When But-2-yne is treated with `H_(2)` / `Pd-BaSO_(4)` the product formed will be

Explanation

To determine the product formed when But-2-yne is treated with \( H_2 \) in the presence of \( Pd-BaSO_4 \) (Lindlar's catalyst), we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reactant**:
But-2-yne is an alkyne with the structure:
\[ CH_3-C \equiv C-CH_3 \]
This means there is a triple bond between the second and third carbon atoms.

2. **Understand the Catalyst**:
The catalyst \( Pd-BaSO_4 \) is known as Lindlar's catalyst. It is used for the partial hydrogenation of alkynes to alkenes.

3. **Reaction Process**:
When But-2-yne is treated with hydrogen gas (\( H_2 \)) in the presence of Lindlar's catalyst, the triple bond is partially hydrogenated to form a double bond.

4. **Determine the Product**:
The product of this reaction will be an alkene. Specifically, the hydrogenation of But-2-yne will yield But-2-ene.

5. **Stereochemistry of the Product**:
Lindlar's catalyst preferentially produces the cis isomer of the alkene. Therefore, the product will be cis-but-2-ene.

6. **Write the Final Product**:
The final product can be represented as:
\[ \text{cis-}CH_3-CH=CH-CH_3 \]

### Final Answer:
The product formed when But-2-yne is treated with \( H_2 \) / \( Pd-BaSO_4 \) is **cis-but-2-ene**.

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