Which of the acids cannot be prepared by Grignard reagent?

Options

A. Acetic acid
B. Succinic acid
C. Formic acid
D. All of these

Answer

To determine which acid cannot be prepared by Grignard reagent, we need to understand the general reaction mechanism of how Grignard reagents work in synthesizing carboxylic acids.

### Step-by-Step Solution:

1. **Understanding Grignard Reagents**:
Grignard reagents are organomagnesium compounds represented as RMgX, where R is an alkyl or aryl group and X is a halogen.

2. **Reaction with Carbon Dioxide**:
When a Grignard reagent reacts with carbon dioxide (CO2), it forms a carboxylate salt. The general reaction can be represented as:
\[ RMgX + CO2 \rightarrow RCOOMgX \]

3. **Hydrolysis**:
Upon hydrolysis (reaction with water), the carboxylate salt is converted into a carboxylic acid:
\[ RCOOMgX + H2O \rightarrow RCOOH + Mg(OH)X \]

4. **Analyzing the Carboxylic Acids**:
The resulting carboxylic acid (RCOOH) will have one carbon from the Grignard reagent (R) and one carbon from the carbon dioxide (CO2). Therefore, the carboxylic acid produced will always have at least two carbon atoms.

5. **Identifying the Acids**:
- **Acetic Acid (CH3COOH)**: Can be prepared by using methyl magnesium halide (CH3MgX).
- **Succinic Acid (C4H6O4)**: Can be prepared by using a Grignard reagent with a longer alkyl chain.
- **Formic Acid (HCOOH)**: This acid has only one carbon atom.

6. **Conclusion**:
Since formic acid (HCOOH) has only one carbon atom, it cannot be prepared using a Grignard reagent, which requires at least two carbon atoms in the final product.

### Final Answer:
**Formic acid cannot be prepared by Grignard reagent.**

---