In organic synthesis, it’s essential to know how to reduce a nitro group (NO2) to an amino group (NH2). There are two primary methods for doing this: catalytic hydrogenation and metal-acid reductions.
Catalytic hydrogenation uses hydrogen gas in the presence of a catalyst like palladium in charcoal, platinum, or nickel. These conditions are considered neutral because they do not involve acid or base. An alternative approach is to use metals such as iron, zinc, or tin with hydrochloric acid under acidic conditions.
Reducing Carbonyls Connected to a Benzene Ring
Another crucial reaction in organic synthesis involves reducing a carbonyl group (C=O) attached to a benzene ring to a methylene group (CH2). There are three methods for this conversion: the Clemensen reduction, the Wolf-Kishner reduction, and catalytic hydrogenation.
The Clemensen reduction uses zinc and mercury as an amalgam in the presence of HCl under acidic conditions. In contrast, the Wolf-Kishner reduction employs hydrazine with potassium hydroxide and heat under basic conditions. Lastly, catalytic hydrogenation uses hydrogen gas in the presence of a metal catalyst like palladium in charcoal, platinum, or nickel under neutral conditions.
Example Reactions
Here are some example reactions demonstrating these reductions:
- Reduction of a nitro group without affecting an aldehyde carbonyl using tin and HCl under acidic conditions.
- Conversion of a carbonyl group to a CH2 group via the Wolf-Kishner reduction with hydrazine, potassium hydroxide, and heat under basic conditions.
- Reduction of a carbonyl group directly connected to a benzene ring using zinc and mercury in the presence of HCl under acidic conditions.
- Catalytic hydrogenation of a carbonyl group with an OH group present using platinum under neutral conditions.
