Which is the best reagent among the following to convert but-2-yne to trans-but-2-ene?

Options

A. `H_(2)//Pd`-charcoal
B. `H_(2)//"Raney nickel"`
C. `H_(2)//Pd-BaSO_(4)`
D. `Na//"liq".NH_(3)`

Answer

To convert but-2-yne to trans-but-2-ene, we need to choose the best reagent from the given options. Let's analyze each option step-by-step.

### Step 1: Understand the structure of but-2-yne
But-2-yne has the structure:
\[ \text{CH}_3 - \text{C} \equiv \text{C} - \text{CH}_3 \]
This means it has a triple bond between the second and third carbon atoms.

### Step 2: Identify the target product
The target product is trans-but-2-ene, which has the structure:
\[ \text{CH}_3 - \text{C} = \text{C} - \text{CH}_3 \]
In this structure, the two methyl groups (CH₃) are on opposite sides of the double bond, which is characteristic of the trans isomer.

### Step 3: Analyze the reagents
1. **Option A: H₂ with Palladium on Charcoal**
- This reagent can reduce the triple bond to a double bond but does not guarantee the formation of the trans isomer. It could lead to a mixture of cis and trans products.

2. **Option B: H₂ with Rhenium Nickel**
- Rhenium Nickel is a strong hydrogenation catalyst that will fully reduce but-2-yne to butane, which is not our desired product.

3. **Option C: H₂ with Palladium Barium Sulfate (Lindlar Catalyst)**
- Lindlar Catalyst is known to selectively hydrogenate alkynes to cis-alkenes. Therefore, it would yield cis-but-2-ene, which is not the desired trans product.

4. **Option D: Sodium in Liquid Ammonia**
- This reagent is known for converting alkynes to trans-alkenes. When but-2-yne reacts with sodium in liquid ammonia, it will yield trans-but-2-ene.

### Step 4: Conclusion
Based on the analysis, the best reagent to convert but-2-yne to trans-but-2-ene is **Option D: Sodium in Liquid Ammonia**.

### Final Answer
**The best reagent to convert but-2-yne to trans-but-2-ene is Sodium in Liquid Ammonia (Option D).**

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