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Master Organic Chemistry Reaction Guide

Birch Reduction of Aromatic Rings

Description: The Birch reduction is the reduction of an aromatic ring with sodium (or potassium) metal in ammonia (NH₃) in the presence of an alcohol. The result is a 1,4-cyclohexadiene.

description of the birch reduction of aromatics to 1 4 dienes

Notes: Don’t confuse this with NaNH₂/NH₃, which is formation of a benzyne.

The substitution pattern of 1,4-cyclohexadiene product depends on whether the ring is attached to an electron-withdrawing group (EWG) or an electron donating group (EDG).

Examples:

-examples of the birch reduction of aromatics to give 1 4 dienes

Notes: The presence of an electron-withdrawing group tends to stabilize an anion directly adjacent to it, resulting in patterns like those seen in examples 1, 3, and 4.

With an electron-donating group (such as OCH3) the intermediate anion is formed as far away as possible, resulting in the formation of an enol ether in conjugation with the new double bond.

For more on this, see: Birch Reduction of Aromatic Rings

Mechanism:

The role of ammonia as solvent is to dissolve the sodium metal, which ends up forming a deep blue solution. The active reagent in the Birch reduction is a free electron, solvated by NH₃, which is responsible for the strong color.

formation of electride salt in the Birch

The first step is addition of the electron to the aromatic ring (Step 1) leading to the formation of a “radical anion”. In the next step, the carbanion is protonated by the alcohol (Step 2, arrows A and B) forming a new C-H bond and leaving behind a free radical. The radical is reduced to another carbanion (Step 3), which is then protonated again (Step 4, arrows C and D) giving the 1,4 cyclohexadiene product.

reduction of aromatic ring with sodium metal and alcohol