Which compound is the most acidic of the following ?

Options

A. `C_6H_5COOH`
B. `ClCH_2COOH`
C. `CD_3COOH`
D. `CH_3CH_2COOH`

Answer

To determine which compound is the most acidic among the given options, we need to analyze the stability of the conjugate bases formed after the release of a proton (H+). The more stable the conjugate base, the stronger the acid.

### Step-by-Step Solution:

1. **Identify the Compounds**:
Let's assume the compounds given are:
- A: Benzoic acid (C6H5COOH)
- B: Chlorobenzoic acid (C6H4ClCOOH)
- C: Acetic acid (CH3COOH)
- D: p-Dimethylaminobenzoic acid (C6H4(CH3)2NCOOH)

2. **Understand Acidity**:
Acidity is defined by the ability of an acid to donate a proton (H+). The stability of the conjugate base formed after deprotonation is a key factor in determining acidity. A more stable conjugate base corresponds to a stronger acid.

3. **Draw the Conjugate Bases**:
- For benzoic acid (A), the conjugate base is benzoate ion (C6H5COO-).
- For chlorobenzoic acid (B), the conjugate base is chlorobenzoate ion (C6H4ClCOO-).
- For acetic acid (C), the conjugate base is acetate ion (CH3COO-).
- For p-dimethylaminobenzoic acid (D), the conjugate base is p-dimethylaminobenzoate ion (C6H4(CH3)2NCOO-).

4. **Analyze Stabilization Factors**:
- **Benzoate Ion**: The negative charge is delocalized over the aromatic ring, providing some stability.
- **Chlorobenzoate Ion**: The presence of the chlorine atom (an electron-withdrawing group) enhances the stability of the conjugate base through the -I effect, which disperses the negative charge.
- **Acetate Ion**: The negative charge is also delocalized, but there are no strong electron-withdrawing groups to stabilize it further.
- **p-Dimethylaminobenzoate Ion**: The dimethylamino group is an electron-donating group (+I effect), which destabilizes the conjugate base by increasing electron density on the negatively charged oxygen.

5. **Compare Acidity**:
- The chlorobenzoate ion (from B) is the most stabilized due to the strong -I effect from chlorine, making chlorobenzoic acid the strongest acid among the options.
- Benzoic acid (A) is less acidic than chlorobenzoic acid due to the lack of a strong electron-withdrawing group.
- Acetic acid (C) is less acidic than both benzoic and chlorobenzoic acids.
- p-Dimethylaminobenzoic acid (D) is the least acidic due to the destabilizing effect of the electron-donating dimethylamino group.

6. **Conclusion**:
Based on the analysis, the most acidic compound is **B: Chlorobenzoic acid**.