Question

Some meta-directing substituents in aromatic substitution are given. Which one is most deactivating ?

• A. -C ≡ N
• B. -SO₃H
• C. -COOH
• D. -NO₂

Answer 1

The Correct Option is D

 To determine which meta-directing substituent is the most deactivating in aromatic substitution, we need to understand the electronic effects these substituents have on the aromatic ring.

In aromatic chemistry, substituents can either activate or deactivate the benzene ring and direct incoming substituents to certain positions (ortho, meta, or para). Meta-directing groups generally withdraw electron density from the aromatic ring and are usually deactivating groups.

1. Electronic Effects: Meta-directing groups often have strong electron-withdrawing properties. These groups stabilize the positive charge (those groups stabilize cations) on the intermediate species through resonance or inductive effects.
2. Comparison of Groups:

-C ≡ N:

• The cyano group is strongly electron-withdrawing due to its triple bond and its electronegative nitrogen atom.

-SO₃H:

• The sulfonic acid group is also electron-withdrawing, though less so than -NO₂.

-COOH:

• The carboxyl group withdraws electrons via resonance and inductive effects but is not as strong as the nitro group.

-NO₂:

• The nitro group is one of the strongest electron-withdrawing groups due to the resonance effect and the high electronegativity of oxygen, making it extremely deactivating.

1. Conclusion: Among the given groups,

-NO₂

1. is the most deactivating meta-directing substituent because it has a very strong electron-withdrawing capacity, greatly reducing the electron density on the aromatic ring and hindering further substitution.

Therefore, the correct answer is

-NO₂

.