Problem 72 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following, the strongest base is (a) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{NH}_{2}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2}-\mathrm{NH}_{2}\) (c) \(\mathrm{p}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) (d) \(\mathrm{m}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\)

Step-by-Step Solution

Answer

(b) \( \mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\mathrm{-NH}_2\) is the strongest base.

Step 1: Identify the Functional Groups

First, identify the amine group (\(\mathrm{-NH}_2\)) in each compound, as this group is responsible for basicity due to its lone pair of electrons.

Step 2: Compare Electron-withdrawing and Electron-donating Groups

Assess how other functional groups attached to the aromatic ring influence the amine group's basicity. Electron-withdrawing groups, such as \(\mathrm{-NO}_2\), decrease basicity by pulling electron density away, whereas electron-donating groups increase basicity.

Step 3: Analyze Each Compound

(a) \(\mathrm{C}_6\mathrm{H}_3\mathrm{NH}_2\) has no additional groups influencing basicity significantly beyond typical aromatic effects. (b) \(\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\mathrm{-NH}_2\) has a benzyl group, which is slightly electron-donating, increasing the basicity of the amine. (c) and (d), both contain \(\mathrm{-NO}_2\) groups that withdraw electron density, significantly reducing the basicity.

Step 4: Determine the Strongest Base

The compound in option (b) contains a benzyl group that increases electron density on the nitrogen atom, making \( \mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\mathrm{-NH}_2\) the strongest base among the given options.

Key Concepts

Electron-withdrawing Groups

Electron-withdrawing groups are crucial players in the chemistry of amines, especially when discussing their basicity. These groups, such as the nitro group \(\mathrm{-NO}_2\), act by pulling electron density away from the amine nitrogen atom.
This reduction in electron density on nitrogen decreases the availability of the lone pair of electrons for bonding, which in turn weakens the amine's basicity.

• They are often electron-deficient groups with elements like nitrogen, oxygen, or halogens.
• They create a partial positive charge on the atom they are attached to, which attracts electrons from nearby atoms, including the nitrogen of an amine group.

When an electron-withdrawing group is attached to an aromatic ring along with an amine group, the basic nature of the amine decreases significantly. For instance, in the case of \(\mathrm{p}-\mathrm{NO}_2-\mathrm{C}_6 \mathrm{H}_4 \mathrm{NH}_2\) and \(\mathrm{m}-\mathrm{NO}_2-\mathrm{C}_6 \mathrm{H}_4 \mathrm{NH}_2\), the presence of the \(\mathrm{-NO}_2\) group drastically diminishes their ability to function as bases.

Electron-donating Groups

Electron-donating groups provide a boost to the basicity of amines by increasing electron density on the nitrogen atom. A classic example is the benzyl group \(\mathrm{-CH}_2\-\), which acts as a mild electron-donating group. It fortifies the nitrogen's lone pair availability, making the entire molecule more basic.
Such groups, when bound to aromatic rings, can either donate electrons through resonance or inductively push electron density towards the nucleophilic site.

• Typically include alkyl groups, hydroxyl \(\mathrm{-OH}\), and amino \(\mathrm{-NH}_2\) groups.
• These groups are effective due to their ability to donate electrons either through resonance or inductive mechanisms.

As seen in \(\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 -\mathrm{NH}_2\), the benzyl group's electron-donating effect increases the basicity properties of the amine considerably. It's this increase in electron availability that makes \(\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 -\mathrm{NH}_2\) the strongest base among the options provided.

Functional Groups Influence on Basicity

The interaction between amines and functional groups plays a vital role in determining basicity. Amines, with their characteristic lone pair of electrons on nitrogen, can interact differently based on what groups are present on their structure. This interaction either enhances or diminishes their ability to act as bases.
The functional group's position and type are key. Functional groups can modulate the basicity of amines by influencing the electron density on the nitrogen atom.

• Electron-withdrawing groups reduce electron density, thereby reducing basicity.
• Electron-donating groups increase electron density and thereby increase basicity.

Ultimately, the overall molecular environment defined by these groups can either increase the nucleophilic character of the nitrogen or render it less reactive towards protonation. By understanding these interactions, we can predict and even manipulate the basicity of different amine structures, tailoring compounds for specific chemical reactions and applications in synthetic chemistry.