Addition of `H_2` to 2-butyne in presence of Lindlar's catalyst . Gives cis-2-butene while with Na/in liq. `NH_3` gives trans-2-butene. Why so ? Explain.

Explanation

To understand why the addition of \( H_2 \) to 2-butyne in the presence of Lindlar's catalyst gives cis-2-butene, while the addition with sodium in liquid ammonia gives trans-2-butene, let's break down the reactions step by step.

### Step 1: Understanding the Reactants
2-butyne is an alkyne with the formula \( C_4H_6 \). It has a triple bond between the second and third carbon atoms. The goal is to convert this alkyne into an alkene by adding hydrogen (\( H_2 \)).

### Step 2: Reaction with Lindlar's Catalyst
Lindlar's catalyst is a poisoned palladium catalyst that facilitates the partial hydrogenation of alkynes to cis-alkenes. When 2-butyne reacts with \( H_2 \) in the presence of Lindlar's catalyst, the reaction proceeds as follows:

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