Which of the following dicarboxylic acids contains the most acidic hydrogen ?

Options

A. Maleic acid
B. Fumaric acid
C. Succinic acid
D. Malonic acid

Answer

To determine which dicarboxylic acid contains the most acidic hydrogen, we need to analyze the stability of the conjugate bases formed when the acidic hydrogen is removed. The more stable the conjugate base, the more acidic the hydrogen.

### Step-by-Step Solution:

1. **Identify the Dicarboxylic Acids**:
We have four dicarboxylic acids to consider. Let's denote them as:
- Maleic Acid (cis-2-butenedioic acid)
- Fumaric Acid (trans-2-butenedioic acid)
- Succinic Acid (butanedioic acid)
- Oxalic Acid (ethanedioic acid)

2. **Draw the Structures**:
- **Maleic Acid**: HOOC-CH=CH-COOH
- **Fumaric Acid**: HOOC-CH=CH-COOH (same as maleic but trans)
- **Succinic Acid**: HOOC-CH2-CH2-COOH
- **Oxalic Acid**: HOOC-COOH

3. **Analyze the Conjugate Bases**:
- When an acidic hydrogen is removed from each acid, we form their respective conjugate bases:
- Maleic Acid: COO⁻-CH=CH-COOH
- Fumaric Acid: COO⁻-CH=CH-COOH
- Succinic Acid: COO⁻-CH2-CH2-COOH
- Oxalic Acid: COO⁻-COO⁻

4. **Evaluate Stability of Conjugate Bases**:
- **Maleic and Fumaric Acids**: Both can stabilize their negative charge through resonance. However, maleic acid has a cis configuration that allows for intramolecular hydrogen bonding, which can reduce the acidity slightly.
- **Succinic Acid**: The conjugate base does not have resonance stabilization and is less acidic compared to maleic and fumaric acids.
- **Oxalic Acid**: The conjugate base has two COO⁻ groups, which can stabilize the negative charge through resonance and inductive effects, making it very stable.

5. **Conclusion**:
After analyzing the stability of the conjugate bases, we find that **Oxalic Acid** has the most stable conjugate base due to the presence of two carboxylate groups that can delocalize the negative charge. Therefore, it contains the most acidic hydrogen among the given dicarboxylic acids.

### Final Answer:
**Oxalic Acid contains the most acidic hydrogen.**