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21.7: Reduction of Carbonyl Compounds and Acid Chlorides Through Catalytic Hydrogenation

  • Page ID
    216886
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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.

    clipboard_e4612c4d86bb063eb49e7b4dae741efe5.png

    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.

    clipboard_e683b70c60452d7f1d0ec755c924151b9.png

    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)

    EXAMPLE:

    clipboard_ed688d61e757eeb35e2d3e9c96a2999ad.png