Question

Which of the following reagents is used to convert $\text{C}\equiv \text{C}$ triple bond to $\text{C=C}$ double bond to give Cis isomer of alkene?

• A. $\text{ZnCl}_{2}/\text{HCl}$
• B. Pd-C/quinoline
• C. Na liquid $\text{NH}_{3}$
• D. Na/Hg in $\text{H}_{2}\text{O}$

Hint:

Stereoselective Alkyne Reduction:
Lindlar's Catalyst (Pd/C + poison) $\rightarrow$ Syn-addition $\rightarrow$ CIS Alkene.
Birch Reduction (Na in liquid $\text{NH}_3$) $\rightarrow$ Anti-addition $\rightarrow$ TRANS Alkene.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
We must select the specific chemical reagent capable of performing a stereoselective partial reduction (hydrogenation) of an alkyne, specifically stopping at the alkene stage and yielding the cis stereoisomer exclusively.

Step 2: Detailed Explanation:
Reducing an alkyne directly to an alkane is easy (using raw $\text{H}_2/\text{Pd}$). Stopping exactly at the alkene stage requires a specially "poisoned" (deactivated) catalyst. The stereochemistry depends entirely on the reagent chosen:
(c) Na in liquid $\text{NH_3$:} This is the famous Birch Reduction. The reaction proceeds via a radical anion mechanism in solution. Because the intermediates repel each other, it strictly leads to the thermodynamic anti-addition of hydrogen, yielding exclusively the trans-alkene.
(a) $\text{ZnCl_2\text{/HCl}$:} This is Lucas reagent, used for substituting hydroxyl groups with chlorine in alcohols. It does not reduce alkynes.
(d) Na/Hg in $\text{H_2\text{O}$:} A generic amalgam reducing agent, not used for stereoselective alkyne reduction.
(b) Pd-C/quinoline: This is a variation of Lindlar's catalyst (Palladium deposited on carbon or barium sulfate, poisoned heavily with quinoline or sulfur). Because the reaction occurs on the flat solid surface of the metal catalyst, both hydrogen atoms must be delivered to the alkyne from the exact same side (syn-addition). This restricted geometry rigidly forces the formation of the cis-alkene exclusively.

Step 3: Final Answer:
The reagent used is Pd-C/quinoline, matching option (b).