Equilibration of Cycloalkanes 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. 

Before attempting to summarise the debates on mechanisms, a little further background is needed. Cycloalkane exchange is faster than for linear and branched alkanes, and with metal films and powders it is frequently possible to work at subambient temperatures, even as low as 77 K. Deactivation by carbon deposition is however often a problem its removed by treatment with hydrogen shows 

The mechanisms that have been clearly identified as occurring in cyclopentane exchange are those responsible for forming the following products. 

Cyclopentane-d2 by a single operation of the ap mechanism or perhaps by dissociate adsorption to give the 1,2-diadsorbed molecule. 

Cyclopentanes-fi(6 to -d o by inversion of the molecule, followed by multiple exchange on the other side. 

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