Birch Reduction

Birch Reduction reduces aromatic rings to 1,4-dihydro derivatives using sodium and liquid ammonia in synthesis.

Purpose of Birch Reduction:

• Reduces aromatic rings (like benzene) to non-conjugated cyclohexadienes.
• Partial reduction – breaks aromaticity but doesn’t fully saturate the ring.

Reagents:

• Alkali metal (Na, Li, or K) in liquid ammonia (NH₃)
• Proton source (like ethanol or tert-butanol)

Mechanism Steps (for benzene):

1. Step 1: Electron Addition

   • Sodium donates an electron to the aromatic ring → radical anion.
     • C₆H₆  +  e⁻  →  [C₆H₆]⁻•

2. Step 2: Protonation

   • Proton source (ROH) protonates one carbon bearing the radical.
     • [C₆H₆]⁻•  +  ROH  →  C₆H₅H• (radical)

3. Step 3: Second Electron Transfer

   • Another electron is added → carbanion.
     • C₆H₅H•  +  e⁻  →  C₆H₅H⁻

4. Step 4: Final Protonation

   • Carbanion is protonated again.
   • 
   • Product: 1,4-cyclohexadiene derivative (with specific regioselectivity).

Example of Birch Reduction:

• • Benzene  →  1,4-cyclohexadiene (with Na/NH₃ and EtOH)
  • Toluene  →  1,4-cyclohexadiene derivative

Regioselectivity:

• Electron-withdrawing groups (e.g., COOH): reduction occurs at the opposite positions.
• Electron-donating groups (e.g., OCH₃): reduction occurs ortho and para to the group.

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