Dissolving-metal reduction
Birch reduction of benzene
How the Birch reduction of benzene works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Na (or Li), liquid NH₃, EtOH (or t-BuOH). Conditions: −33 °C, THF cosolvent. Bonds formed: 2 C–H; 1,4-cyclohexadiene; bonds broken: aromatic π system (two double bonds retained). Stereochemistry: Substituents: EDG stay on sp² carbons; EWG end on sp³ carbon. Rate and selectivity: First electron transfer / protonation of the radical anion are rate-limiting.
At a glance
Step 1: Electron transfer gives the radical anion
Sodium metal dissolved in liquid ammonia transfers a single electron directly to the aromatic ring, forming a resonance-stabilized radical anion — the ring temporarily loses aromaticity to accept the charge.
Step 2: Alcohol protonates the radical anion
An alcohol (the proton source) protonates the radical anion at a ring carbon, forming a neutral, resonance-stabilized pentadienyl radical.
Step 3: Second electron, second protonation
A second electron transfer, followed by a second protonation, gives a 1,4-cyclohexadiene — reduced at two specific positions, with the ring's remaining double bonds landing in a substituent-dependent, predictable pattern.
Abbreviations and terms
- EWG
- Electron-withdrawing group. A substituent (C=O, NO₂, CN…) that pulls electron density away, stabilising anions and activating electrophiles.
- EDG
- Electron-donating group. A substituent (OR, NR₂, alkyl) that pushes electron density in, stabilising cations and activating rings toward electrophiles.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- EtOH
- Ethanol. Polar protic solvent.
- SET
- Single-electron transfer. Movement of one electron (not a pair), generating radicals/radical ions.
How it compares with related reactions
| Birch reduction of benzene | Bouveault-Blanc reduction | Catalytic reduction of a nitro group to an amine | |
|---|---|---|---|
| Reagents | Na (or Li), liquid NH₃, EtOH (or t-BuOH) | Na metal, absolute EtOH (excess) | ArNO₂, H₂ (1–5 atm), 5–10% Pd/C or PtO₂ |
| Conditions | −33 °C, THF cosolvent | Reflux in ethanol (or toluene/EtOH) | EtOH or EtOAc, 25–60 °C |
| Stereochemistry | Substituents: EDG stay on sp² carbons; EWG end on sp³ carbon | No stereocentre in simple esters | No stereocentre; usually chemoselective for NO₂ over aryl C–Cl only with care |
| Rate / selectivity | First electron transfer / protonation of the radical anion are rate-limiting | SET to the ester C=O is rate-limiting; aldehyde reduced faster than ester | Highly exothermic; hydroxylamine can accumulate if catalyst is poisoned |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Birch reduction of benzene?
Na (or Li), liquid NH₃, EtOH (or t-BuOH). Conditions: −33 °C, THF cosolvent.
Which bonds are formed and which are broken?
Formed: 2 C–H; 1,4-cyclohexadiene. Broken: aromatic π system (two double bonds retained).
What is the stereochemical outcome?
Substituents: EDG stay on sp² carbons; EWG end on sp³ carbon
What controls the rate or the selectivity?
First electron transfer / protonation of the radical anion are rate-limiting
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go