Hydrogen as a reducing agent catalytic hydrogenation

The catalysts used to make hydrogen react with double bonds are transition metals usually palladium (as in this example) or platinum, but sometimes nickel, rhodium, or ruthenium. We will talk about several different reductions in this section, but the mechanisms of all of them are similar, and very different from those involved in carbonyl reductions. 

Catalytic hydrogenation takes place on the surface of the metal. The metal must therefore be finely divided, and is usually dispersed on the surface of an inert support. This is what Pd/C means—finely divided palladium carried on a charcoal support. The first step is chemical absorption of hydrogen onto the metal surface, a process that results in breakage of the H—H bonds and distributes hydrogen atoms where they can react with the organic substrate. Now the alkene can also bond to the metal, and hydrogen can be transferred from the metal to the alkene. 

Palladium on charcoal (Pd/C) is usually 5-10% by mass Pd and 90-95% C. It is made by suspending charcoal powder in a PdCl2 solution, and then reducing the PdCl2 to Pd metal, usually with H gas, but sometimes with formaldehyde, HCHO (which is oxidized to formic acid, HCO2H). The palladium metal precipitates on to the charcoal which can be filtered off and dried. The fine Pd particles present maximum surface area to the reaction they catalyse and, while Pd is an expensive metal it is recyclable since the Pd/C is insoluble and can be recovered by filtration. 

Hydrogenation with a palladium or a piatinum cataiyst is the most common way of reducing alkenes. You may find our mechanism rather unsatisfactory, but it is hard to draw curly arrows for the reactions involved here. There is, however, pienty of evidence that the hydrogenation occurs like this, for example the alkene below reacts in such a way that the major product receives both hydrogen atoms on the same face of the molecule—just what we would expect from a reaction at a surface. 

Plants such as soya, rapeseed, cottonseed, and sunflower are useful sources of edible vegetable ollsi but these oils are unsuitable as butter substitutes because of their low melting points. Their low melting points relative to animal fats are largely due to cis double bonds that disrupt the packing of the alkyl chains in 

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