Question

Which of the following is the strongest base?

• A. Aniline
• B. N-methyl aniline
• C. O-methyl aniline
• D. Benzylamine

Hint:

Key Exam Tip:
Basicity Order of Amines:
• Aliphatic amines (like alkylamines, benzylamine) are generally more basic than aromatic amines (like aniline).
• Electron-donating groups (like -CH$_3$, -OCH$_3$) on the aromatic ring increase basicity of aromatic amines.
• Electron-withdrawing groups (like -NO$_2$, -Cl) on the aromatic ring decrease basicity of aromatic amines.
• Resonance delocalization of the lone pair on nitrogen significantly reduces basicity. Benzylamine has a localized lone pair on nitrogen due to the intervening CH$_2$ group, making it the strongest base here.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Basicity in amines is determined by the availability of the lone pair of electrons on the nitrogen atom to accept a proton (\( H^+ \)).
Step 2: Detailed Explanation:
1. Aromatic Amines (Aniline and its derivatives): In Aniline (\( C_6H_5NH_2 \)), the lone pair on nitrogen is in resonance with the pi-electron system of the benzene ring. This delocalization makes the electrons less available for donation to a proton.
2. Inductive Effects: In N-methyl aniline, the \( -CH_3 \) group adds electron density (\( +I \) effect), but the lone pair is still delocalized. In o-methyl aniline, steric hindrance also plays a role.
3. Benzylamine (\( C_6H_5CH_2NH_2 \)): Although it contains a benzene ring, the \( -NH_2 \) group is attached to an \( sp^3 \) carbon (\( -CH_2- \)), not directly to the ring.
4. Because there is a methylene bridge, the lone pair on Nitrogen cannot participate in resonance with the ring. It remains localized.
Step 3: Final Answer:
Since the lone pair in benzylamine is localized and not delocalized by resonance, it is much more available for protonation. Thus, benzylamine is a significantly stronger base than aniline or its substituted derivatives.