Which is the weakest base :

Options

A.
B.
C.
D. `CH_(3)NH_(2)`

Answer

To determine which of the given compounds is the weakest base, we need to analyze the basicity of each compound based on their structure and hybridization. Here is a step-by-step solution:

### Step 1: Identify the Compounds
The compounds mentioned are:
1. Aniline (C6H5NH2)
2. NH (Ammonia)
3. Cyclohexylamine (C6H11NH2)
4. Methylamine (CH3NH2)

### Step 2: Understand Basicity
Basicity refers to the ability of a compound to donate a lone pair of electrons. The stronger the base, the more readily it can donate its lone pair. Basicity is influenced by:
- Electronegativity of the atoms involved
- Hybridization of the nitrogen atom

### Step 3: Analyze Hybridization and S Character
- **Aniline**: The nitrogen in aniline is attached to a benzene ring (C6H5), which is sp² hybridized. The sp² hybridization has 33% s character.
- **Ammonia (NH3)**: The nitrogen in ammonia is sp³ hybridized, with 25% s character.
- **Cyclohexylamine**: Similar to ammonia, the nitrogen is also sp³ hybridized (25% s character).
- **Methylamine**: The nitrogen in methylamine is also sp³ hybridized (25% s character).

### Step 4: Compare S Character
- Aniline (sp²): 33% s character
- Ammonia (sp³): 25% s character
- Cyclohexylamine (sp³): 25% s character
- Methylamine (sp³): 25% s character

### Step 5: Determine Basicity
Since basicity is inversely proportional to s character:
- Aniline has the highest s character (33%) and thus the weakest ability to donate its lone pair, making it the weakest base among the four compounds.
- The other three (ammonia, cyclohexylamine, and methylamine) have lower s character (25%) and are stronger bases compared to aniline.

### Conclusion
The weakest base among the given options is **Aniline**.

---