Reagent: Na (or Li) in liquid NH3 with an alcohol (EtOH) as proton source.

Product: 1,4-Cyclohexadiene (unconjugated diene — NOT 1,3-cyclohexadiene).

Substituent effects on regioselectivity:

• Electron-donating groups (-OR, -NR2, -R): Reduction occurs at positions NOT bearing the substituent. Product retains the substituent on a double bond.
• Electron-withdrawing groups (-COOH, -COR, -COOR): Reduction occurs at positions bearing the substituent. Product has the substituent on a saturated carbon.

Mechanism: Electron addition → radical anion → protonation → radical → electron addition → carbanion → protonation. The 1,4-product is kinetically favored.