Problem 5 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Consider the relative basicity of these three amines. Which statement is true? (a) Cyclohexylamine is the strongest base and aniline is the weakest base (b) Cyclohexylamine is the strongest base and 4 -nitroaniline is the weakest base (c) Aniline is the strongest base and cyclohexylamine is the weakest base (d) Nitroaniline is the strongest base and cyclohexylamine is the weakest base

Step-by-Step Solution

Answer

Option (b) is true: Cyclohexylamine is the strongest base and 4-nitroaniline is the weakest base.

Step 1: Identify the Amines

The amines in question are cyclohexylamine, aniline, and 4-nitroaniline. Each has different structures and substituents that affect their basicity.

Step 2: Understand Basicity

The basicity of an amine is determined by its ability to donate a pair of electrons. More electron density on the nitrogen atom means higher basicity.

Step 3: Analyze Cyclohexylamine

Cyclohexylamine is a simple aliphatic amine with a cyclohexane ring. Its nitrogen is not delocalized with any aromatic system, making it a stronger electron donor.

Step 4: Analyze Aniline

In aniline, the nitrogen's lone pair is partially delocalized into the benzene ring, reducing the availability of these electrons for protonation, and thus reducing basicity in comparison to aliphatic amines.

Step 5: Analyze 4-Nitroaniline

4-Nitroaniline contains a nitro group, which is a strong electron-withdrawing group. This further reduces the electron density on nitrogen, making it even less basic than aniline.

Step 6: Compare Basicities

Cyclohexylamine, not having its electron pair delocalized or withdrawn, will be the strongest base, while 4-nitroaniline, due to the nitro group, will be the weakest.

Step 7: Choose the Correct Statement

From the analysis, cyclohexylamine is the strongest base due to no delocalization or electron-withdrawing groups, and 4-nitroaniline is the weakest due to the electron-withdrawing nitro group.

Key Concepts

Cyclohexylamine

Cyclohexylamine is an aliphatic amine, characterized by its structure: a six-membered cyclohexane ring attached to an amine group (-NH2). This particular configuration is crucial in defining its basicity.
Unlike aromatic amines, cyclohexylamine does not share its lone pair of electrons with any aromatic system. This lack of delocalization signifies that the nitrogen atom has high electron density, making it a potent electron donor.
The ability to donate electrons is directly linked to the strength of an amine's basicity. Therefore, cyclohexylamine is a stronger base compared to its aromatic counterparts, as its nitrogen is not influenced by additional electron-withdrawing or delocalizing factors.

Aniline

Aniline is an aromatic amine where the amine group is attached directly to a benzene ring. This connection impacts its basicity significantly.
In aniline, the nitrogen’s lone pair can participate in resonance with the benzene ring. This means the electrons are shared between the nitrogen and the aromatic system, leading to partial delocalization.
This resonance stability lowers the electron density on the nitrogen, meaning it is less available to bond with protons and act as a base. Thus, while aniline is still basic, it is less so than aliphatic amines like cyclohexylamine.

Nitroaniline

4-Nitroaniline features an additional group, the nitro group (-NO2), in para position to the amine group on the benzene ring.
The nitro group is known for being a strong electron-withdrawing group due to its ability to stabilize negative charge through its resonance structures. It pulls electron density away from the nitrogen, further reducing its basicity.
Hence, among the given amines, 4-nitroaniline is the weakest base. The electron-withdrawing property of the nitro group means the nitrogen atom is even less likely to donate its electrons for bond formation, rendering it significantly less basic compared to cyclohexylamine and even aniline.