Question

Which among the following compounds is a weakest base?

• A. Phenylmethanamine
• B. N-Methylaniline
• C. Benzenamine
• D. N,N-Dimethylaniline

Hint:

When ranking the basicity of aniline derivatives, remember this straightforward trend: basic strength increases with the number of alkyl groups on the nitrogen atom because of their electron-donating inductive effect. Therefore, the ordering goes: $\text{Aniline (weakest)} < \text{N-Methylaniline} < \text{N,N-Dimethylaniline}$.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We are given four different amine compounds (one aliphatic-aromatic and three purely aromatic amines). We need to determine which molecule acts as the weakest base.

Step 2: Key Formula or Approach:
The basic strength of an amine depends on the availability of the lone pair of electrons on the nitrogen atom to accept a proton ($\text{H}^+$). If the lone pair is delocalized into an aromatic ring via resonance, it becomes less available, severely dropping the basic strength. For aromatic amines, electron-donating alkyl groups attached to the nitrogen atom increase electron density through $+I$ (inductive) effects, slightly boosting basicity compared to the unsubstituted aniline.

Step 3: Detailed Explanation:
Let's analyze each option systematically: 1.

Phenylmethanamine (Benzylamine, $\text{C}_6\text{H}_5\text{CH}_2\text{NH}_2$): The $-\text{NH}_2$ group is attached to an $sp^3$ hybridized aliphatic carbon. The nitrogen lone pair is not in conjugation with the benzene ring, making it highly localized and available. This is the strongest base in the group.
2.

Benzenamine (Aniline, $\text{C}_6\text{H}_5\text{NH}_2$): The lone pair on nitrogen is in direct conjugation with the $\pi$-system of the benzene ring. It gets heavily delocalized into the ring via resonance, which dramatically lowers its availability to capture protons.
3.

N-Methylaniline ($\text{C}_6\text{H}_5\text{NHCH}_3$): Although the lone pair undergoes resonance delocalization into the ring, the presence of one methyl group ($-\text{CH}_3$) attached directly to the nitrogen provides an electron-donating $+I$ effect, which increases the electron density on nitrogen relative to plain aniline.
4.

N,N-Dimethylaniline ($\text{C}_6\text{H}_5\text{N(CH}_3)_2$): Here, two electron-donating methyl groups exert an even stronger combined $+I$ inductive effect on the nitrogen atom, making it more basic than both aniline and N-methylaniline.
Comparing the three aniline derivatives, unsubstituted benzenamine (aniline) lacks any reinforcing $+I$ alkyl groups to compensate for its resonance losses, leaving its nitrogen atom the least electron-dense. Thus, benzenamine is the weakest base.

Step 4: Final Answer:
The weakest base among the options is benzenamine, matching option (C).