Problem 84 - A Complete Resource Book in Chemistry for JEE Main

Question

Aniline is less basic than (a) 4-nitroaniline (b) 4 -aminobenzaldehyde (c) anilinium hydrochloride (d) dimethyl amine

Step-by-Step Solution

Answer

Aniline is less basic than dimethylamine (d).

Step 1: Understanding Aniline

Aniline is a primary aromatic amine, and its basicity is influenced by the resonance of the lone pair of electrons on the nitrogen with the benzene ring. This delocalization reduces the electron density on the nitrogen, making it less available for protonation.

Step 2: Comparing with 4-Nitroaniline

4-Nitroaniline has a nitro group at the para position, which is a strong electron-withdrawing group that reduces the electron density on the nitrogen even more than aniline itself. Thus, aniline is more basic than 4-nitroaniline.

Step 3: Comparing with 4-Aminobenzaldehyde

In 4-aminobenzaldehyde, an aldehyde group is present at the para position, which pulls electron density from the benzene ring, albeit less strongly than a nitro group. This makes 4-aminobenzaldehyde less basic than aniline.

Step 4: Comparing with Anilinium Hydrochloride

Anilinium hydrochloride is aniline protonated with an additional HCl group, existing as a salt. It is definitely less basic since it is already in the cationic form, prone to lose its basicity.

Step 5: Comparing with Dimethylamine

Dimethylamine is an aliphatic amine with no involvement of resonance and has greater electron density around the nitrogen atom, making it more basic than aniline.

Key Concepts

Aniline

Aniline is an aromatic amine where a benzene ring is directly attached to an amino group (\(-NH_{2}\)). This connection influences its basic properties. One of the key characteristics of aniline is the resonance interaction of the nitrogen’s lone pair of electrons with the benzene ring.

This electron delocalization spreads the electron density over the aromatic system. As a result, the lone pair on the nitrogen is less available to accept protons, reducing its basicity.

The aromatic nature distinguishes aniline from aliphatic amines, like ethylamine, due to this unique resonance effect.

Resonance in Aromatic Amines

Resonance in aromatic amines like aniline significantly impacts their chemical behavior. This phenomenon occurs when the lone pair of electrons on the nitrogen interacts with the electrons in the benzene ring.

Because of resonance, electrons are distributed across the delocalized system, stabilizing the molecule but decreasing the availability of the lone pair for reaction with acids.

• This results in a lower tendency for protonation compared to aliphatic amines.
• The balance between the aromatic ring and the electron donation by nitrogen is crucial.

Understanding this interplay is key to predicting basicity in such amines.

Electron-withdrawing Groups

Electron-withdrawing groups, when attached to the benzene ring, can further affect the basicity of aromatic amines. Groups like the nitro group in 4-nitroaniline withdraw electron density through the ring via resonance and inductive effects.

These groups enhance the deficiency of electrons around the nitrogen atom, making it even less available for protonation.

• This results in amines being less basic compared to those with electron-donating groups.
• Even structural variances in position, like ortho, meta, or para, influence the overall basicity.

Such variations are crucial when comparing different amines.

Comparative Basicity

Comparative basicity involves evaluating the ability of different amine compounds to accept protons.

Factors such as resonance, electron-donating, or withdrawing groups, and the hybridization state of nitrogen greatly affect basicity levels.

• Aliphatic amines, like dimethylamine, lack electron delocalization, resulting in higher electron density on nitrogen and stronger basicity.
• Aromatic amines are often less basic due to the influence of resonance and substituents like nitro groups, which exacerbate electron deficiency.

By understanding these influences, one can better predict reactions and behavior of different amine compounds in chemical solutions.

Protonation

Protonation is the process where an atom gains a proton, forming a positive ion. In the context of amines, it occurs when the nitrogen atom accepts a proton.

For aniline, protonation is less favorable due to the delocalization of nitrogen's lone pair with the benzene ring, weakening its proton-accepting ability.

• This is why the formation of anilinium ion, such as in anilinium hydrochloride, showcases a reduced basic character.
• In comparison, protonation is more efficient in aliphatic amines, which lack the electron-delocalizing effects of aromatic systems.

Understanding protonation is fundamental in assessing the reactivity and strength of basicity in amines.