Cycloalkenes, hydrogenation pressure

Carbonaceous residues could act as bridges for top-side addition to adsorbed cycloalkenes, or in solution adsorbed solvent molecules could perform the same role. (6) Finally - a possibility not previously considered, but one which follows logically from (5) - a hydrogen or deuterium atom from an alkyl radical may add top-side to a chemisorbed alkene as one step in the alkyl-alkene-alkyl chain shown in process 7.K, or by alkyl disproportionation. This is most likely to happen when species are packed closely together, i.e. when they are small or at low hydrogen pressure. 

The stereochemistry of hydrogenation of 1,2-dimethylcyclQhexene and 1,2-dimethylcyclopentene is instructive. Each of these substances would be expected to yield only the ci5-l,2-dimethylcycloalkane via cis addition. Both cis and trans isomers, however, are formed from either of these two cycloalkenes when hydrogenated in the liquid phase (acetic acid) over reduced platinum oxide—one of the more stereoselective catalysts [57, 58). The ratio of isomers which is produced is a function of the pressure of hydrogen, the proportion of cis increasing with increasing pressure (Fig. 5). This fact implies that the trans isomer is formed via a... 

Although the transition state for the exchange reaction may be described as the critical complex for the conversion of the half-hydrogenated state to either a jr-complexed olefin or an eclipsed vicinal diadsorbed alkane, the stereochemistry of hydrogenation of cycloalkenes on platinum at low pressures can be understood if the transition state has a virtually saturated structure. 

Isolated double bonds in alkenes and cycloalkenes are best reduced by catalytic hydrogenation. Addition of hydrogen is very easy and takes place at room temperature and atmospheric pressure over almost any noble metal catalyst, over Raney nickel, and over nickel catalysts prepared in special ways such as P-1 nickel [13] or complex catalysts, referred to as Nic [49]. 

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