21.7: Reduction of Carbonyl Compounds and Acid Chlorides Through Catalytic Hydrogenation
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Another way to reduce carbonyl groups and acid chlorides is through the catalytic addition of hydrogen. Just like the C=C bond, the C=O bond is capable of adding one mole of hydrogen. The catalyst typically used to accomplish this is called Raney Nickel.
If there are any C=C bonds present in the molecule, obviously they will also take up hydrogen. If selective reduction of the carbonyl group is desired, use NaBH4 instead.
As with the case of hydride reductions, the above reactions also go through the aldehyde stage, but cannot stop due to the high reactivity of the H2 /catalyst mixture. However, just as was the case in the addition of hydrogen to triple bonds, the process can be stopped at the aldehyde stage by the use of a reduced reactivity version of the H2 /catalyst mixture. This is accomplished by the addition of a “poison,” just as it was done with alkynes to stop at the alkene stage. It turns out that Lindlar’s catalyst works in this case as well.
"Poisoned" catalysts for hydrogenation
Pd / BaSO4 / S Pd / BaSO4 / quinoline (Lindlar's catalyst)