Problem 146 - A Complete Resource Book in Chemistry for JEE Main

Question

Amongst the following, the most basic compound is (a) benzylamine (b) aniline (c) acetanilide (d) p-nitroaniline

Step-by-Step Solution

Answer

Benzylamine is the most basic compound among the options.

Step 1: Understanding Basicity

Basicity refers to the tendency of a compound to accept protons. In organic chemistry, this often relates to the availability of a lone pair of electrons on nitrogen atoms that can bind to a proton. The less hindered the lone pair, the more basic the compound.

Step 2: Analyzing Benzylamine

Benzylamine ( exttt{C}_6 exttt{H}_5 exttt{-CH}_2 exttt{-NH}_2) has an amino group attached to a benzyl group. The electron-donating nature of the benzyl group helps stabilize the positive charge on the nitrogen atom when it accepts a proton, making benzylamine a relatively strong base.

Step 3: Analyzing Aniline

Aniline ( exttt{C}_6 exttt{H}_5 exttt{-NH}_2) consists of an amino group directly attached to a phenyl ring. The phenyl ring resonates the lone pair of electrons from the nitrogen into the ring, reducing its availability to bond with protons. Therefore, aniline is less basic than benzylamine.

Step 4: Analyzing Acetanilide

Acetanilide ( exttt{C}_6 exttt{H}_5 exttt{-NHCOCH}_3) has an amide linkage, significantly reducing the basicity of the nitrogen due to the electron-withdrawing nature of the carbonyl group and delocalization of lone pair electrons into the carbonyl.

Step 5: Analyzing p-Nitroaniline

p-Nitroaniline ( exttt{C}_6 exttt{H}_5 exttt{-NH}_2 exttt{-NO}_2) contains a nitro group, which is a strong electron-withdrawing group. This further decreases the availability of nitrogen's lone pair, making it the least basic among the given compounds.

Step 6: Conclusion on Basicity

By comparing these compounds, benzylamine, with its unencumbered nitrogen and electron-stabilizing benzyl group, is the most basic. Aniline is less basic due to resonance stabilization, acetanilide is significantly less basic due to electron-withdrawing and resonance effects, and p-nitroaniline is the least basic owing to the strong electron-withdrawing nitro group.

Key Concepts

Understanding Benzylamine's Basicity

Benzylamine is known for its relatively high basicity compared to other related compounds. This is due to the structure of benzylamine, which includes a benzyl group attached to an amino group. The presence of the benzyl group is crucial for its basic nature.

• The benzyl group is electron-donating. This means it helps to stabilize the nitrogen atom when it accepts a proton.
• The lone pair of electrons on the nitrogen atom in benzylamine is quite accessible because it's not redistributed into a ring, making it more available for bonding with protons.

These factors combined make benzylamine a strong base among the compounds considered.

Exploring Aniline's Limited Basicity

Aniline is an aromatic amine where the amino group is directly connected to a phenyl ring, significantly affecting its basicity. Aniline's structure contributes to its moderate basic nature as follows:

• The lone pair of electrons on the nitrogen can resonate or delocalize into the benzene ring.
• Because of this resonation, the electron pair is less available to bond with protons.

This resonance effect means aniline is less basic compared to benzylamine, as the electron pair on the nitrogen is less free to engage with additional protons.

Evaluating Acetanilide's Basicity

Acetanilide offers a look at how the introduction of functional groups can affect the basicity of a compound. Here, an amide linkage is introduced, considerably changing its basic character:

• This amide linkage includes a carbonyl group which is electron-withdrawing.
• The lone pair of electrons on the nitrogen can also be delocalized into this carbonyl group, further diminishing their availability to bond with protons.

Because of these factors, acetanilide is much less basic than both benzylamine and aniline.

Analyzing the Weak Basicity of p-Nitroaniline

In p-nitroaniline, the structure is significantly affected by a nitro group. Here's how it influences its basic nature:

• The nitro group is one of the strongest electron-withdrawing groups.
• It pulls away electron density from the nitrogen atom, making its lone pair electrons even less available for bonding with protons.

Therefore, p-nitroaniline has the weakest basicity amongst benzylamine, aniline, and acetanilide, due to the severe electron-withdrawing effect of the nitro group.