Cycloalkanes deuterium exchange

Superacid DF-SbFs induces protium-deuterium exchange in isobutane [54b]. Strong acids (BF3, BFj-HjO, HF-BFj, CF3SO3H)catalyze carbonylation of alkanes including methane by carbon monoxide [54c], whereas sulfuric acid can induce carbonylation of iso- and cycloalkanes [54e,d]. In both cases, carboxylic acids are obtained. The elimination of molecular hydrogen from alkyl can occur (see a recent theoretical study of the Hz elimination from CzHs [54f]). 

Nickel has been reported to show behaviour similar to platinum [236], Further, in the reactions of cycloalkenes with deuterium, the product cycloalkanes are much more extensively exchanged over palladium than over nickel or platinum [236]. Such behaviour is not unexpected by comparison with the results obtained in the hydrogenation of linear alkenes (Sect. 3, p. 25). 

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. 

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... 

The isotopic exchange reaction between deuterium and (+)3-methyl-hexane on nickel and palladium catalysts at temperatures above 100° leads to racemization of the optically active hydrocarbon. In exchange between deuterium and cycloalkanes at temperatures between —50° and about 75°, a discontinuity separates the concentrations of C H D and. In cyclopentane at about 50°, for example, exchange... 

TABLE 6.7. Apparent Arrhenius Parameters E, In A), Rates at 423 K and Multiplicities M for the Exchange of Cycloalkanes with Deuterium over Unsupported Metals... 

To appreciate how important alkyl reversal is in the reactions being considered, the reactions of monosubstituted Ce cycles with deuterium are informative the results obtained with carbon-supported metals after 25% addition are given in Table 7.7 Remembering that deuterium numbers M of the cycloalkane greater than two mean more hydrogen exchange than cycloalkene exchange, and vice versa, the results are broadly in line with the characteristics of the three metals as exposed in the earlier sections. A certain amount of alkyl reversal must occur in all cases, but alkene desorption is only important with palladium. With Pt/C... 

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