Q.Arrange the compounds in increasing order of rate of reaction towards nucleophilic substitution. (Note printed in the Exemplar for questions 26 to 29.)

The key idea is that the rate of nucleophilic substitution in aryl halides increases with the number of strong electron-withdrawing groups (like $-\text{NO}_2$) on the ring. These groups stabilise the Meisenheimer complex intermediate by delocalising the negative charge.

Reasoning:

1. Chlorobenzene (a) has no activating groups, so it reacts very slowly.
2. 1-chloro-4-nitrobenzene (b) has one nitro group at the para position, which withdraws electrons by both inductive and resonance effects, moderately increasing the rate. …

Why this problem is about resonance stabilisation, not just induction

In nucleophilic substitution on aryl halides (SNAr mechanism), the rate depends on how easily the nucleophile can attack the carbon bearing the halogen. A chlorine on a plain benzene ring is quite unreactive because the lone pairs on chlorine are delocalised into the ring, giving the C–Cl bond partial double-bond character. But when nitro groups are present, they can stabilise the negative charge that builds up on the ring during the attack — and that changes everything.

The key is that nitro groups are strongly electron-withdrawing by both induction and resonance. However, for SNAr, resonance withdrawal from the ortho and para positions is what matters. A nitro group at the para position can directly accept electron density from the ring when a negative charge develops at the carbon bearing the chlorine. At the meta position, this resonance stabilisation is impossible — so meta-nitro groups are far less effective.

Let’s see how this plays out for the three compounds given.

1. Chlorobenzene (a) — no activating/withdrawing groups. The ring is electron-rich, and the nucleophile faces a high energy barrier. The intermediate Meisenheimer complex (the negatively charged σ-adduct) is not stabilised at all. This is the slowest.

2. 1-Chloro-4-nitrobenzene (b) — one nitro group at the para position. When the nucleophile attacks at the carbon bearing chlorine, the negative charge can be delocalised onto the oxygen atoms of the nitro group via resonance. This stabilises the intermediate significantly. The rate is much faster than chlorobenzene — typically by a factor of $10^5$ or more.

3. 1-Chloro-2,4,6-trinitrobenzene (c) — three nitro groups, all at positions that can stabilise the intermediate (two ortho, one para). The negative charge can be spread across three nitro groups. This compound is so reactive that it undergoes nucleophilic substitution even under mild conditions — it’s the famous picryl chloride. The rate is astronomical compared to the others. …

Method: Effect of Electron-Withdrawing Groups on Nucleophilic Aromatic Substitution ($S_NAr$) in Aryl Halides

This question tests your understanding of how −NO₂ groups influence the stability of the anionic Meisenheimer complex intermediate formed during nucleophilic aromatic substitution of aryl halides.

Step-by-step reasoning

Step 1: Identify the reaction type

The compounds are aryl halides. Aryl halides do not undergo $S_N1$ (no stable phenyl cation forms — the positive charge cannot be resonance-stabilised by the ring) or $S_N2$ (backside attack is blocked by the ring). Instead, they react via the addition-elimination ($S_NAr$) mechanism: the nucleophile first adds to the ring carbon bearing the leaving group, forming a negatively charged Meisenheimer complex, and only then does the halide leave.

Step 2: Recall the key principle

Electron-withdrawing groups (like −NO₂) speed up $S_NAr$ by stabilizing the negative charge of the Meisenheimer complex through resonance. More −NO₂ groups at ortho/para positions relative to the leaving group → more stabilization of the intermediate → faster reaction.

Step 3: Analyze each compound …

Common Mistakes in Nucleophilic Aromatic Substitution ($S_NAr$) Reactivity (Aryl Halides)

Mistake #1: Confusing SN1 with SN2 or Addition-Elimination Mechanism

The error: Students apply SN1 logic (carbocation stability) to aryl halides. But aryl halides do NOT undergo SN1 — the carbocation would be highly unstable (anti-aromatic if at the ring).

The truth: These reactions proceed via addition-elimination (SNAr) mechanism, where electron-withdrawing groups (NO₂) increase reactivity by stabilizing the Meisenheimer complex intermediate.

How to avoid: Always check the substrate type. For aryl halides, think SNAr, not SN1.

Mistake #2: Reversing the Order of Reactivity

The error: Students think more electron density = faster reaction (as in SN2). So they rank chlorobenzene > nitrobenzene > trinitrobenzene.

The correct logic: In SNAr, more electron withdrawal = faster reaction. The nitro groups pull electron density away from the ring, making the carbon more δ⁺ and more susceptible to nucleophilic attack.

Correct order (increasing rate):

• (a) chlorobenzene — slowest (no activating groups)
• (b) 1-chloro-4-nitrobenzene — faster (one NO₂ group)
• (c) 1-chloro-2,4,6-trinitrobenzene — fastest (three NO₂ groups)

So the correct answer is (iv) (a) < (b) < (c).

Mistake #3: Ignoring the Position of Substituents

The error: Students count NO₂ groups but forget that ortho/para positions matter for resonance stabilization of the intermediate.

Why it matters: NO₂ at ortho or para positions can delocalize the negative charge in the Meisenheimer complex via resonance. Meta NO₂ groups are less effective.

In this question: All NO₂ groups are at ortho/para positions relative to Cl, so they all contribute effectively. …