Which of the following compounds is most acidic?

Options

A. `Cl-CH_(2)CH_(2)-OH`
B.
C.
D.

Answer

To determine which of the given compounds is the most acidic, we need to analyze the stability of their conjugate bases after the removal of a proton (H⁺). The more stable the conjugate base, the stronger the acid.

### Step-by-Step Solution:

1. **Identify the Compounds**:
Let's denote the compounds as follows:
- Compound 1: Cl-CH₂-CH₂-OH (Alcohol with a chlorine substituent)
- Compound 2: CH₃-CH₂-OH (Simple alcohol)
- Compound 3: NO₂-Ph-OH (Phenol with a nitro group)
- Compound 4: CH₃-Ph-OH (Phenol with a methyl group)

2. **Remove the Proton (H⁺)**:
When we consider the acidity, we remove H⁺ from each compound to form their respective conjugate bases:
- Conjugate base of Compound 1: Cl-CH₂-CH₂-O⁻
- Conjugate base of Compound 2: CH₃-CH₂-O⁻
- Conjugate base of Compound 3: NO₂-Ph-O⁻
- Conjugate base of Compound 4: CH₃-Ph-O⁻

3. **Analyze the Stability of Conjugate Bases**:
- For Compound 1, the conjugate base is not significantly stabilized by resonance or inductive effects.
- For Compound 2, the conjugate base is slightly stabilized by the alkyl group, but not significantly.
- For Compound 3, the conjugate base (NO₂-Ph-O⁻) is stabilized by resonance with the nitro group, which is a strong electron-withdrawing group. This resonance allows the negative charge to be delocalized onto the nitro group.
- For Compound 4, the conjugate base (CH₃-Ph-O⁻) is destabilized by the methyl group, which is an electron-donating group, making the negative charge less stable.

4. **Determine the Most Acidic Compound**:
The conjugate base of Compound 3 (NO₂-Ph-O⁻) is the most stable due to the resonance stabilization provided by the electron-withdrawing nitro group. Therefore, Compound 3 is the most acidic.

5. **Conclusion**:
The most acidic compound among the given options is **Compound 3: NO₂-Ph-OH**.