Organic Chemistry: Reduction Reactions of Benzene

This note covers two unique reduction reactions affecting the pi electrons in a benzene ring: catalytic hydrogenation and Birch reduction.

The pi electrons in benzene are more stable than those in alkenes or alkynes.
Standard catalytic hydrogenation conditions (like those used for alkenes and alkynes) do not work on benzene.
To reduce benzene to cyclohexane using catalytic hydrogenation, harsh conditions are required:
- High pressure hydrogen (100 atmospheres or 1500 psi)
- Nickel catalyst
- Elevated temperature (150 degrees Celsius)

This reaction is less commonly covered in organic chemistry courses than catalytic hydrogenation.
Birch reduction uses sodium metal in methanol with ammonia.
It converts a benzene ring (aromatic, six pi electrons) to a non-aromatic ring with two sp3 carbons (four pi electrons).
The reaction proceeds through a radical anion intermediate.
- Electron-withdrawing groups stabilize the intermediate.
  - Reduction occurs adjacent to electron-withdrawing groups.
- Electron-donating groups destabilize the intermediate.
  - Reduction occurs NOT adjacent to electron-donating groups.
Understanding the influence of electron-withdrawing and donating groups on the intermediate is key to predicting the product.