Which of the following reagents is used to convert an alkyne into a trans-alkene?

Solution

1. The reduction of an internal alkyne to a trans-alkene is a stereoselective process.
2. Dissolving metal reduction using $\text{Na}$ in liquid ${\text{NH}}_3$ proceeds via a radical anion intermediate.
3. This mechanism favors the formation of the more stable trans-isomer.
4. Hence the answer is (B).