Birch Reduction Mechanism (With EDG)

🌿 Birch Reduction – Overview

The Birch reduction is a partial reduction of aromatic rings using:

• Sodium (Na) or lithium (Li) metal
• In liquid ammonia (NH₃)
• With an alcohol (ROH) as a proton source

It converts benzene or substituted aromatics into 1,4-cyclohexadienes.

⚗️ Mechanism Steps

1. Electron transfer (1st reduction):
   Sodium (or lithium) donates an electron to the aromatic ring → forms a radical anion.
2. Protonation:
   The radical anion is protonated at the position of highest electron density by the alcohol.
3. Second electron transfer:
   Another electron is added to form a carbanion intermediate.
4. Final protonation:
   The carbanion is protonated again → gives the 1,4-cyclohexadiene product.

💡 Effect of Electron-Donating Groups (EDGs)

Examples of EDGs: –OCH₃, –OH, –NH₂, –CH₃

• EDGs donate electron density into the aromatic ring via resonance or induction.
• This increases electron density at the ortho and para positions.
• During reduction, the negative charge of the intermediate (radical anion/carbanion) avoids EDG-substituted carbons, because both are electron-rich.
• Therefore, protonation occurs at positions meta to the EDG.

🔍 Result

• Aromatic rings with electron-donating substituents give 1,4-cyclohexadienes where the double bonds are located near the EDG, and protons are added at meta positions.
• In contrast, electron-withdrawing groups (EWGs) lead to protonation ortho/para to the substituent.

🧪 Example

Anisole (methoxybenzene) + Na/NH₃/EtOH →
→ 1-methoxy-1,4-cyclohexadiene,
where reduction occurs at C-3 and C-5 (meta to –OCH₃).