Major Product Determination in Alkyne Reduction Reaction

To determine the major product formed in the given reaction, we analyze the reaction of a compound containing both a terminal alkyne and an internal alkyne with $\text{NaNH}_2$ followed by $\text{Na / liq. NH}_3$.

Step 1: Deprotonation by $\text{NaNH}_2$

• The starting material contains two types of triple bonds:
  1. A terminal alkyne ($-\text{C}\equiv\text{CH}$), which has an acidic hydrogen ($pK_a \approx 25$).
  2. An internal alkyne ($-\text{C}\equiv\text{C}-$), which lacks acidic acetylenic protons.
• $\text{NaNH}_2$ is a strong base that selectively deprotonates the terminal alkyne to form an alkynide (acetylide) anion: $\text{R}-\text{C}\equiv\text{C}-\text{CH}_2-\text{CH}_2-\text{C}\equiv\text{C}-\text{H} + \text{NH}_2^- \longrightarrow \text{R}-\text{C}\equiv\text{C}-\text{CH}_2-\text{CH}_2-\text{C}\equiv\text{C}^- + \text{NH}_3$

Step 2: Dissolving Metal Reduction ($\text{Na / liq. NH}_3$)

• Internal Alkyne Reduction: Sodium in liquid ammonia ($\text{Na / liq. NH}_3$) reduces internal alkynes via a single-electron transfer (SET) mechanism. This anti-addition reduction selectively yields the stereospecific $(E)$-alkene (trans-alkene).
• Protection of Terminal Alkyne: The terminal alkyne is present in its conjugate base form, the alkynide anion ($-\text{C}\equiv\text{C}^-$). Due to the strong electrostatic repulsion from the existing negative charge, the alkynide anion cannot accept an electron from sodium to form a radical anion. Consequently, the terminal alkyne group is protected against Birch reduction and remains completely unreacted.

Step 3: Protonation (Workup)

• Upon workup (or from proton exchange with $\text{NH}_3$), the terminal alkynide anion is protonated back to regenerate the terminal triple bond ($-\text{C}\equiv\text{CH}$): $\text{R}'-\text{C}\equiv\text{C}^- + \text{NH}_3 \longrightarrow \text{R}'-\text{C}\equiv\text{CH} + \text{NH}_2^-$

Final Product Structure

• Terminal group: Intact terminal alkyne ($-\text{C}\equiv\text{CH}$).
• Internal group: Converted selectively to a trans-alkene (double bond in $E$-configuration).

Comparing this result with the given options, Option (B) correctly shows the molecule with an unreacted terminal alkyne and a trans-internal double bond.

Correct Answer: (B)