Q.Assertion: Nitration of chlorobenzene leads to the formation of m-nitrochlorobenzene.
Reason: –$\mathrm{NO_2}$ group is a m-directing group.

Concept: Inductive and resonance effects in electrophilic aromatic substitution.

Step 1: Chlorobenzene has a chlorine atom that is ortho/para-directing due to resonance donation of lone pairs, but it is deactivating due to its strong –I effect.

Step 2: Nitration of chlorobenzene therefore gives ortho and para nitrochlorobenzene as major products, not meta. The –NO₂ group is indeed meta-directing, but that applies when it is already on the ring — here, the directing group is chlorine. …

The key here is to separate two different substituents: chlorine (on the benzene ring in the starting material) and the nitro group (which is the incoming group in the reaction). The reason talks about the directing effect of the –NO₂ group, but the assertion involves the directing effect of chlorine. Mixing these up is the classic trap.

Let’s break it down.

1. What does the assertion claim?

   Nitration of chlorobenzene means we take chlorobenzene (C₆H₅–Cl) and treat it with a nitrating mixture (HNO₃ + H₂SO₄). The product is said to be m-nitrochlorobenzene.

   But is that true? Chlorine is an ortho-para director (it activates the ring by resonance, though it deactivates by induction — the net effect is ortho/para orientation). So nitration of chlorobenzene gives a mixture of o-nitrochlorobenzene and p-nitrochlorobenzene, with very little m-isomer.

   Therefore, the assertion is false.

2. What does the reason state?

   The –NO₂ group is a meta-directing group. This is absolutely correct. The nitro group is strongly electron-withdrawing (by both induction and resonance), which deactivates the ring and directs incoming electrophiles to the meta position.

   So the reason is true.

3. Does the reason explain the assertion? …

Method: Directive Effects in Electrophilic Aromatic Substitution

This method uses the activating/deactivating nature and directing ability of substituents to predict the major product in nitration reactions.

Steps:

1. Identify the substituent already present on the ring

   In chlorobenzene, the substituent is –Cl (chlorine).

2. Classify the substituent

   –Cl is an ortho-para director (not meta-directing).

   • Reason: Chlorine has lone pairs that donate electrons by resonance (+R effect), even though it withdraws by induction (–I effect). The resonance effect dominates in directing the incoming electrophile to ortho and para positions.

3. Predict the major product

   Nitration of chlorobenzene gives o-nitrochlorobenzene and p-nitrochlorobenzene as major products — not m-nitrochlorobenzene.

4. Evaluate the Assertion

   Assertion says: “Nitration of chlorobenzene leads to m-nitrochlorobenzene.”

   This is wrong — the major products are ortho and para isomers.

5. Evaluate the Reason …

Common Mistakes & How to Avoid Them

Mistake 1: Confusing the directing group in the Assertion

• The mistake: Students think chlorobenzene gives m-nitrochlorobenzene because chlorine is an o/p-director — but here the assertion says the product is m-nitrochlorobenzene.
• Why it happens: They misread the assertion or assume the product is always ortho/para for halogens.
• How to avoid:
  • Chlorine is an o/p-directing group (due to resonance donation of lone pairs).
  • So nitration of chlorobenzene gives o- and p-nitrochlorobenzene, not the meta isomer.
  • Correct fact: The assertion is wrong.

Mistake 2: Thinking the Reason is wrong

• The mistake: Students reject the reason because they think –NO₂ is an o/p-director.
• Why it happens: They confuse –NO₂ (strong deactivator, meta-director) with –NH₂ or –OH (strong activators, o/p-directors).
• How to avoid:
  • –NO₂ group is a strong electron-withdrawing group (by both inductive and resonance effects).
  • It deactivates the ring and directs incoming groups to the meta position.
  • Correct fact: The reason is correct.

Mistake 3: Choosing option (i) — both correct, reason explains assertion

• The mistake: Students match "both seem correct" without checking if the assertion is actually true.
• Why it happens: They see a correct reason and assume the assertion must also be correct.
• How to avoid:
  • Always verify the assertion independently before linking it to the reason. …