Reaction Products and the Single-Atom Site Model

Reaction Products and the Single-Atom Site Model 

On the other hand, 1,4-addition must involve a diadsorbed sp)ecies such as 64. The surface atoms can be MH, MH, or 3mH2, but only one can be the MH2 type. The transfer of one hydrogen atom from each surface site gives the adsorbed olefin with its stereochemistry determined by the mode of adsorption of the diene. The s-cis form of butadiene (65) is about 2.3 kcal/mol less stable than the s-trans form (66). This corresponds to about a 30 70 ratio, which agrees quite well with the observed cis/trans-2-butene ratios formed over Type B catalysts. The likelihood of 1,4-addition is increased when larger catalyst particles are present because more neighboring edge and comer atoms are also present. 

With a palladium catalyst in the hydrogenation of butadiene, the reaction mechanism probably takes place by way of a 7t-allyl intermediate such as 67 or 68. The steric constraints of the cis form, 68, are such that the trans species, 67, is considerably more stable. The low cis/tram 2-butene ratios observed with palladium are a consequence of the difference in energy between these two adsorbed species. 

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