrogen in Reaction 5.4.1. A double replacement or metathesis reaction occurs when there is a two-way exchange of ions between compounds. This happens, for example, when a solution of sulfuric acid reacts with a solution of barium hydroxide to yield solid barium sulfate and water ... 

These considerations show the essentially thermodynamic nature of and it follows that only those metals that form reversible -i-ze = A/systems, and that are immersed in solutions containing their cations, take up potentials that conform to the thermodynamic Nernst equation. It is evident, therefore, that the e.m.f. series of metals has little relevance in relation to the actual potential of a metal in a practical environment, and although metals such as silver, mercury, copper, tin, cadmium, zinc, etc. when immersed in solutions of their cations do form reversible systems, they are unlikely to be in contact with environments containing unit activities of their cations. Furthermore, although silver when immersed in a solution of Ag ions will take up the reversible potential of the Ag /Ag equilibrium, similar considerations do not apply to the NaVNa equilibrium since in this case the sodium will react with the water with the evolution of hydrogen gas, i.e. two exchange processes will occur, resulting in an extreme case of a corrosion reaction. 

Thus, Kohn and Taylor (40) point out that the y irradiation of zinc oxide which speeds up the reaction of hydrogen-deuterium exchange lowers the magnitude of the effect when a donor impurity is introduced into the specimen. 

Freund (44) studied the influence of ultraviolet light on the catalytic activity of zinc oxide in relation to the reaction of hydrogen-deuterium exchange. The author noted that the photocatalytic effect was positive and that it decreased with rising temperature. 

The complexity of the adsorption process, in particular its duality as illustrated above as well as in more recent data of Wicke (12), also shows in the irregular behavior of zinc oxide as a catalyst for the hydrogen-deuterium exchange (13). Thus this reaction proceeds at measurable rates at temperature as low as 14O K., indicating that at least part of the low-temperature adsorption is of the dissociating type. The apparent activation energy at low temperatures is low, but in the temperature... 

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