Problem 80 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the amines I. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) II. \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) III. \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) IV. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) (a) \(\mathrm{I}<\mathrm{IV}<\mathrm{II}<\mathrm{III}\) (b) \(\mathrm{IV}<\mathrm{III}<\mathrm{II}<\mathrm{I}\) (c) \(\mathrm{I}<\mathrm{II}<\mathrm{III}<\mathrm{IV}\) (d) \(\mathrm{II}<\mathrm{III}<\mathrm{IV}<\mathrm{I}\)

Step-by-Step Solution

Answer

The correct ranking is (a) \( \mathrm{I} < \mathrm{IV} < \mathrm{II} < \mathrm{III} \).

Step 1: Identify the amines

Examine the four given amines to understand their structure: 1. \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \) is an aniline, which is an arylamine.2. \( \mathrm{CH}_{3} \mathrm{NH}_{2} \) is methylamine, a primary aliphatic amine.3. \(( \mathrm{CH}_{3})_2 \mathrm{NH} \) is dimethylamine, a secondary aliphatic amine.4. \(( \mathrm{CH}_{3})_3 \mathrm{N} \) is trimethylamine, a tertiary aliphatic amine.

Step 2: Consider the basicity of amines

Amines have varying basicity depending on the alkyl groups attached to the nitrogen and the presence of pi-conjugated systems, such as in aromatic rings. Alkyl groups increase basicity by electron-donating inductive effects, while aromatic systems decrease basicity due to resonance withdrawal.

Step 3: Analyze aniline

Aniline (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \)) has a benzene ring that withdraws electrons from the nitrogen through resonance. This decreases nitrogen's ability to donate \((\mathrm{NH}_2 )\) electrons, lowering its basicity.

Step 4: Analyze aliphatic amines

Methylamine (\( \mathrm{CH}_{3} \mathrm{NH}_{2} \)) is a primary amine. Dimethylamine (\(( \mathrm{CH}_{3})_2 \mathrm{NH} \)) and trimethylamine (\(( \mathrm{CH}_3)_3 \mathrm{N} \)) are more basic due to increased alkyl substitution and electron-donating effects.

Step 5: Compare basicity

The order of basicity in aliphatic amines is typically secondary > primary > tertiary (dimethylamine > methylamine > trimethylamine) in standard conditions, but steric factors favor trimethylamine slightly over dimethylamine.

Step 6: Create final ranking

Considering all factors: - Aniline is the least basic among the given options.- Trimethylamine and dimethylamine are more basic than methylamine under standard conditions.Correct ranking based on basicity considerations: \( \mathrm{I} < \mathrm{IV} < \mathrm{II} < \mathrm{III} \).

Key Concepts

Aryl vs Aliphatic Amines

When comparing amines, it is crucial to distinguish between aryl and aliphatic types because they exhibit different properties. Aryl amines have the amine group directly attached to an aromatic ring, such as benzene, making them significantly less basic than their aliphatic counterparts. In the case of anilines or arylamines, such as \[\text{C}_6\text{H}_5\text{NH}_2\]\( \}\beta-222 LaTeX{ace}, the presence of the benzene ring affects the basicity. This contrast is primarily due to resonance effects, which are not present in aliphatic amines. Instead, aliphatic amines like methylamine (\[\text{CH}_3\text{NH}_2\]\) benefit from the inductive effect of alkyl groups, where the electron density is pushed towards the nitrogen atom. This results in increased basicity compared to their aryl counterparts. Thus, when comparing aniline with any aliphatic amine, the latter will generally be more basic due to the lack of electron-withdrawing resonance from an aromatic ring.

Inductive Effect in Amines

The inductive effect plays a significant role in determining the basicity of amines. It involves the shifting of electron density through sigma bonds and is typically observed in the presence of alkyl substituents. Alkyl groups, like those found in methylamine, dimethylamine, and trimethylamine, donate electrons to the nitrogen atom.
This donation occurs via the inductive effect, enhancing the electron density around the nitrogen atom and, consequently, increasing the amine's ability to donate electrons.

• The greater the number of alkyl groups attached to the nitrogen, the stronger the inductive effect, often making secondary amines more basic than primary ones.
• Tertiary amines, like trimethylamine, also benefit from this effect, but steric hindrance can slightly reduce their basicity compared to secondary amines.

This principle helps explain the basicity order observed in the given amines: dimethylamine being more basic than both methylamine and trimethylamine.

Resonance in Aniline

Aniline \[\text{(C}_6\text{H}_5\text{NH}_2\text{)}\]\ ited in basicity due to resonance interactions with its benzene ring. This resonance occurs when one of the lone pairs on the nitrogen participates in conjugation with the aromatic ring.
As a result, the electron density on the nitrogen atom decreases, reducing its tendency to donate lone pair electrons. This phenomenon contrasts with the action of the inductive effect seen in aliphatic amines.

• In aniline, the stabilization provided by resonance outweighs the inductive electron release, making the nitrogen less nucleophilic.
• This means that aniline’s ability to accept protons—a key factor in basicity—is diminished.

Understanding the balance between resonance and inductive effects is essential to predict the behavior of amines in various chemical environments.