Which of the following is a meta directing group?
Show Hint
- \( F \)
- \( \text{NHCH}_3 \)
- \( \text{NH}_2 \)
- \( \text{CF}_3 \)
The Correct Option is D
Approach Solution - 1
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.
Approach Solution -2
Substituents on a benzene ring are classified as ortho/para directors or meta directors based on whether they donate or withdraw electron density through resonance. Checking each group:
- \( F \): Fluorine withdraws electron density inductively because it is highly electronegative, but its lone pairs can also feed back into the ring by resonance, giving it a net electron-donating mesomeric effect. This resonance donation directs incoming electrophiles to the ortho and para positions, making fluorine an ortho/para director despite its electronegativity.
- \( \text{NHCH}_3 \): This is a secondary amine group where the nitrogen lone pair delocalises strongly into the ring by resonance. That strong electron donation raises electron density most at the ortho and para positions, making it a powerful ortho/para director, not a meta director.
- \( \text{NH}_2 \): The amino group behaves the same way, donating its lone pair into the ring through resonance and pushing electron density toward the ortho and para carbons. It is one of the strongest ortho/para directing groups known, so it does not direct to the meta position.
- \( \text{CF}_3 \): The trifluoromethyl group has no lone pair to donate into the ring and instead pulls electron density away purely through the inductive effect of its three fluorine atoms. With no resonance donation to offset this withdrawal, electron density drops most sharply at the ortho and para positions, leaving the meta position comparatively more electron rich and therefore the preferred site for electrophilic attack.
The deciding factor is resonance: F, NHCH3, and NH2 can all feed electron density back into the ring through a lone pair, even if some also withdraw inductively, and that resonance donation makes them ortho/para directors. CF3 has no such lone pair to donate, so its inductive withdrawal is left unopposed and it directs incoming groups to the meta position instead.
So the correct answer is \( \text{CF}_3 \).
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