In the conversion of alkyne to trans-alkene by Birch reduction using alkali metals (such as Na or K) in liquid NH₃ and alcohol (MeOH or EtOH),

the mechanism takes place in the formation of intermediate species in the following sequence:

 

• A

  $\text{ Radical anion }\to \text{ vinylic radical }\to \text{ trans-vinylic dianion }\to \text{ trans-alkene }$

• B

  $\text{ Radical anion }\to \text{ trans-vinylic anion }\to \text{ vinylic radical }\to \text{ trans-alkene }$

• C

  $\text{ Vinylic radical }\to \text{ radical anion }\to \text{ trans-vinylic anion }\to \text{ trans-alkene }$

• D

  $\text{ Vinvlic radical }\to \text{ trans-vinylic anion }\to \text{ radical anion }\to \text{ trans-alkene }$