Which acid derivative is a functional isomer of carboxylic acids?

Options

A. Acid chlorides
B. Esters
C. Acid amides
D. Acid anhydrides

Answer

To determine which acid derivative is a functional isomer of carboxylic acids, we need to analyze the given options based on their molecular formulas and functional groups.

### Step-by-Step Solution:

1. **Understanding Functional Isomers**:
- Functional isomers are compounds that have the same molecular formula but different functional groups. This means they will exhibit different chemical properties.

2. **Identifying the Structure of Carboxylic Acids**:
- The general structure of a carboxylic acid is represented as \( R-C(=O)-OH \), where \( R \) is a hydrocarbon group. The molecular formula for a carboxylic acid is \( C_nH_{2n+1}COOH \) or \( C_nH_{2n+2}O_2 \).

3. **Analyzing the Options**:
- **Option 1: Acid Chlorides**:
- Structure: \( R-C(=O)-Cl \)
- Molecular formula: \( C_nH_{2n+1}ClO \)
- Conclusion: Different molecular formula, so it is not a functional isomer.

- **Option 2: Esters**:
- Structure: \( R-C(=O)-O-R' \)
- Example: Ethanoic acid (CH₃COOH) and its ester (CH₃COOCH₃).
- Molecular formula: Both have \( C_2H_4O_2 \) (for ethanoic acid) and \( C_2H_4O_2 \) (for the ester).
- Conclusion: Same molecular formula and different functional groups, so esters are functional isomers of carboxylic acids.

- **Option 3: Acid Amides**:
- Structure: \( R-C(=O)-NH_2 \)
- Molecular formula: \( C_nH_{2n+1}NO \)
- Conclusion: Different molecular formula, so it is not a functional isomer.

- **Option 4: Acid Anhydrides**:
- Structure: \( R-C(=O)-O-C(=O)-R \)
- Molecular formula: \( C_{2n}O_3 \)
- Conclusion: Different molecular formula, so it is not a functional isomer.

4. **Final Conclusion**:
- The only acid derivative that is a functional isomer of carboxylic acids is **Esters**.

### Answer:
The acid derivative that is a functional isomer of carboxylic acids is **Esters**.