`CH_3 C-= C CH_3 overset(H_2//Pt)to A overset(D_2//Pt)to B` The compounds A and B, respectively are

Options

A. cis-butene-2 and rac-2, 3-dideuterobutane
B. trans-butene-2 and rac-2, 3-dideuterobutane
C. cis-butene-2 and meso-2, 3-dideuterobutane
D. trans-butene-2 and meso-2, 3-dideuterobutane

Answer

To solve the problem, we need to determine the compounds A and B from the given reaction sequence involving the starting material \( CH_3C \equiv CCH_3 \) (which is 2-butyne) and the reactions with \( H_2/Pt \) and \( D_2/Pt \).

### Step 1: Identify the starting compound
The starting compound is \( CH_3C \equiv CCH_3 \), which is known as 2-butyne. It has a triple bond between the two carbon atoms in the center.

### Step 2: Reaction with \( H_2/Pt \)
When 2-butyne is treated with hydrogen gas (\( H_2 \)) in the presence of platinum (\( Pt \)), it undergoes hydrogenation. This reaction converts the triple bond into a double bond, resulting in the formation of a cis-alkene. The product A is therefore:

\[ A = CH_3CH=CHCH_3 \]

This compound is known as cis-2-butene.

### Step 3: Reaction with \( D_2/Pt \)
Next, we take compound A (cis-2-butene) and react it with deuterium gas (\( D_2 \)) in the presence of platinum. In this reaction, deuterium, which is an isotope of hydrogen, will add across the double bond. The result will be the addition of deuterium atoms to the same side of the double bond, leading to the formation of a meso compound. The product B is therefore:

\[ B = CH_3C(D)C(D)CH_3 \]

This compound is known as meso-2,3-dideuterobutane.

### Summary of Results
- Compound A: **Cis-2-butene** (or simply cis-butene)
- Compound B: **Meso-2,3-dideuterobutane**