Question

Given below are two statements:
Statement (I) : The NH₂ group in Aniline is ortho and para directing and a powerful activating group.
Statement (II) : Aniline does not undergo Friedel- Craft’s reaction (alkylation and acylation). In the light of the above statements
Choose the most appropriate answer from the options given below :

• A. Both Statement I and Statement II are correct
• B. Both Statement I and Statement II are incorrect
• C. Statement I is incorrect but Statement II is correct
• D. Statement I is correct but Statement II is incorrect

Answer 1

The Correct Option is A

This question asks us to evaluate two statements regarding the chemical properties of aniline and choose the correct option describing their validity.

Concept Used:

The analysis requires understanding the principles of Electrophilic Aromatic Substitution (EAS) and the role of substituents on the benzene ring.

1. Activating and Directing Effects: Substituents on a benzene ring can influence its reactivity towards electrophiles. Activating groups (electron-donating groups) increase the electron density of the ring, making it more reactive. They typically direct incoming electrophiles to the ortho and para positions. Deactivating groups (electron-withdrawing groups) decrease the ring's electron density, making it less reactive.
2. Resonance Effect (+R/-R) and Inductive Effect (+I/-I): These are the primary mechanisms through which substituents exert their electronic effects. The amino group (-NH₂) has a lone pair on the nitrogen atom, which it can donate to the ring via a strong positive resonance effect (+R).
3. Friedel-Crafts Reaction: This is a type of EAS used for alkylating or acylating an aromatic ring. It requires a strong Lewis acid catalyst, such as anhydrous aluminum chloride (\(\text{AlCl}_3\)).
4. Lewis Acids and Bases: A Lewis acid is an electron-pair acceptor (e.g., \(\text{AlCl}_3\)), and a Lewis base is an electron-pair donor (e.g., aniline, due to the lone pair on nitrogen).

Step-by-Step Solution:

Step 1: Analyze Statement (I).

"The NH₂ group in Aniline is ortho and para directing and a powerful activating group."

The nitrogen atom in the amino group (–NH₂) has a lone pair of electrons. This lone pair can participate in resonance with the benzene ring, delocalizing the electrons into the ring. This is known as the +R (or +M) effect.

The resonance structures show that the electron density is specifically increased at the ortho and para positions. This makes these positions highly susceptible to attack by an electrophile, thus the –NH₂ group is an ortho and para director.

Furthermore, because the +R effect donates electron density to the ring as a whole, it makes the ring much more reactive towards electrophiles than benzene itself. The +R effect of the –NH₂ group is very strong, making it a powerful activating group.

Therefore, Statement (I) is a correct statement.

Step 2: Analyze Statement (II).

"Aniline does not undergo Friedel-Crafts reaction (alkylation and acylation)."

The Friedel-Crafts reaction requires a Lewis acid catalyst, typically anhydrous \(\text{AlCl}_3\). Aniline, with its lone pair of electrons on the nitrogen atom, acts as a Lewis base.

When aniline is mixed with the \(\text{AlCl}_3\) catalyst, a strong acid-base reaction occurs before the alkylation or acylation can take place. The basic nitrogen atom of aniline donates its lone pair to the Lewis acid \(\text{AlCl}_3\), forming a salt.

\[ \text{C}_6\text{H}_5\ddot{\text{N}}\text{H}_2 + \text{AlCl}_3 \longrightarrow \text{C}_6\text{H}_5\stackrel{+}{\text{N}}\text{H}_2\text{--}\stackrel{-}{\text{Al}}\text{Cl}_3 \]

In this salt, the nitrogen atom now bears a formal positive charge. The resulting anilinium-type group (\(–\text{N}^+\text{H}_2\text{--}\)) becomes a very strong electron-withdrawing group (strong -I effect). This group strongly deactivates the benzene ring towards electrophilic attack. The ring becomes so electron-deficient that it fails to react with the electrophile (carbocation or acylium ion) required for the Friedel-Crafts reaction.

Therefore, Statement (II) is also a correct statement.

Final Result:

Both Statement (I) and Statement (II) are factually correct. Statement (I) describes the general activating and directing nature of the amino group, while Statement (II) describes a specific exception to this reactivity due to an interaction with the Lewis acid catalyst used in Friedel-Crafts reactions.

Thus, Both Statement (I) and Statement (II) are true.