Q.Assertion: Presence of a nitro group at ortho or para position increases the reactivity of haloarenes towards nucleophilic substitution.
Reason: Nitro group, being an electron withdrawing group decreases the electron density over the benzene ring.

The key idea is that a nitro group activates the ring toward nucleophilic substitution by withdrawing electron density through both inductive and resonance effects, making the carbon attached to the halogen more electrophilic.

Step 1: In nucleophilic aromatic substitution (SNAr), the rate depends on how easily the nucleophile can attack the carbon bearing the halogen. An electron-withdrawing group (EWG) at the ortho or para position stabilises the intermediate Meisenheimer complex by delocalising its negative charge.

Step 2: The nitro group is a strong EWG — it withdraws electrons inductively and also by resonance (through its $-M$ effect). This reduces electron density on the ring, especially at ortho and para positions, making the haloarene more susceptible to attack by a nucleophile. …

Why haloarenes are normally slow to undergo nucleophilic substitution

In an ordinary haloarene like chlorobenzene, the ring is electron-rich (helped along by the halogen's own lone-pair donation into the ring), so an incoming nucleophile finds the ring carbon unattractive to attack. Harsh conditions are usually needed.

What a nitro group changes

A nitro group is a powerful electron-withdrawing group, both through induction and through resonance. As the diagram shows, the ring's own $\pi$ electrons can shift toward the nitro group, leaving a formal positive charge on the ring carbon and extra negative charge on a nitro oxygen. This withdrawal lowers the electron density on the ring overall — the exact statement the reason makes.

Why that withdrawal is the correct explanation …

Method: Resonance and Inductive Effect Analysis for Nucleophilic Aromatic Substitution

Step 1 – Identify the reaction type

The question deals with nucleophilic aromatic substitution (SNAr) in haloarenes.

In SNAr, the rate depends on how easily the leaving group (halogen) can be displaced by a nucleophile.

Step 2 – Role of the nitro group

A nitro group ($-NO_2$) is a strong electron-withdrawing group by both inductive effect and resonance effect.

• Inductive effect: pulls electron density through sigma bonds.
• Resonance effect: delocalizes the negative charge from the ring into the nitro group, especially at ortho and para positions.

Step 3 – Effect on reactivity

When a nitro group is at ortho or para position relative to the halogen:

• It stabilizes the Meisenheimer complex (the negatively charged intermediate) by delocalizing the negative charge.
• This lowers the activation energy for the nucleophilic attack.
• Hence, reactivity increases.

Step 4 – Evaluate the Assertion and Reason

• Assertion: Correct — ortho/para nitro groups increase reactivity in SNAr.
• Reason: Correct — nitro group is electron-withdrawing and decreases electron density on the ring.
• But: The reason is incomplete — it only mentions the inductive effect. The key explanation is the resonance stabilization of the intermediate, not just the decrease in electron density. So, the reason is not the correct explanation of the assertion.

Step 5 - Final answer …

Here are the common mistakes students make on this exact question, along with how to avoid each one.

Mistake 1: Confusing "Reactivity" with "Acidity"

• The Error: Students see "nitro group" and "electron withdrawing" and immediately think of acidity (e.g., comparing $pK_a$ of phenols or carboxylic acids).
• Why It's Wrong: This question is about nucleophilic substitution in haloarenes, not about acid strength. The electron withdrawal here affects the rate of $S_NAr$ (nucleophilic aromatic substitution), not proton donation.
• How to Avoid: Always check the context of the question. If it mentions "haloarenes" and "nucleophilic substitution," think about how easily a nucleophile can attack the ring carbon bearing the halogen, and how well the resulting negatively-charged intermediate (the Meisenheimer complex) is stabilised.

Mistake 2: Thinking the Assertion Is False

• The Error: Some students think the assertion is false because they recall that nitrobenzene is less reactive than benzene in electrophilic substitution, and assume the same logic applies here.
• Why It's Wrong: That's a different reaction type. For nucleophilic substitution, an electron-withdrawing group like nitro at ortho/para increases reactivity — it's the opposite trend from electrophilic substitution.
• How to Avoid: Clearly separate electrophilic vs nucleophilic substitution in your mind:
  • Electrophilic substitution: Nitro group deactivates the ring (electrophiles want an electron-rich ring).
  • Nucleophilic substitution: Nitro group activates the ring at ortho/para (nucleophiles want an electron-poor, attackable ring carbon).

Mistake 3: Dismissing the Reason as "Too Vague" and Marking It False

• The Error: Students feel the reason ("decreases electron density over the ring") is incomplete — since it doesn't explicitly mention the Meisenheimer complex or resonance — and conclude it must be the "wrong statement," picking (iii) or (iv). …