Hydrogen as a reducing agent: catalytic hydrogenation
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص534-535
2025-06-21
399
The simplest reducing agent is hydrogen itself, H2. Hydrogen can’t generally be used as a reducing agent for carbonyl compounds—it isn’t nucleophilic enough. However, it can act as a reducing agent for other, weaker, double and triple bonds, such as C=C, C=N, C≡C and C≡N. To do these reactions, a metal catalyst is required and the process is known as catalytic hydrogenation. The hydrogen is provided by a cylinder, perhaps via a balloon, or can be made by electrolysis and pumped with the substrate over the catalyst. In the example below, the alkene is reduced while the aldehyde remains untouched.
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.
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