Catalytic hydrogenation involves addition of hydrogen to a ` C = C or -C -= C- `bond in presence of a metal catalyst such as Ni, Pt, Pd, etc. These are called heterogeneous catalysts. Both `H_2` and alkene/alkyne get adsorbed on the surface of the metal , and syn-addition of hydrogen atoms takes place. The relative rates of hydrogenation follows the order:
`-C-=C- gt gt C=C lt gt C= o lt C_6H_6`
If sodium or lithium in liquid ammonia is used, anti-addition of H-atoms occurs across `-C-=C-`bond. These reducing agents can also be used to reduce double bonds of benzene rings and conjugated dienes but not of isolated dienes.
`[A]underset"Catalyst"overset"Lindlar's"larrCH_3-C-=C-CH_3underset(liq. NH_3)overset" Na in"to [B]`
[A] and [ B] are respectively

Options

A. cis, trans-2-butene
B. both trans-2-butene
C. trans,cis-2-butene
D. both cis-2-butene

Explanation

To solve the problem of identifying the products [A] and [B] from the given reaction involving catalytic hydrogenation and the use of sodium or lithium in liquid ammonia, we will follow these steps:

### Step 1: Identify the Reactant
The reactant given is 2-butyne, which has the structure:
\[ \text{CH}_3 - \text{C} \equiv \text{C} - \text{CH}_3 \]

### Step 2: Understand the Catalyst Used
The catalyst mentioned is Lindlar's catalyst, which is palladium (Pd) supported on calcium carbonate (CaCO3). This catalyst is used for the partial hydrogenation of alkynes to alkenes, resulting in a cis (or syn) addition of hydrogen.

### Step 3: Perform the Reaction with Lindlar's Catalyst
When 2-butyne reacts with hydrogen in the presence of Lindlar's catalyst, it undergoes syn-addition, leading to the formation of cis-2-butene. The structure of cis-2-butene is:
\[ \text{CH}_3 - \text{CH} = \text{CH} - \text{CH}_3 \]
This product is denoted as [A].

### Step 4: Understand the Second Reaction with Liquid Ammonia
Next, we consider the reaction of 2-butyne with sodium (Na) in liquid ammonia (NH3). This reaction leads to anti-addition of hydrogen, resulting in the formation of trans-2-butene. The structure of trans-2-butene is:
\[ \text{CH}_3 - \text{CH} = \text{CH} - \text{CH}_3 \]
This product is denoted as [B].

### Step 5: Summarize the Products
Thus, the products can be summarized as follows:
- [A] = cis-2-butene
- [B] = trans-2-butene

### Final Answer
- [A] = cis-2-butene
- [B] = trans-2-butene