Which of the following is least soluble in water at 298 K?

Options

A. `CH_(3)NH_(2)`
B. `(CH_(3))_(3)N`
C. `(CH_(3))_(2)NH`
D. `C_(6)H_(5)NH_(2)`

Explanation

To determine which of the given compounds is least soluble in water at 298 K, we need to analyze the solubility of each compound based on their structure and the presence of functional groups.

### Step-by-Step Solution:

1. **Identify the Compounds**:
Let's denote the compounds as follows:
- A: Methylamine (CH₃NH₂)
- B: N,N-Dimethylmethanamine (C₂H₇N)
- C: N-Methylmethanamine (C₃H₉N)
- D: Aniline (C₆H₅NH₂)

2. **Understand Solubility Principles**:
- Amines generally have good solubility in water due to their ability to form hydrogen bonds with water molecules.
- Smaller amines are typically more soluble because they have less hydrophobic character.
- As the size of the hydrocarbon (alkyl or aryl group) increases, the solubility in water decreases due to increased hydrophobicity.

3. **Analyze Each Compound**:
- **Methylamine (A)**: A small amine with one methyl group, which is water-soluble due to hydrogen bonding.
- **N,N-Dimethylmethanamine (B)**: Contains two methyl groups, still relatively small and water-soluble.
- **N-Methylmethanamine (C)**: Contains one methyl group and one larger hydrocarbon part, but still small enough to be water-soluble.
- **Aniline (D)**: An aromatic amine with a benzene ring, which is a large hydrocarbon part. The hydrophobic character of the benzene ring makes it less soluble in water.

4. **Conclusion**:
Based on the analysis, Aniline (D) is the compound with the largest hydrophobic character due to the presence of the benzene ring, making it the least soluble in water at 298 K.

### Final Answer:
The compound that is least soluble in water at 298 K is **D: Aniline**.

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