Question

Which of the following is a meta directing group?

• A. \( F \)
• B. \( \text{NHCH}_3 \)
• C. \( \text{NH}_2 \)
• D. \( \text{CF}_3 \)

Hint:

- Electron-withdrawing groups (e.g., NO\(_2\), CF\(_3\), COOH) are meta-directors because they reduce electron density at ortho/para positions. - Electron-donating groups (e.g., NH\(_2\), OH, OCH\(_3\)) are ortho/para directors due to their resonance donating effect. - Fluorine and other halogens are exceptions: they withdraw inductively (-I) but donate mesomerically (+M), making them ortho/para directors.

Answer 1

The Correct Option is D

A meta-directing group is an electron-withdrawing group that decreases electron density at the ortho and para positions of a benzene ring, making electrophilic substitution more likely at the meta position. Among the given options, \( \text{CF}_3 \) is a strong electron-withdrawing group due to the high electronegativity of fluorine atoms. 
It exerts a -I (inductive withdrawing) effect, pulling electron density away from the benzene ring and making the ortho and para positions less reactive to electrophiles. As a result, substitution occurs at the meta position. 

On the other hand: - Fluorine (F) is an electron-withdrawing group by inductive effect (-I) but has a +M (mesomeric donating) effect, making it an ortho/para director. 
- Amines (NH\(_2\), NHCH\(_3\)) are strong electron-donating groups due to their +M effect (resonance donation), which increases electron density at the ortho and para positions, making them ortho/para directors. 

Since CF\(_3\) is the only strong electron-withdrawing group without a significant resonance donating effect, it is the correct meta-directing group.