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 Concept:
Partial reduction of alkynes to alkenes can be stereoselective depending on the reagent used.

Step 2: Formula Application:
1. Lindlar's Catalyst (Pd/CaCO$_3$ or Pd-C deactivated with quinoline/sulfur) $\to$ Cis-alkene. 2. Birch Reduction (Na or Li in liquid $NH_3$) $\to$ Trans-alkene.

Step 3: Explanation:
Using palladium on carbon deactivated (poisoned) with quinoline allows for the addition of hydrogen to the same side of the triple bond, resulting in the Cis isomer. This reagent is often called Lindlar's catalyst.

Step 4: Final Answer:
Pd-C/quinoline is the reagent used.