The trans-alkenes are formed by the reduction of alkynes with

Options

A. `H_2-Pd//C ,BaSO_4`
B. `NaBH_4`
C. Na/liq. `NH_3`
D. `Sn - HCl`

Answer

To solve the question regarding the formation of trans-alkenes from alkynes, we will follow these steps:

### Step 1: Understand the Reaction
We need to reduce alkynes (which have a triple bond) to form alkenes (which have a double bond). Specifically, we want to form trans-alkenes, where the substituents on the double bond are on opposite sides.

### Step 2: Identify the Reduction Method
The reduction of alkynes can be achieved using various catalysts. The key is to find a method that specifically leads to the formation of trans-alkenes.

### Step 3: Analyze the Options
We have several catalysts to consider:
1. Hydrogen with palladium (H2/Pd)
2. Hydrogen with palladium on barium sulfate (H2/Pd-BaSO4)
3. Sodium in liquid ammonia (Na/NH3)
4. Sodium borohydride (NaBH4)

### Step 4: Evaluate Each Catalyst
- **H2/Pd**: This catalyst typically leads to the formation of cis-alkenes due to the syn-addition of hydrogen.
- **H2/Pd-BaSO4**: This is a Lindlar catalyst, which also results in the formation of cis-alkenes.
- **Na/NH3**: Sodium in liquid ammonia is known to reduce alkynes to trans-alkenes through a radical mechanism, allowing for the formation of the desired product.
- **NaBH4**: This is a milder reducing agent that does not typically reduce alkynes to alkenes effectively.

### Step 5: Conclusion
The only catalyst that leads to the formation of trans-alkenes from alkynes is **sodium in liquid ammonia (Na/NH3)**. Therefore, the correct answer is option 3.

### Final Answer
**The trans-alkenes are formed by the reduction of alkynes with sodium in liquid ammonia (Na/NH3).**

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