Among the given pairs, in which pair second compound is more acidic than first ?

Options

A. `BrCH_2NO_2 and CH_3CH_3`
B. `CH_3-overset(O)overset(||)"CCH"_2CN and CH_3-overset(O)overset(||)C-CH_3`
C.
D.

Explanation

To determine which pair of compounds has the second compound more acidic than the first, we need to analyze the influence of substituents on the acidity of the compounds. The acidity of a compound is affected by the presence of electron-withdrawing groups (EWGs) and electron-donating groups (EDGs).

### Step-by-Step Solution:

1. **Understand Acidity and Substituent Effects**:
- Acidity increases with the presence of electron-withdrawing groups (EWGs) because they stabilize the negative charge on the conjugate base formed after losing a proton (H⁺).
- Acidity decreases with the presence of electron-donating groups (EDGs) because they destabilize the negative charge on the conjugate base.

2. **Analyze Each Pair**:
- **Pair 1**: Br NO2-CH2 vs. CH3
- Both Br and NO2 are EWGs. The first compound is more acidic due to the presence of these groups.
- **Pair 2**: CH3COCH2CN vs. CH3COCH3
- The CN group is an EWG, while CH3 is an EDG. The first compound is more acidic due to the CN group.
- **Pair 3**: COCH3-phenol vs. phenol
- The COCH3 group is an EWG, making the first compound more acidic.
- **Pair 4**: CH3-phenol vs. phenol
- The CH3 group is an EDG, which decreases acidity. Thus, phenol is more acidic than the CH3-substituted phenol.

3. **Identify the Correct Pair**:
- After analyzing all pairs, we find that in **Pair 4**, phenol is more acidic than CH3-phenol, making it the correct answer where the second compound (phenol) is more acidic than the first (CH3-phenol).

### Conclusion:
The pair where the second compound is more acidic than the first is **Pair 4: CH3-phenol vs. phenol**.