Which of the following compounds is most acidic ?

Options

A. `C_(6)H_(5)COOH`
B. `ClCH_(2)COOH`
C. `BrCH_(2)COOH`
D. `O_(2)NCH_(2)CH_(2)COOH`

Answer

To determine which of the given compounds is the most acidic, we need to analyze the stability of their conjugate bases. The compounds in question are:

1. Benzoic acid (C6H5COOH)
2. Chlorocytic acid (ClC6H4COOH)
3. Bromide acid (BrC6H4COOH)
4. Nitro-propanoic acid (NO2C3H6COOH)

### Step-by-Step Solution:

**Step 1: Identify the conjugate bases of each acid.**
- For benzoic acid, the conjugate base is benzoate ion (C6H5COO⁻).
- For chlorocytic acid, the conjugate base is chlorobenzoate ion (ClC6H4COO⁻).
- For bromide acid, the conjugate base is bromobenzoate ion (BrC6H4COO⁻).
- For nitro-propanoic acid, the conjugate base is nitro-propanoate ion (NO2C3H5COO⁻).

**Hint:** The conjugate base is formed when the acid donates a proton (H⁺).

**Step 2: Analyze the stability of each conjugate base.**
- The stability of the conjugate base is crucial for determining the acidity of the corresponding acid. The more stable the conjugate base, the stronger the acid.
- Benzoate ion is stabilized by resonance, as it has two equivalent resonance structures.
- Chlorobenzoate ion is also stabilized by resonance, but the presence of the electron-withdrawing chlorine atom increases its stability further compared to benzoate ion.
- Bromobenzoate ion is less stable than chlorobenzoate because bromine is a weaker electron-withdrawing group than chlorine.
- Nitro-propanoate ion has the nitro group, which is a strong electron-withdrawing group, but its position relative to the carboxyl group affects its overall stability.

**Hint:** Electron-withdrawing groups stabilize the conjugate base by delocalizing negative charge.

**Step 3: Compare the stability of the conjugate bases.**
- Benzoate ion: Stable due to resonance.
- Chlorobenzoate ion: More stable than benzoate due to the electron-withdrawing effect of chlorine.
- Bromobenzoate ion: Less stable than chlorobenzoate due to the weaker electron-withdrawing effect of bromine.
- Nitro-propanoate ion: While the nitro group is a strong electron-withdrawing group, its position (alpha position) may not stabilize the conjugate base as effectively as the chlorine in chlorobenzoate.

**Hint:** The position of substituents matters; electron-withdrawing groups are more effective when they are closer to the carboxyl group.

**Step 4: Conclusion on acidity.**
After analyzing the stability of the conjugate bases, we conclude that chlorocytic acid (chlorobenzoate ion) has the most stable conjugate base due to the strong electron-withdrawing effect of chlorine. Therefore, chlorocytic acid is the most acidic among the given compounds.

**Final Answer:** Chlorocytic acid is the most acidic compound.