Question

Which of the following is a meta directing group:

• A. F
• B. \(NHCH_3\)
• C. \(NH_2\)
• D. \(CF_3\)

Answer 1

The Correct Option is D

The correct option is (D): \(CF_3\).

Answer 2

The question asks which of the given substituents directs further electrophilic substitution to the meta position of a benzene ring. This depends on whether the group can donate electrons into the ring by resonance or only withdraw them inductively. Let's check each group.

1. F: Fluorine carries lone pairs that can be donated into the ring by resonance, even though the atom also withdraws electron density inductively because of its high electronegativity. The net resonance donation wins out for directing purposes, making fluorine an ortho and para director despite being an overall deactivating group.
2. \(NHCH_{3}\): The nitrogen here carries a lone pair that conjugates directly into the aromatic ring, strongly activating it and sending new substituents to the ortho and para positions.
3. \(NH_{2}\): Like the methylamino group, the amino nitrogen's lone pair delocalizes into the ring through resonance, making this one of the strongest ring-activating, ortho and para directing groups.
4. \(CF_{3}\): The carbon of the trifluoromethyl group has no lone pair available to donate into the ring. The three highly electronegative fluorine atoms only pull electron density away through the inductive effect. With no resonance donation possible, this group purely withdraws electron density and sends incoming electrophiles to the meta position.

Only the trifluoromethyl group lacks any lone pair capable of resonance donation into the ring, which is what makes it behave differently from the other three substituents.

The correct answer is \(CF_{3}\).