Question

The ortho-, para-directing and deactivating group in aromatic electrophilic substitution reaction is

• A. -CH\(_3\)
• B. -OH
• C. -Cl
• D. -NO\(_2\)
• E. -COOH

Hint:

Halogens are unique: they are deactivating but ortho/para directing.

Answer 1

The Correct Option is C

Step 1: Understanding Substituent Effects:
In electrophilic aromatic substitution, substituents on the benzene ring influence both the rate of reaction (activation/deactivation) and the position of the incoming electrophile (orientation).
- Activating groups: Increase the rate of reaction compared to benzene. They are typically ortho, para-directing.
- Deactivating groups: Decrease the rate of reaction. Most are meta-directing, but there is a key exception.
- Ortho, para-directing groups: Direct the incoming electrophile to the positions ortho and para to themselves.
- Meta-directing groups: Direct the incoming electrophile to the meta position.
The question asks for a group that is both deactivating and ortho, para-directing.
Step 2: Analysis of Groups:
Let's analyze the groups based on their electronic effects:
(A) -CH\(_{3}\) (Alkyl group): Donates electron density through the +I (inductive) effect. It is a weak activating group and is ortho, para-directing.
(B) -OH (Hydroxyl group): Strongly donates electron density through the +R (resonance) effect, which outweighs its -I effect. It is a strong activating group and is ortho, para-directing.
(D) -NO\(_{2}\) and (E) -COOH: Both are strongly electron-withdrawing through -R and -I effects. They are strong deactivating groups and are meta-directing.
(C) -Cl (Halogen): Halogens present a unique case. They are strongly electronegative, so they withdraw electron density from the ring through the -I effect, making the ring less reactive towards electrophiles (deactivating). However, they also have lone pairs of electrons that can be donated to the ring via the +R effect. This resonance effect stabilizes the carbocation intermediates formed during ortho and para attack more than meta attack. Thus, halogens are ortho, para-directing. The inductive effect determines the rate (deactivation), while the resonance effect determines the orientation (ortho, para).
Step 3: Final Answer:
Halogens, such as -Cl, are the classic examples of groups that are deactivating yet ortho, para-directing. This corresponds to option (C).