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

Correct arrangement: (c) 1 > 3 > 4 > 2.

Step 1: Assess Basicity of Unsubstituted Aniline

Aniline (compound 1: \( \mathrm{C}_6 \mathrm{H}_5-\mathrm{NH}_2 \)) is the standard reference for basicity in aromatic amines. Without electron-withdrawing or donating groups, aniline is more basic than its nitro-substituted derivatives. Thus, it is expected to have the highest basicity in comparison.

Step 2: Impact of Nitro Group on Basicity

The presence of a nitro group (\( \mathrm{NO}_2 \)) is an electron-withdrawing group, which decreases the electron density on the nitrogen atom of the amine group. This reduces the ability of the nitrogen atom to donate a pair of electrons, thus decreasing the basicity.

Step 3: Ordering by Position of Nitro Substitution

The nitro group's position affects its electron-withdrawing effect. The ortho (2) position experiences strong resonance and inductive effects, making it the least basic. The para (4) position also experiences strong resonance effects. The meta (3) position, however, primarily experiences the inductive effect, making it less impactful than the ortho or para positions. Thus, in terms of basicity: 3 > 4 > 2.

Step 4: Combine and Order All Compounds

Combining the previously discussed points, the order of decreasing basicity is 1 > 3 > 4 > 2. This is because the unsubstituted aniline (1) is the most basic, followed by the meta-substituted amine (3) which is less impacted by the nitro group's electronegativity than the para (4), and the ortho (2) is the least basic due to strong resonance and inductive contributions.

Key Concepts

Aniline

Aniline is a simple aromatic amine with a chemical structure \( \mathrm{C}_6 \mathrm{H}_5-\mathrm{NH}_2 \). In terms of basicity, aniline serves as the reference point for comparing the basicity of substituted aniline compounds. Its structure consists of a benzene ring attached to an amino group. This lone nitrogen atom is key to its basicity because it has a pair of electrons available for donation. The absence of additional electron-withdrawing or donating groups makes aniline comparatively more basic than its derivatives that have such substitutions.

In practical terms:

• Aniline is more basic than any of its nitro-substituted counterparts.
• Its basic strength stems from the ability of the nitrogen atom to donate its lone pair.

Electron Withdrawing Groups

Electron-withdrawing groups, like the nitro group (\( \mathrm{NO}_2 \)), are pivotal in decreasing the basicity of aromatic amines. These groups pull electron density away from the nitrogen atom, crucial for its ability to donate electrons and thereby exhibit basic characteristics.

The effect can be observed as:

• Reduction in electron availability at the nitrogen site.
• Decrease in the nitrogen atom's propensity to share its lone pair of electrons.

By weakening the electron-donating capability of the nitrogen, electron-withdrawing groups like the nitro group reduce amine basicity significantly. This makes compounds with these groups less effective in proton acceptance compared to aniline.

Resonance and Inductive Effects

Both resonance and inductive effects play a crucial role in determining the basicity of substituted anilines. These effects vary depending on the position of substitution on the aromatic ring.

Let's break down these effects:

• Resonance Effect: The delocalization of electrons across the molecule that can stabilize or destabilize the compound. In the case of ortho and para positions, resonance effects are stronger, further diminishing electron density at the nitrogen.
• Inductive Effect: The electron-withdrawing power through sigma bonds. This effect is most evident at the ortho position but less impactful at the meta position.

These concepts explain why the position of the electron-withdrawing nitro group significantly influences the basicity. Resonance generally has a more extended impact than induction, particularly when the substituent is in resonance-affecting positions like ortho or para.

Nitro Group Substitution

Nitro group substitution, depending on its position, has a distinct impact on the basicity of aniline derivatives. The presence of a nitro group in different positions relative to the amino group (ortho, meta, or para) alters the electron-withdrawing effects experienced by the nitrogen.

Key insights about nitro substitution:

• Ortho position: Experiences the strongest electron-withdrawing effects due to both resonance and inductive interactions. Hence, it makes the compound least basic.
• Meta position: Primarily influenced by inductive effects, making it less impacting than ortho or para and relatively more basic.
• Para position: Strong resonance effects lead to decreased electron density, hence reduced basicity, more than meta, less than ortho.

In the context of basicity, the order affected by the position of the nitro group can be crucially influential. Ortho is the most extreme in diminishing basicity, while meta is the least impactful.