Problem 136 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following amines: (1) \(\mathrm{C}_{6} \mathrm{H}_{3}-\mathrm{NH}_{2}\) (2) \(0-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4}-\mathrm{NH}_{2}\) (3) \(\mathrm{m}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4}-\mathrm{NH}_{2}\) (4) \(\mathrm{p}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4}-\mathrm{NH}_{2}\) Arrange these compounds in decreasing order of basicity. (a) \(1>2>3>4\) (b) \(2>1>3>4\) (c) \(1>3>4>2\) (d) \(1>3>2>4\)

Step-by-Step Solution

Answer

(d) 1 > 3 > 2 > 4

Step 1: Understanding Basicity in Amines

Basicity in amines is largely determined by the availability of the lone pair of electrons on the nitrogen atom to accept a proton. In aromatic amines, the electron-withdrawing or electron-donating effects of substituents can affect this availability.

Step 2: Analyze the Structure of Aniline

The compound (1) is \(\mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{NH}_{2}\), also known as aniline. This compound has a basic lone pair on nitrogen, and no substituents that significantly withdraw electron density, making its basicity fairly standard among the group.

Step 3: Analyze the Structure of o-Nitroaniline

The compound (2) is \(0-\mathrm{NO}_{2}-\mathrm{C}_{6}\mathrm{H}_{4}-\mathrm{NH}_{2}\), which is an aniline with a nitro group in the ortho position. The nitro group is strongly electron-withdrawing and decreases the availability of the nitrogen lone pair by resonance and induction, significantly reducing the basicity.

Step 4: Analyze the Structure of m-Nitroaniline

The compound (3) is \(m-\mathrm{NO}_{2}-\mathrm{C}_{6}\mathrm{H}_{4}-\mathrm{NH}_{2}\), which places the nitro group at the meta position. The electron-withdrawing effect is less directly involved with the nitrogen lone pair in this position, but it still decreases basicity more than an unsubstituted aniline.

Step 5: Analyze the Structure of p-Nitroaniline

The compound (4) is \(p-\mathrm{NO}_{2}-\mathrm{C}_{6}\mathrm{H}_{4}-\mathrm{NH}_{2}\), which has the nitro group at the para position. This placement allows for maximum resonance interaction between the nitro group and the lone pair on nitrogen, thus reducing basicity the most.

Step 6: Compare and Arrange Basicity Levels

Based on the electron-withdrawing effects and positions of the nitro groups, aniline (1) is the most basic, followed by m-nitroaniline (3), then o-nitroaniline (2), and the least basic is p-nitroaniline (4). Thus, the order is: \(1 > 3 > 2 > 4\).

Key Concepts

Electron-withdrawing effects

Electron-withdrawing effects play a crucial role in determining the basicity of amines. Basicity in amines refers to the ability of the nitrogen atom within the amine group \((\mathrm{NH}_{2})\) to accept a proton. A lone pair of electrons on the nitrogen atom is responsible for this proton acceptance. If any substituents on the aromatic ring are electron-withdrawing, they tend to pull electron density away from the nitrogen. This action makes the lone pair on nitrogen less available for protonation, thereby reducing the basicity of the compound.

Electron-withdrawing groups (EWGs) include substituents such as the nitro group \((\mathrm{NO}_{2})\). These groups can withdraw electrons either through inductive effects or resonance effects. Inductive effects involve the transmission of the electron-withdrawing character through the sigma bonds, while resonance effects involve electron delocalization through pi bonds, which can significantly affect the molecule's basicity. By understanding these effects, we can predict how each molecule's structure influences its basicity.

Aromatic amines

Aromatic amines, such as aniline, consist of an amine group attached to an aromatic ring like benzene. The basicity of aromatic amines differs notably from aliphatic amines due to the interaction between the lone pair of electrons on nitrogen and the aromatic system. In amines like aniline (compound 1: \(\mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{NH}_{2}\)), there are no strong electron-withdrawing groups directly attached to the nitrogen. Thus, the lone pair on the nitrogen is relatively more available to accept a proton, making it more basic compared to its substituted counterparts.

However, when strong EWGs such as nitro groups are attached to the aromatic ring at different positions relative to the amine group, they alter the basicity by affecting electron availability. The position of substituents such as ortho, meta, and para can significantly alter the electronic environment of the nitrogen, influencing the basicity. For example, the ortho (o) and para (p) positions allow more significant interaction of the substituent with the nitrogen's lone pair, unlike the meta (m) position, which has a relatively less direct interaction. Understanding these differences helps us evaluate and compare the basicity of different aromatic amines.

Nitro group influence

The presence and position of the nitro group \((\mathrm{NO}_{2})\) significantly influence the basicity of aromatic amines. Nitro groups are powerful electron-withdrawing groups because of their ability to withdraw electron density both via inductive and resonance effects. In compounds such as o-nitroaniline (compound 2), m-nitroaniline (compound 3), and p-nitroaniline (compound 4), the nitro group's influence is critical in determining basicity.

• **Ortho (o) position**: In o-nitroaniline, the nitro group is positioned adjacent to the amine group. This close proximity allows for both resonance and inductive interactions that significantly reduce the nitrogen's electron density, leading to decreased basicity.
• **Meta (m) position**: In m-nitroaniline, the nitro group does not engage in resonance with the nitrogen's lone pair as efficiently as when it is at the ortho or para positions. This reduces the electron-withdrawing effect slightly but still lowers basicity compared to unsubstituted aniline.
• **Para (p) position**: In p-nitroaniline, the nitro group is positioned such that it can maximize resonance interaction with the lone pair on the nitrogen. This position allows the most significant reduction in electron density and thus results in the lowest basicity among the nitroanilines.

Understanding these positional effects is critical in predicting the reactivity and properties of these nitrogen-containing aromatic compounds. The most notable impact is observed in p-nitroaniline, where the para positioning allows maximum electron withdrawal, leading to the lowest basicity in the set of compounds analyzed.