Birch Reduction: Mechanism & Regioselectivity | Chemca

Organic Chemistry

Birch Reduction: Mechanism & Regioselectivity

By Chemca Editorial Team • Last Updated: January 2026 • 10 min read

The Birch Reduction is a reaction used to convert aromatic rings into 1,4-Cyclohexadienes (non-conjugated dienes). It uses alkali metals (Sodium or Lithium) dissolved in liquid ammonia in the presence of an alcohol (proton source).

1. General Reaction

Benzene is reduced to 1,4-cyclohexadiene. Note that the aromaticity is lost, and the double bonds are isolated (not conjugated).

$$ \text{Benzene} + 2Na + 2ROH \xrightarrow{\text{Liquid } NH_3} \text{1,4-Cyclohexadiene} + 2RO^-Na^+ $$

Conditions:

Reagents: Na or Li metal + Liquid Ammonia ($NH_3$).
Proton Source: Alcohol (Ethanol or t-Butanol).
Mechanism: Dissolving Metal Reduction (Radical Anion Mechanism).

2. Detailed Mechanism

The reaction involves the transfer of electrons from the metal to the aromatic ring.

Step 1: Electron Transfer

Sodium donates an electron to the benzene ring, forming a Radical Anion.

$$ Na \rightarrow Na^+ + e^- $$ $$ C_6H_6 + e^- \rightarrow [C_6H_6]^{\bullet -} \text{ (Radical Anion)} $$

Step 2: Protonation

The radical anion accepts a proton from the alcohol ($ROH$) to form a neutral radical.

Step 3: Second Electron Transfer & Protonation

Another electron is added to form an anion, which is then protonated again to yield the final 1,4-diene.

3. Regioselectivity (Crucial for Exams)

The position of the double bonds in substituted benzenes depends on the nature of the substituent.

Case A: Electron Donating Groups (EDG)

Examples: $-R$ (Alkyl), $-OR$ (Alkoxy), $-NH_2$.
Outcome: The substituent stays on the Double Bond (reduction occurs at ortho and meta positions).
Reason: The electron density is highest at the substituted carbon, so the incoming electron prefers other positions.

Case B: Electron Withdrawing Groups (EWG)

Examples: $-COOH$, $-CONH_2$.
Outcome: The substituent ends up on the Saturated Carbon (reduction occurs at ipso and para positions).
Reason: The EWG stabilizes the negative charge at the ipso position.

4. Examples

A. Reduction of Anisole (EDG)

Product: 1-Methoxy-1,4-cyclohexadiene.

The $OCH_3$ group is attached to a double-bonded carbon.

B. Reduction of Benzoic Acid (EWG)

Product: 2,5-Cyclohexadiene-1-carboxylic acid.

The $COOH$ group is attached to a saturated (sp3) carbon.

5. Comparison

Substituent Type	Examples	Product Structure
EDG	$-CH_3, -OCH_3$	Substituent on $C=C$
EWG	$-COOH, -C(O)R$	Substituent on $-CH_2-$ (sp3)

Birch Reduction Quiz

Test your concepts on Regioselectivity. 10 MCQs with explanations.

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