Which of the following reagent is used for the conversion of 2 - hexyne into trans 2- hexene

Options

A. `H_2,PtO_2`
B. `NaBH_4`
C. `H_2//Pd//BaSO_4`
D. `Li-NH_3//C_2H_5OH`

Answer

To convert 2-hexyne into trans-2-hexene, we need to use a reagent that facilitates the reduction of the triple bond in a way that results in the formation of the trans isomer. Here’s a step-by-step solution to identify the correct reagent:

### Step 1: Understand the Structure of 2-Hexyne
2-Hexyne is an alkyne with the following structure:
- It has a triple bond between the second and third carbon atoms in a six-carbon chain.
- The structure can be represented as CH3-CH≡C-CH2-CH2-CH3.

### Step 2: Identify the Desired Product
The target product is trans-2-hexene, which has a double bond between the second and third carbon atoms and is in the trans configuration. The structure of trans-2-hexene is:
- CH3-CH=CH-CH2-CH2-CH3, where the two methyl groups (CH3) are on opposite sides of the double bond.

### Step 3: Choose the Appropriate Reagent
To convert 2-hexyne to trans-2-hexene, we need a reagent that can selectively reduce the alkyne to the trans alkene. The best choice for this transformation is:
- **Lithium in Liquid Ammonia (Li/NH3)**: This reagent is known for its ability to reduce alkynes to trans alkenes through a radical mechanism.

### Step 4: Eliminate Other Options
1. **H2/Pt**: This would lead to syn addition, resulting in the cis isomer.
2. **NaBH4**: This is a mild reducing agent and does not effectively reduce alkynes to alkenes.
3. **H2/Pd-BaSO4**: This is a poisoned catalyst that typically leads to the formation of the cis isomer.

### Conclusion
The reagent that successfully converts 2-hexyne into trans-2-hexene is **Lithium in Liquid Ammonia (Li/NH3)**.

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