Problem 62 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Statement-1 : In strongly acidic solutions, aniline becomes more reactive towards electrophilic reagents. Statement-2 : The amino group being completely protonated in strongly acidic solution, the lone pair of electrons on the nitrogen isno longer available for resonance. [2001S] (a) Statement-1 is True, Statement-2 is True; Statement-2 is a correct explanation for Statement-1. (b) Statement- 1 is True, Statement-2 is True; Statement-2 is NOT a correct explanation for Statement-1 (c) Statement- 1 is True, Statement- 2 is False (d) Statement-1 is False, Statement-2 is True

Step-by-Step Solution

Answer

Option (d): Statement-1 is False, Statement-2 is True.

Step 1: Understanding Protonation of Aniline

Aniline has an amino group (-NH2) that can get protonated in an acidic solution to form anilinium ion (-NH3+). By accepting a proton, the lone pair on nitrogen is used, making it unavailable for resonance with the benzene ring.

Step 2: Analyzing Reactivity in Strongly Acidic Solution

In strongly acidic solutions, aniline reacts differently than in neutral or basic media. The lack of electron pair due to protonation means that the electron-donating effect of the amino group is diminished. This typically makes aniline less reactive towards electrophilic aromatic substitution, contrary to what Statement-1 suggests.

Step 3: Evaluate Statement-1

Statement-1 claims aniline becomes more reactive towards electrophiles in strongly acidic solutions. Given that protonation of the nitrogen removes electron donation ability, Statement-1 is False because the reactivity is actually reduced.

Step 4: Evaluate Statement-2

Statement-2 correctly describes that the lone pair on nitrogen is no longer available for resonance in strongly acidic solutions because the amino group is protonated.

Step 5: Final Decision on Statements

With Statement-1 being false and Statement-2 being true, evaluate the options. Option (d) is correct as it matches our conclusion: Statement-1 is False, Statement-2 is True.

Key Concepts

Protonation of Aniline

In chemistry, aniline is an organic compound consisting of a benzene ring attached to an amino group (-NH2). The protonation of aniline occurs when this amino group accepts a proton (H⁺) in strongly acidic environments, forming the anilinium ion (-NH3⁺). This process is crucial to understanding the reactivity of aniline.

When aniline is protonated, the lone pair of electrons on the nitrogen atom becomes engaged in bonding with the hydrogen ion. As a result, these electrons are no longer free to participate in resonance with the benzene ring. This alters the electronic configuration of aniline, impacting how it engages in further chemical reactions.

• The protonated form is known as the anilinium ion.
• Protonation uses up the lone pair on nitrogen, hindering resonance.
• Protonated aniline has different chemical behavior compared to its non-protonated form.

Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS) is a reaction in which an electrophile replaces a hydrogen atom in an aromatic system, such as benzene. Aniline, in its non-protonated state, is quite reactive in such reactions due to the electron-donating effects of its amino group.

The amino group in aniline usually increases the electron density of the benzene ring, enhancing its reactivity towards electrophiles. However, when aniline is protonated, the scenario changes dramatically:

• The electron-donating effect is minimized or negated because the lone pair of electrons is bonded to a proton.
• This leads to decreased reactivity towards electrophilic aromatic substitution.
• Unlike the more common cases where electron-donating groups increase reactivity, protonation of aniline results in reduced reactivity under strongly acidic conditions.

This explains why, contrary to some expectations, protonated aniline is less reactive in EAS.

Resonance in Chemistry

Resonance is a concept used to describe delocalized electrons in certain molecules, which can spread out over multiple atoms, usually within conjugated systems. In aniline, resonance is possible when the lone pair of electrons on the nitrogen atom aligns with the pi-bonding network of the benzene ring.

This sharing of electrons through resonance balances the electron distribution across the molecule, enhancing its overall stability.

• In its natural state, aniline can exhibit resonance involving its amino group.
• Protonation of aniline disrupts this resonance because the lone pair is now occupied.
• Lack of resonance makes the molecule less stabilized and affects its chemical properties.

Therefore, resonance plays a key role in defining the characteristics and reactivity of aromatic compounds like aniline before and after protonation.