Which of the following is Weakest acid ?

Options

A.
B.
C.
D.

Answer

To determine which of the following acids is the weakest, we need to analyze the given options based on their structure and the stability of their conjugate bases. The options provided are:

1. Benzoic acid
2. Paranitrobenzoic acid
3. Metanitrobenzoic acid
4. Orthonitrobenzoic acid

### Step-by-step Solution:

**Step 1: Understand the concept of acidity and conjugate bases.**
- Acidity is determined by the ability of an acid to donate a proton (H⁺). The strength of an acid is often related to the stability of its conjugate base after the proton is lost.

**Step 2: Identify the conjugate bases of the acids.**
- The conjugate base of benzoic acid (C₆H₅COOH) is benzoate ion (C₆H₅COO⁻).
- The conjugate bases of the nitro-substituted benzoic acids (paranitro, metanitro, and orthonitro) will also be similar, but with the nitro group (NO₂) affecting their stability.

**Step 3: Analyze the effects of substituents on the conjugate bases.**
- The nitro group (NO₂) is an electron-withdrawing group that stabilizes the negative charge on the conjugate base through resonance and inductive effects.
- In the case of benzoic acid, the conjugate base only has resonance stabilization from the oxygen atom.

**Step 4: Compare the stability of the conjugate bases.**
- For paranitrobenzoic acid, metanitrobenzoic acid, and orthonitrobenzoic acid, the presence of the nitro group stabilizes the conjugate base more effectively than in benzoic acid.
- Therefore, the conjugate bases of the nitro-substituted benzoic acids are more stable due to the electron-withdrawing effect of the nitro group.

**Step 5: Conclude which acid is the weakest.**
- Since benzoic acid has a less stable conjugate base compared to the nitro-substituted acids, it is the weakest acid among the options provided.

### Final Answer:
**Benzoic acid is the weakest acid among the given options.**