Which of the following exists as a zwitterion?

Options

A. p-Aminophenol
B. Salicylic acid
C. Sulphonilic acid
D. p-Aminoacetophenone

Explanation

To determine which of the given compounds exists as a zwitterion, we need to understand what a zwitterion is and analyze each option accordingly.

### Step-by-Step Solution:

1. **Understanding Zwitterions**:
- A zwitterion is a molecule that has both positive and negative charges but is overall electrically neutral. This typically occurs in amino acids where there is an amine group (-NH2) that can accept a proton (H+) and a carboxylic acid group (-COOH) that can donate a proton.

2. **Analyzing the Options**:
- We have the following compounds to consider:
1. Para-amino phenol
2. Sulfonic acid
3. Sulfonyl acid
4. Para-amino aceto phenol

3. **Option 1: Para-amino phenol**:
- Structure: Contains an amine group (-NH2) and a hydroxyl group (-OH).
- Analysis: While it has basic (amine) and slightly acidic (phenolic -OH) properties, it does not form a zwitterion because it lacks a strong acidic group like -COOH.

4. **Option 2: Sulfonic acid**:
- Structure: Contains a sulfonic acid group (-SO3H).
- Analysis: This compound is acidic but does not have a basic group to balance the charge, so it cannot exist as a zwitterion.

5. **Option 3: Sulfonyl acid**:
- Structure: Contains an amine group (-NH2) and a sulfonic acid group (-SO3H).
- Analysis: The sulfonic acid can donate a proton (H+) while the amine can accept a proton, allowing this compound to exist as a zwitterion.

6. **Option 4: Para-amino aceto phenol**:
- Structure: Contains an amine group (-NH2) and an acetyl group (C=O with CH3).
- Analysis: Similar to para-amino phenol, it lacks a strong acidic group to form a zwitterion.

7. **Conclusion**:
- Among the options provided, **sulfonyl acid** (Option 3) is the only compound that can exist as a zwitterion due to the presence of both an acidic sulfonic acid group and a basic amine group.

### Final Answer:
**The compound that exists as a zwitterion is Sulfonyl acid (Option 3).**