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

To determine the strongest base among the given options, we need to consider the factors that affect basicity. Basicity is related to the availability of the lone pair of electrons on the nitrogen atom to accept a proton (H$^+$). Let's analyze each option: (1) Aniline (C$_6$H$_5$NH$_2$): In aniline, the lone pair of electrons on the nitrogen atom is delocalized into the benzene ring through resonance. This delocalization makes the lone pair less available to accept a proton, thus reducing its basicity. The resonance structures show the electron density being spread across the ring. (2) N-methyl aniline (C$_6$H$_5$NHCH$_3$): In N-methyl aniline, one hydrogen atom on the nitrogen is replaced by a methyl group (-CH$_3$). The methyl group is an electron-donating group (EDG) by inductive effect. This inductive effect pushes electron density towards the nitrogen atom, making the lone pair slightly more available for protonation compared to aniline. However, the lone pair is still delocalized into the benzene ring. (3) O-methyl aniline (o-toluidine) (CH$_3$-C$_6$H$_4$-NH$_2$): In o-methyl aniline, a methyl group is attached to the ortho position of the benzene ring. The methyl group is an electron-donating group (EDG) via the inductive effect. This inductive effect increases the electron density on the nitrogen atom. However, the ortho position also introduces steric hindrance, which can slightly impede the approach of a proton. Despite steric effects, the inductive donation generally increases basicity compared to aniline. Compared to N-methyl aniline, the electron donation is to the ring, indirectly affecting the nitrogen. (4) Benzylamine (C$_6$H$_5$CH$_2$NH$_2$): In benzylamine, the amino group (-NH$_2$) is attached to a methylene (-CH$_2$-) group, which is then attached to the benzene ring. The lone pair of electrons on the nitrogen atom is primarily localized on the nitrogen and is not directly involved in resonance with the benzene ring. The -CH$_2$- group is an electron-donating group by induction. This inductive effect increases the electron density on the nitrogen atom, making its lone pair more available for protonation. Compared to aniline and its derivatives where the lone pair is delocalized into the ring, the lone pair in benzylamine is much more localized and available. Comparison:
• Aniline is the least basic due to significant resonance delocalization of the lone pair.
• N-methyl aniline and O-methyl aniline are more basic than aniline due to the electron-donating inductive effect of the methyl group. However, resonance in the aniline part of the molecule still reduces basicity.
• Benzylamine is an aliphatic amine (attached via a CH$_2$ group) and therefore its lone pair is significantly more localized and available for protonation. The electron-donating inductive effect of the methylene group further enhances its basicity. Therefore, benzylamine is the strongest base among the given options. Final Answer: \(\boxed{4}\)