All aliphatic amines are more basic than ammonia but due to delocalization of lone pair of electrons of the nitrogen atom on the benzene ring, aniline is a weaker base than ammonia. The basic strength of the substituted anilines, however, depends upon the nature of the substituents. Whereas electron-donating groups tend to increase, electron-withdrawing groups tend to decrease the basic strength. the base strenghening effect of the electron-donating groups and base weakening effect of the electron-withdrawing groups is, however, more pronounced at p-than at m-position. However, due to ortho effect, o-substituted anilines are weaker bases than anilines regardless of the nature of substituent whether electron-donating or electron-withdrawing.
Q. Among the following, weakest base is

Options

A. `C_(6)H_(5)CH_(2)NH_(2)`
B. `C_(6)H_(5)CH_(2)NHCH_(3)`
C. `O_(2)N-CH_(2)NH_(2)`
D. `CH_(3)NHCHO`

Explanation

To determine the weakest base among the given options of substituted anilines, we will analyze the basicity of each compound based on the effects of substituents and resonance.

### Step-by-Step Solution:

1. **Understanding Basicity**:
- Basicity in amines is influenced by the availability of the lone pair of electrons on the nitrogen atom. The more available the lone pair, the stronger the base.
- Aliphatic amines are generally more basic than ammonia due to the electron-donating effect of alkyl groups.

2. **Analyzing Aniline**:
- Aniline (C6H5NH2) is less basic than ammonia because the lone pair on nitrogen is delocalized into the benzene ring, reducing its availability for protonation.

3. **Effect of Substituents**:
- **Electron-Donating Groups (EDGs)**: Increase basicity by donating electron density to the nitrogen atom, making the lone pair more available.
- **Electron-Withdrawing Groups (EWGs)**: Decrease basicity by pulling electron density away from the nitrogen atom, making the lone pair less available.

4. **Position of Substituents**:
- The effect of substituents is more pronounced at the para position than at the meta position.
- Ortho-substituted anilines are generally weaker bases due to steric hindrance and the ortho effect, regardless of whether the substituent is electron-donating or withdrawing.

5. **Evaluating Each Option**:
- **Option 1: CH2NH2** (Alkyl amine): The lone pair is not delocalized, making it a strong base.
- **Option 2: CH2NH(CH3)**: The methyl group is an EDG, enhancing basicity. Thus, it is also a strong base.
- **Option 3: CH2NH2-NO2**: The nitro group is an EWG, which decreases basicity. This compound is weaker than the previous two.
- **Option 4: CH3NHCHO**: The lone pair on nitrogen is involved in resonance with the carbonyl group (C=O), significantly reducing its availability for protonation. This makes it the weakest base.

6. **Conclusion**:
- Among the options provided, the weakest base is **Option 4: CH3NHCHO** due to the resonance effect of the carbonyl group that delocalizes the lone pair on nitrogen.

### Final Answer:
The weakest base among the given options is **Option 4: CH3NHCHO**.

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