Alkenes, hydrogenation dissociative chemisorption

In the foregoing it has been discus.sed how a metal can dissociate H2. Fig. 3.6 explains the principle of catalysis with an example of the hydrogenation of ethylene, for which dissociative chemisorption of hydrogen is an elementary step in the catalytic cycle. The adsorption of alkenes, on the other hand, is non-dissociative. 

The mechanism of heterogenous hydrogenation involves (1) dissociative chemisorption of H2 on the catalyst, (2) coordination of the alkene to the surface of the catalyst, and (3) addition of the two hydrogen atoms to the activated 7t-bond in a syn-manner. 

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... 

Dissociative chemisorption occurs preferably with alkenes in which the aUyUc methyl group is highly activated (e. g., propene). Hydrogen abstraction gives an allyl radical, which can be bound as follows ... 

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