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 query requires assessing the validity of two assertions concerning aniline's chemical behavior and selecting the option that accurately reflects their correctness.

Concept Used:

The evaluation hinges on the principles of Electrophilic Aromatic Substitution (EAS) and the impact of substituents on the benzene ring's reactivity.

1. Activating and Directing Effects: Substituents modify the benzene ring's susceptibility to electrophilic attack. Activating groups (electron-donating) enhance ring electron density, increasing reactivity and typically directing electrophiles to the ortho and para positions. Deactivating groups (electron-withdrawing) reduce ring electron density, diminishing reactivity.
2. Resonance Effect (+R/-R) and Inductive Effect (+I/-I): These are the primary mechanisms by which substituents influence electronic properties. The amino group (–NH₂) possesses a nitrogen lone pair, which it can donate to the ring via a potent positive resonance effect (+R).
3. Friedel-Crafts Reaction: This EAS subtype involves alkylation or acylation of an aromatic ring, necessitating a strong Lewis acid catalyst, such as anhydrous aluminum chloride (\(\text{AlCl}_3\)).
4. Lewis Acids and Bases: A Lewis acid accepts an electron pair (e.g., \(\text{AlCl}_3\)), while a Lewis base donates one (e.g., aniline, due to the nitrogen lone pair).

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 amino group (–NH₂) in aniline features a nitrogen atom with a lone pair of electrons. This lone pair can engage in resonance with the benzene ring, delocalizing electron density within the ring. This phenomenon is termed the +R (or +M) effect.

Resonance structures demonstrate an increased electron density specifically at the ortho and para positions. Consequently, these positions are highly prone to electrophilic attack, establishing the –NH₂ group as an ortho and para director.

Moreover, the +R effect's donation of electron density to the entire ring renders it significantly more reactive towards electrophiles than benzene itself. The pronounced +R effect of the –NH₂ group classifies it as a powerful activating group.

Therefore, Statement (I) is accurate.

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 nitrogen lone pair, functions as a Lewis base.

Upon mixing aniline with the \(\text{AlCl}_3\) catalyst, a strong acid-base reaction ensues before alkylation or acylation can occur. The basic nitrogen atom of aniline donates its lone pair to the Lewis acid \(\text{AlCl}_3\), forming a salt.

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

This salt formation results in the nitrogen atom carrying a formal positive charge. The resultant anilinium-type group (\(–\text{N}^+\text{H}_2\text{--}\)) acts as a potent electron-withdrawing group (strong -I effect), severely deactivating the benzene ring towards electrophilic attack. The ring becomes so electron-deficient that it cannot react with the electrophile (carbocation or acylium ion) necessary for the Friedel-Crafts reaction.

Therefore, Statement (II) is also accurate.

Final Result:

Both Statement (I) and Statement (II) are factually correct. Statement (I) accurately describes the general activating and directing properties of the amino group, while Statement (II) highlights a specific instance where reactivity is inhibited due to interaction with the Lewis acid catalyst employed in Friedel-Crafts reactions.

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