Which of the following reagents is most suitable for the conversion of ${\text{CH}}_3\text{C}\equiv {\text{CCH}}_3$ to ${\text{trans-CH}}_3\text{CH}={\text{CHCH}}_3$?

Solution

1. The reduction of an internal alkyne to a trans-alkene is a stereospecific reaction.
2. Dissolving metal reduction using $\text{Na}$ in liquid ${\text{NH}}_3$ proceeds via a radical anion intermediate.
3. The trans-isomer is thermodynamically more stable than the cis-isomer, leading to the trans-alkene product.
4. Hence the answer is (B).