Carboxylic acid in which Hell-Volhard-Zelinsky reaction cannot be carried out?

Options

A.
B.
C.
D.

Answer

To determine which carboxylic acid cannot undergo the Hell-Volhard-Zelinsky (HVZ) reaction, we need to analyze the requirements of this reaction and the structure of the carboxylic acids.

### Step-by-Step Solution:

1. **Understand the Hell-Volhard-Zelinsky Reaction**:
The HVZ reaction involves the halogenation of carboxylic acids at the alpha position (the carbon adjacent to the carboxyl group). This reaction typically requires the presence of alpha hydrogen atoms.

2. **Identify the Structure of Carboxylic Acids**:
A general carboxylic acid can be represented as R-COOH, where R is a hydrocarbon chain or group. The alpha carbon is the carbon atom next to the carboxyl group (COOH).

3. **Determine the Role of Alpha Hydrogen**:
For the HVZ reaction to occur, the carboxylic acid must have at least one alpha hydrogen atom. This hydrogen is essential for the formation of the alpha halo acid (RCHXCOOH).

4. **Examine Formic Acid**:
Formic acid has the chemical formula HCOOH. In this structure:
- R = H (there is no alkyl group)
- The alpha carbon is directly attached to the carboxyl group, and it has no hydrogen atoms (since it is bonded to another hydrogen).

5. **Conclusion**:
Since formic acid (HCOOH) does not have any alpha hydrogen atoms (it lacks an alkyl group), it cannot undergo the Hell-Volhard-Zelinsky reaction. Therefore, it is the carboxylic acid for which the HVZ reaction cannot be carried out.

### Final Answer:
The carboxylic acid in which the Hell-Volhard-Zelinsky reaction cannot be carried out is **formic acid (HCOOH)**.

---