Ethylene hydrogenation steric factor

The methylvinyl radical (IV) can abstract a hydrogen atom from a feed or product molecule to form propylene or it can lose a hydrogen atom to form allene or propadiene as products. For the 2-butenes, steric factors inhibit methyl radical addition thus C5 products are formed to a far lesser extent than from 1-butene. While ethylene may be formed by a sequential decomposition of propylene, this cannot be the only path for its formation, as the yield of ethylene in the high conversion region increases about twice as rapidly as does the methane yield. An additional source of ethylene is the symmetrical cleavage of butadiene to vinyl radicals. 

In the reaction of m-substituted substrates, carbonylation takes place exclusively at the less-congested position (i.e., the 6-position), irrespective of the electronic nature of the substituents, indicating that the regioselectivity is determined by steric factors. In the case of 2-naphthylpyridine, carbonylation takes place selectively at the 3-position, presumably because of steric hindrance by the peri-hydrogen on the naphthalene ring. In sharp contrast to the a and /3 carbonylations described above, this reaction is restricted to ethylene as the olefin partner. The use of 1-hexene resulted in no reaction. 

Twigg and Rideal (50) indicated that the spacing of catalyst atoms would be an important factor in determining the activity of metal surfaces in ethylene hydrogenation, and predicted both that different crystal faces of the same metal would show different catalytic activity, and that variation of activity among metals might be attributed to the steric factor. These predictions were verified by the work of Beeck,... 

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