Birch reduction: practice and mechanism
Na in liquid NH₃ with ethanol reduces the benzene ring only partly: a 1,4-cyclohexadiene forms. The two remaining double bonds are not conjugated.
Example
Sodium in liquid ammonia transfers electrons to the ring and ethanol protonates. The ring is only partly reduced, giving a 1,4-cyclohexadiene (not conjugated). Electron-donating groups such as OCH₃ or CH₃ stay on a double bond.
Mechanism step by step
- 1
Sodium gives an electron to the benzene ring: a radical anion forms.
- 2
Ethanol protonates the radical anion; a second electron and a second proton follow.
- 3
The protons end up on opposite carbons (1 and 4): 1,4-cyclohexadiene forms, with two isolated double bonds.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Full reduction to cyclohexane needs H₂ with a catalyst under pressure. The Birch reduction stops at the 1,4-cyclohexadiene because isolated double bonds do not accept further electrons.
Often confused with: Catalytic hydrogenation
Frequently asked questions
Where do the double bonds go with substituents?
Electron-donating groups (OCH₃, alkyl) stay on a double bond; electron-withdrawing groups (COOH) end up on the sp³ carbon.
Why ethanol?
NH₃ is too weakly acidic as a proton source. Ethanol delivers protons fast enough without being so acidic that it consumes the sodium at once.