Exchange of alkanes with deuterium

It will not have escaped notice that adsorbed species implicated in the exchange of alkanes with deuterium are formally the same as those invoked in the hydrogenation of alkenes indeed the reiteration of the alkyl-alkene transformation (process 6.J) held responsible for multiple exchange in linear and branched alkanes, and designated the afi exchange mechanism, is on the face it of identical with the old and well-tried Horiuti-Polanyi mechanism for alkene hydrogenation. This will be discussed further in the next chapter (sections 7.1 and 7.21), but briefly it supposes the sequential addition of two hydrogen atoms to some adsorbed form of the alkene, e.g. 

In view of the proven multistep catalytic process, transport of the reacting species from one type of site to another before desorption as an alkane, cycloalkane, or benzene seems necessary. Since desorbed olefin plays a significant role in exchange of cycloalkanes with deuterium on Pd films even at ambient temperatures, olefins and even dienes could be responsible in the transport steps. It is useful to recall that this is, in fact, the basis of the classic theory of dualfunctional catalysis. 

To the best of our knowledge, there are no published data on the reactions of alkanes with H2 over palladium single crystals. Deuterium exchange reactions with various alkanes over palladium films were extensively investigated by Kemball et al. (39-42). Because those results were already discussed in several excellent review articles (43-46), we do not intend to dwell upon analyzing all the data at greater length. We only specify here the basic characteristics of Pd films exhibited in D2/alkane reactions ... 

TABLE 6A. Apparent Arrhenius Parameters (E, In A), Rates at 423 K and Multiplicities (M) for the Exchange of Higher Linear Alkanes with Deuterium... 

G. A. Olah, Y. Halpem, J. Shen, Y. K. Mo, Electrophilic reactions at single bonds. XII. Hydrogen-deuterium exchange, protolysis (deuterolysis), and oligocondensation of alkanes with superacids, J. Am. Chem. Soc., 1973, 95, 4960-4969. 

Much of the work on the catalytic activation of C—H bonds in alkanes follows from the important observation of Garnett and Hodges (21) that, in the presence of the PtCl42- ion, aromatic compounds exchange hydrogen with deuterium in the D20—CH3C02D solvent. Temperatures in the range 80°-100°C are used and the solvent also contains a mineral acid to stabilize the platinum(II) from disproportionation ... 

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 ... 

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