Which of the following carboxylic acids is highly insoluble in water

Options

A. Propanoic acid
B. Butanoic acid
C. Pentanoic acid
D. Decanoic acid

Explanation

To determine which carboxylic acid is highly insoluble in water, we need to analyze the solubility of carboxylic acids based on their molecular structure and size.

### Step-by-Step Solution:

1. **Understanding Carboxylic Acids**: Carboxylic acids have the general formula R-COOH, where R is an alkyl group. The solubility of these acids in water is influenced by the size of the alkyl group.

2. **Solubility Trend**:
- The first four carboxylic acids (methanoic acid, ethanoic acid, propanoic acid, and butanoic acid) are highly soluble in water. This is due to their ability to form hydrogen bonds with water molecules.
- As the number of carbon atoms in the carboxylic acid increases (beyond four), the solubility in water decreases. This is because larger alkyl groups are hydrophobic and do not interact favorably with water.

3. **Identifying the Options**:
- Let's consider the options given in the question (not explicitly listed in the transcript, but we can assume common carboxylic acids):
- Propanoic acid (C3)
- Butanoic acid (C4)
- Pentanoic acid (C5)
- Decanoic acid (C10)

4. **Analyzing Each Option**:
- **Propanoic Acid (C3)**: Soluble in water.
- **Butanoic Acid (C4)**: Soluble in water.
- **Pentanoic Acid (C5)**: Less soluble in water compared to the first four acids, but still has some solubility.
- **Decanoic Acid (C10)**: Highly insoluble in water due to its larger molecular size and the predominance of the hydrophobic alkyl chain.

5. **Conclusion**: Among the given options, **Decanoic acid (C10)** is the most insoluble in water due to its larger size and hydrophobic nature.

### Final Answer:
The carboxylic acid that is highly insoluble in water is **Decanoic acid (C10)**.