Problem 188 - A Complete Resource Book in Chemistry for JEE Main

Question

Amongst the following the most basic compound is [2005] (a) benzylamine (b) aniline (c) acetanilide (d) p-nitroanil

Step-by-Step Solution

Answer

Benzylamine is the most basic compound.

Step 1: Understand 'Basicity' in Chemistry

Basicity in chemistry refers to the ability of a compound to accept protons (H⁺ ions). A compound's basicity is influenced by factors like the availability of lone pairs on nitrogen atoms and the compound's structure, which can either donate electrons into or withdraw them from the nitrogen atom.

Step 2: Analyze Each Compound

(a) Benzylamine: Has an aliphatic nature; the lone pair on nitrogen is readily available for protonation. (b) Aniline: Has a nitrogen atom attached to an aromatic benzene ring, reducing availability of its lone pair due to partial delocalization into the ring. (c) Acetanilide: The lone pair on nitrogen is involved in resonance with the acyl group, reducing its availability. (d) p-Nitroaniline: The nitro group withdraws electrons through resonance, making nitrogen less able to donate electrons.

Step 3: Compare Basicity

Benzylamine is more basic than the others due to the availability of its lone pair for proton donation without the influence of resonance or electron-withdrawing groups. Aniline, acetanilide, and p-nitroaniline are less basic due to resonance with the benzene ring or electron-withdrawing effects.

Key Concepts

Nitrogen Lone Pair Availability

In organic chemistry, the basicity of a compound is often linked to the availability of a nitrogen atom's lone pair of electrons. This lone pair is crucial because it can readily accept protons (H⁺ ions), driving the compound's basic properties.
Benzylamine, an aliphatic compound, showcases nitrogen with a fully available lone pair. This means there are no interfering factors, such as aromatic rings, to reduce the nitrogen's ability to attract a proton. As a result, benzylamine is a strong base.
On the other hand, in compounds like aniline, the nitrogen's lone pair interacts with a benzene ring. This interaction, known as delocalization, limits the lone pair’s availability. Essentially, whenever the lone pair is engaged in stabilizing interactions, its potential for accepting protons diminishes.

Resonance Effects

Resonance effects play a crucial role in determining base strength in molecules containing nitrogen atoms. Resonance involves the delocalization of electrons across adjacent atoms and can significantly impact lone pair availability.
For example, in aniline, the nitrogen lone pair participates in resonance with the aromatic benzene ring. This delocalization weakens its ability to attract additional protons, thus making aniline less basic compared to aliphatic amines.

• Aniline: Nitrogen's lone pair partially participates in the benzene ring's resonance.
• Acetanilide: Here, the nitrogen lone pair is involved in resonance with the carbonyl group of the acyl moiety, further limiting nitrogen’s basicity.

Understanding resonance and its effects is essential, as compounds with extensive resonance often display reduced basicity due to limited electron availability at the nitrogen atom.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are functional groups that decrease electron density on nearby atoms through inductive or resonance effects. These groups are critical in assessing the basicity of compounds with nitrogen.
In the case of p-nitroaniline, the nitro group ( NO_2 ) is a powerful electron-withdrawing group. It exerts its influence by pulling electron density away from the nitrogen atom through resonance. This reduction renders the nitrogen less likely to donate its lone pair to incoming protons.
Generally speaking, when a nitrogen atom is in proximity to EWGs, its lone pair is more likely to be less available for bonding with protons, thus reducing the basicity of the compound. Accurately assessing the presence and impact of these groups is vital for understanding and predicting the behavior of nitrogen-containing bases in reactions.