Hydrogen availability selective, conjugated dienes

Hydrogenation studies of conjugated dienes show that the diene is more strongly adsorbed than the monoene produced on partial hydrogenation. This means that the monoolefin is displaced from the catalyst surface by the diene. Also, the double bond of the monoene does not isomerize in the presence of the diene.9 " Therefore, maximum selectivity is obtained when the reaction conditions are such that sufficient diene is available to the catalyst to displace the monoene product. Low hydrogen availability can enhance selectivity at higher conversions. 

The partial hydrogenation of conjugated dienes generally takes place more easily than the saturation of a monoalkene, which improves the prospect for selective hydrogenation of the conjugated species. This reaction is an example of Type III selectivity (Eqn. 15.40) (Chapter 5). Active catalysts can enhance organic substrate diffusion limitations and decrease the selectivity of monoalkene formation. Low hydrogen availability will minimize oversaturation. 

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