Acid strength is measured by the position of equilibrium of ionization in water. In other words acid strength is the function of the stability of conjugate base of the acid. More is the stability of conjugate base of the acid, more is the acidity of acid.
Which one of the following is strongest acid?

Options

A. `CI-CH_(2) - CH_(2) - COOH`
B. `CI-CH_(2)-COOH`
C. `CI-CH_(2) - CH_(2) - CH_(2) - COOH`
D. `CH_(3) -COOH`

Answer

To determine which of the given acids is the strongest, we need to analyze the stability of their conjugate bases. The strength of an acid is directly related to the stability of its conjugate base; the more stable the conjugate base, the stronger the acid.

### Step-by-Step Solution:

1. **Identify the Acids and Their Conjugate Bases:**
- **Option 1:** ClCH2CH2COOH → Conjugate Base: ClCH2CH2COO⁻
- **Option 2:** ClCH2COOH → Conjugate Base: ClCH2COO⁻
- **Option 3:** ClCH2CH2CH2COOH → Conjugate Base: ClCH2CH2CH2COO⁻
- **Option 4:** Cl3COOH → Conjugate Base: Cl3COO⁻

2. **Analyze the Effect of Electron-Withdrawing Groups:**
- Electron-withdrawing groups (EWGs) stabilize the conjugate base by delocalizing negative charge. In this case, the chlorine (Cl) atom acts as an EWG.
- The position of the EWG relative to the carboxylate group (COO⁻) is crucial. The closer the EWG is to the COO⁻, the stronger the acid.

3. **Evaluate Each Option:**
- **Option 1 (ClCH2CH2COOH):** The Cl is at the alpha position. The inductive effect is present but less effective due to distance.
- **Option 2 (ClCH2COOH):** The Cl is at the beta position, which means it has a stronger inductive effect on the COO⁻, making it more stable and thus a stronger acid.
- **Option 3 (ClCH2CH2CH2COOH):** The Cl is at the gamma position, which is further away from the COO⁻, resulting in a weaker inductive effect and less stability of the conjugate base.
- **Option 4 (Cl3COOH):** The presence of three Cl atoms provides a strong electron-withdrawing effect, but the structure may lead to steric hindrance which can affect the acidity.

4. **Conclude the Strongest Acid:**
- Comparing the stability of the conjugate bases, **Option 2 (ClCH2COOH)** is the strongest acid because the Cl atom is closest to the carboxylate group, providing a strong inductive effect that stabilizes the conjugate base (ClCH2COO⁻).

### Final Answer:
**The strongest acid among the options is Option 2: ClCH2COOH.**