Among the following compounds the weakest base is

Options

A. `C_(6)H_(5)NH_(2)`
B. `p-NO_(2)C_(6)H_(5)NH_(2)`
C. `m-NO_(2)C_(6)H_(4)NH_(2)`
D. `C_(6)H_(5)CH_(2)NH_(2)`

Answer

To determine which among the given compounds is the weakest base, we need to analyze the structures of the compounds and their ability to donate a lone pair of electrons to a proton (H⁺). The strength of a base is influenced by factors such as resonance and inductive effects.

### Step-by-Step Solution:

1. **Identify the Compounds**:
Let's denote the compounds as follows:
- Compound 1: Aniline (C₆H₅NH₂)
- Compound 2: Para-nitroaniline (C₆H₄(NH₂)(NO₂))
- Compound 3: Meta-nitroaniline (C₆H₄(NH₂)(NO₂))
- Compound 4: Benzylamine (C₆H₅CH₂NH₂)

2. **Analyze Resonance Effects**:
- **Compound 1 (Aniline)**: The lone pair on the nitrogen can participate in resonance with the benzene ring, making it less available to bond with H⁺.
- **Compound 2 (Para-nitroaniline)**: The nitro group (NO₂) is a strong electron-withdrawing group due to resonance and inductive effects. It pulls electron density away from the nitrogen, making the lone pair less available for bonding with H⁺.
- **Compound 3 (Meta-nitroaniline)**: The nitro group is still electron-withdrawing, but its position (meta) means it has less resonance interaction with the nitrogen compared to the para position. Thus, the lone pair is somewhat more available than in para-nitroaniline.
- **Compound 4 (Benzylamine)**: The lone pair on the nitrogen is not involved in resonance with the benzene ring, making it more available for protonation compared to aniline.

3. **Consider Inductive Effects**:
- The nitro group in both para and meta positions exerts a -I (inductive) effect, which decreases the basicity of the compounds.
- The para-nitroaniline experiences a stronger -R (resonance) effect compared to meta-nitroaniline, leading to a greater decrease in basicity.

4. **Comparison of Basicity**:
- **Aniline** has some basicity due to resonance but is weakened by resonance.
- **Benzylamine** is a stronger base compared to aniline because its lone pair is not involved in resonance.
- **Para-nitroaniline** is the weakest base due to the strong -R effect of the nitro group, which significantly reduces the availability of the lone pair.
- **Meta-nitroaniline** is less basic than aniline but more basic than para-nitroaniline due to the lesser effect of the nitro group.

5. **Conclusion**:
Based on the above analysis, **the weakest base among the given compounds is para-nitroaniline**.