The strongest acid among the following is

Options

A. `p-Cl-C_6 H_4 COOH `
B. `p-OHC _6 H_4 COOH `
C. `C_6 H_5 COOH `
D. `p-NO_2 C_6 H_4 COOH `

Answer

To determine the strongest acid among the given options (A, B, C, and D), we need to analyze the stability of the conjugate bases formed after the acid donates a proton (H+). The stronger the acid, the more stable its conjugate base will be.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Let’s denote the compounds as follows:
- A: Chloro-substituted benzoic acid (C6H4ClCOOH)
- B: Hydroxy-substituted benzoic acid (C6H4(OH)COOH)
- C: Benzoic acid (C6H5COOH)
- D: Nitro-substituted benzoic acid (C6H4(NO2)COOH)

2. **Understand Acid Strength**:
- The strength of an acid is determined by its ability to donate a proton (H+) and the stability of the resulting conjugate base (A-).
- A more stable conjugate base corresponds to a stronger acid.

3. **Analyze Conjugate Bases**:
- When each acid donates a proton, we get the following conjugate bases:
- A-: C6H4ClCOO-
- B-: C6H4(OH)COO-
- C-: C6H5COO-
- D-: C6H4(NO2)COO-

4. **Evaluate Electron-Withdrawing Groups**:
- The stability of the conjugate bases is influenced by substituents on the aromatic ring:
- **Chloro Group (A)**: This is an electron-withdrawing group through the -I effect (inductive effect), which stabilizes the conjugate base.
- **Hydroxy Group (B)**: The -OH group can donate electrons through resonance, which may destabilize the conjugate base.
- **Nitro Group (D)**: The nitro group (-NO2) is a strong electron-withdrawing group through both -M (mesomeric) and -I effects, significantly stabilizing the conjugate base.
- **Benzoic Acid (C)**: Lacks any strong electron-withdrawing groups, making it less stable compared to the others.

5. **Comparing Stability**:
- The nitro group in compound D provides the greatest stabilization to the conjugate base due to its strong -M effect, making D the strongest acid.
- The chloro group in compound A provides some stabilization but is less effective than the nitro group.
- The hydroxy group in compound B destabilizes the conjugate base due to its electron-donating nature.
- Compound C, lacking any strong electron-withdrawing groups, is the weakest.

6. **Conclusion**:
- Based on the analysis, the strongest acid among the options is **D (Nitro-substituted benzoic acid)**.