Birch reduction template

Solvated electrons add to benzene one at a time; each anion takes a proton from ethanol. The second protonation happens at the centre of the pentadienyl anion, so the product is the non-conjugated 1,4-diene.

Mechanism step by step

Sodium in liquid ammonia with an alcohol reduces benzene to cyclohexa-1,4-diene

1. Electron transfer – A solvated electron from Na in NH₃ adds: a radical anion
2. Protonation – Ethanol protonates the anion: a cyclohexadienyl radical
3. Second electron – Another electron: a pentadienyl anion with most charge at the centre
4. Protonation at the centre – The middle carbon takes H⁺: the 1,4-diene, not the conjugated 1,3-diene

Key points

• The blue colour of Na in liquid NH₃ comes from solvated electrons.
• Electron-donating groups (OMe) end up on a C=C; electron-withdrawing groups (CO₂H) end up on an sp³ carbon.
• The alcohol is a proton source more acidic than NH₃ but weak enough not to destroy the electrons quickly.
• Anisole gives 1-methoxycyclohexa-1,4-diene, which hydrolyses to cyclohex-2-enone: a route to enones.

Drawn with 74 atoms, 9 arrows, 6 shapes – every one editable in MolDraw.

Sources

• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012 Textbook