Alkenes, hydrogenation alkene exchange mechanism

Lest it be thought that the Horiuti-Polanyi scheme, or some modification of it, provides all the answers to the problems of alkene hydrogenation and exchange, it must be said that there is considerable evidence that the latter process may involve a quite different mechanism, involving the dissociative chemisorption of the alkene, as for example... 

Apparently, the exchange patterns can be explained qualitatively by reference to either structure for the adsorbed olefin, the eclipsed 1,2-diadsorbed alkane or the olefin tt complex. This argument should, of course, refer to the transition state for the formation of chemisorbed olefin from monoadsorbed alkane, the critical step in the a,)3 exchange mechanism however the revised argument would be much the same. Nevertheless we are provided with two alternative descriptions of the chemisorbed alkene under conditions closely related to those employed in hydrogenation studies. 

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. 

The hydrogenation mechanisms discussed above indicate the reversible formation of intermediates. The degree of reversibility depends on the nature of the catalyst and the alkene, and on reaction conditions. Some nonreducible internal alkenes undergo slow hydrogen exchange and isomerization, indicating that reversible steps can occur in these cases. The reversible formation of alkylmetal intermediates provides a ready explanation for the isomerization of alkenes in the presence of certain metal complexes (see Section 4.3.2). 

The early use of deuterium in place of hydrogen in the study of catalytic hydrogenation led to the recognition that the process was not simply the addition of H2 to the double bond. Horiuti and Polanyi proposed that both H2 and alkene (1) are bound to the catalyst surface and transformed to products by a sequence of elementary steps, which they represented as shown in Scheme 1, where an asterisk ( ) represents a vacant site on the catalyst.The last step, (d), is virtually irreversible under the usual hydrogenation conditions, but can be observed in the exchange reactions of D2 with alkanes. The mechanism accounts for the isomerization of an alkene if the reversal of step (c), which involves the formation of the alkyl intermediate (3), involves the abstraction of a hydrogen atom other than the one first added, and is coupled with the desorption of the alkene, (2) - (1). At present, the bond between the alkene and the metal often is represented as a ir-complex (4), as in equation (7). ... 

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