Reduction of Aldehydes and Ketones to Alcohols: Catalytic Hydrogenation

Aldehydes can be reduced to primary alcohols, and ketones can be reduced to secondary alcohols, using different reagents and reaction conditions. One of the oldest methods of reducing aldehydes and ketones is catalytic hydrogenation. The catalytic hydrogenation method adds hydrogen atoms across the carbon-oxygen π bond, reducing it to alcohol. This reaction is conducted in the presence of transition metal catalysts and hydrogen gas at high pressure.

One commonly used catalyst for catalytic hydrogenation is Raney Nickel saturated with hydrogen. It is prepared from the Nickel-Aluminum alloy. The Ni-Al alloys are treated with a strong base like sodium hydroxide. The aluminum reacts to generate hydrogen, which in turn saturates the nickel catalyst. Apart from the Raney nickel, other transition metals such as palladium and platinum are also used as catalysts for this reduction.

Compared to the reduction of olefins, the C=O reduction using this method requires more energy, thereby requiring higher temperature and pressure. Hence, this method is more popular for reducing olefins compared to carbonyls. Due to its ability to efficiently reduce olefins, this method cannot selectively reduce carbonyl groups in the presence of other multiple bonds. For instance, if a compound contains both C=O bond and C=C bond, both the bonds will get reduced during catalytic hydrogenation.