Hydrogen-deuterium exchange, with hydrocarbons

These clusters are stabilized by the formation of surface complexes with naphthalene. The hydrogen-deuterium exchange in hydrocarbon, RH, takes place... 

The intermediate tricyclic ketones 495 and 496 have been transformed to the methoxy-substituted derivative 97284,285) latter ketone is subject to hydrogen-deuterium exchange only under basic conditions and appears to exist entirely in the keto form despite the ready formation of its anion and successful methylation on oxygen . In agreement with the aromatic nature of 490, the hydrocarbon undergoes electrophilic substitution reactions... 

Hydrogen-deuterium exchange in alkanes shows the same dependence on the composition of solvent, and concentration of mineral acid, of platinum( 11) and of chloride ions as does the exchange in aromatic hydrocarbons. The dependence of the exchange rate on solvent composition has been discussed in Section III,B,1. The rate is independent of mineral acid concentration, increases nonlinearly with increase in the concentration of platinum 11), and decreases nonlinearly with increase in the concentration of chloride ions (55). The nonlinear dependences are due to the solvolysis of the PtCL,2- ion ... 

Evidence that the presence of water is important in these reactions has been obtained by Hansford (82). Pretreatment of the catalyst with a stream of predried air at 500°C resulted in a marked decrease in the rate of cracking. Further, if deuterium oxide was substituted for the water in the catalyst a large percentage of the deuterium exchanged with hydrogen atoms contained in the hydrocarbon undergoing reaction. Hansford also pointed out that effective catalysts for cracking reactions are always prepared from one or more hydrous oxides. 

The rate of hydrogen exchange depends on the protolytic properties of both the solvent and the substrate. In fact there is a correspondence between the magnitude of the rate constants for deuterium exchange with ND3 and the conventional ionization constants of hydrocarbons which were used by Conant and Wheland (1932) and by McEwen (1936) to obtain the first quantitative estimates of the acidity of hydrocarbons. To do this, they determined the equilibrium of metallation of hydrocarbons by organo-alkali metal compounds. This reaction was described by Shorygin (1910) and is represented by the equation... 

When deuterium instead of hydrogen is used, reversal of the half-hydrogenated intermediate accounts for deuterium exchange in alkenes and the formation of saturated hydrocarbons with more than two deuterium atoms in the molecule18,52 (Scheme 11.1). 

The amides of alkali and alkaline-earth metals catalyse hydrogen exchange in hydrocarbons even in the absence of liquid ammonia. For example, the heterogeneous deuterium exchange of benzene and 2-methylbutene-l occurs with a considerable velocity on solid KND2 and Ca(ND2)2 at 70°. This gives rise to the isomerization of 2-methyl-butene-1 to 2-methylbutene-2 (Shatenshtein et al., 1958a). 

It would be interesting to carry out analogous experiments with unsaturated alicyclic hydrocarbons whose side-chain is separated from the ring by a tertiary carbon atom. It would also be desirable to measure the kinetics of hydrogen exchange for non-equivalent atoms in alkene molecules, and to compare the rates of deuterium exchange and isomerization, and also the rates of all these processes in hydrocarbons of similar composition with double and triple bonds, and so on. 

For any given catalytic reaction the active surface area is normally only a small fraction of the area of the active component (active phase). The term active sites is often applied to the sites effective for a particular heterogeneous catalytic reaction. The terms active site and active centre are often used as synonyms, but active centre may also be used to describe an ensemble of sites at which a catalytic reaction takes place. There is evidence that the centres required for some catalytic reactions are composed of a collection of several metal atoms (ensemble). This appears to be the case for such reactions as, for example, hydrogenolysis, hydrogenation of CO, and certain deuterium-exchange processes with hydrocarbons. 

Evidence from the kinetics of both olefin 27, 31) and acetylene 71) hydrogenation and from the exchange of cycloalkanes with deuterium 12) (in which 7r-allylic intermediates are formed) has shown almost unequivocally that molecular hydrogen can interact with adsorbed hydrocarbon species. When this is the case, addition takes place from above the axis of unsaturation. The importance of molecular hydrogen interaction, as a general phenomenon, has yet to be determined. However, the evidence is sufficiently strong to compel the consideration of steps such as... 

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