Birch Reduction

$$\text{R--C}\equiv\text{C--R'} + 2[\text{H}] \xrightarrow{\text{Na or Li in liq. NH}_3} trans\text{-alkene}$$

Concept: Alkynes undergo selective stereospecific reduction using alkali metals dissolved in liquid ammonia to form trans-alkenes (anti-addition).

Reagents & Conditions: Sodium or Lithium dissolved in liquid ammonia at low temperatures (around $$-33^\circ\text{C}$$).

Key Notes:

• Proceeds via a radical anion mechanism with electron transfer followed by proton transfer.
• The hydrogen addition occurs from opposite sides (anti-addition), selectively yielding the thermodynamically more stable trans isomer.
• Terminal alkynes do not undergo Birch reduction to alkenes; instead, they form alkynide salts ($$\text{R--C}\equiv\text{C}^-\text{Na}^+$$) due to the acidic nature of the terminal proton.