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

Concept: Nucleophilic Aromatic Substitution ($S_NAr$) Reactivity — the rate depends on how well the ring stabilises the NEGATIVELY charged Meisenheimer intermediate. Electron-donating groups like methyl push MORE electron density onto a ring that needs to ACCEPT electron density from the incoming nucleophile, so they slow the reaction down. More methyl groups mean more destabilisation and a slower reaction -- the opposite of what "activation" would suggest for an electrophilic reaction.

Step 1: Chlorobenzene (a) has no methyl substituent — nothing works against it, so it is the fastest of the three.

Step 2: 1-chloro-4-methylbenzene (b) has one methyl group at the para position — exactly one of the positions where the Meisenheimer complex's negative charge concentrates, so it meaningfully slows the reaction compared to (a). …

Why the Meisenheimer complex direction matters

SNAr does not follow SN1/SN2 logic. The nucleophile adds to the ring FIRST, at the carbon bearing the halogen, building a negatively charged intermediate (the Meisenheimer complex) before the halide ever leaves. Whatever stabilises that negative charge speeds up the rate-determining step; whatever destabilises it slows the reaction down.

A methyl group is electron-donating (+I effect and hyperconjugation). Donating extra electron density INTO a ring that is already trying to hold a negative charge makes that charge less stable, not more — the ring becomes reluctant to accept even more electron density from the incoming nucleophile. So methyl groups at the ortho/para positions (the positions where the Meisenheimer complex's negative charge is actually delocalised) slow SNAr down. This is the reverse of electrophilic aromatic substitution, where a methyl group speeds the reaction up by stabilising a positively charged intermediate — SNAr and EAS intermediates carry opposite charges, so the same substituent has opposite effects.

Working through the three compounds

• (a) Chlorobenzene — no substituents at all, so nothing works against the Meisenheimer complex forming. This is the fastest. …

Method: Meisenheimer Complex Stability ($S_NAr$ Pathway)

Nucleophilic substitution on haloarenes proceeds through an anionic Meisenheimer intermediate (the nucleophile adds first, then the leaving group departs) -- NOT through a carbocation. The rate depends on how well the ring tolerates that extra NEGATIVE charge.

Step-by-step reasoning

1. Identify the mechanism — these are aryl chlorides; direct ionisation to a phenyl cation essentially never happens (phenyl cations are extremely unstable and this is not how haloarene substitution proceeds). The relevant pathway is addition-elimination through an anionic intermediate.

2. Effect of methyl groups — methyl groups are electron-DONATING (hyperconjugation + inductive effect). Since the reaction needs the ring to ACCEPT extra electron density from the incoming nucleophile, donating MORE electron density onto the ring makes that harder, not easier. More methyl groups = more destabilisation = SLOWER reaction, the opposite of what a naive "electron-rich ring reacts faster" intuition suggests for this ANIONIC-intermediate mechanism.

3. Position matters — the Meisenheimer complex's negative charge concentrates at positions ortho and para to the carbon bearing Cl. Compound (c) has methyls at BOTH of those positions (2,4-dimethyl relative to the 1-chloro), maximising the destabilisation.

4. Apply to the three compounds: …

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, which do not undergo SN1 at all. Aryl halides react via addition-elimination (benzyne or $S_NAr$) mechanisms.

How to avoid: Remember:

• Aryl halides never form carbocations (too unstable)
• The rate depends on stabilization of the Meisenheimer complex (negative charge) in $S_NAr$, not carbocation stability
• For benzyne mechanism, rate depends on C–X bond strength and ortho-directing effects

Mistake #2: Ignoring the Effect of Electron-Donating Groups (EDGs)

The error: Students think more methyl groups = faster reaction, because "more electron density helps."

The truth: In $S_NAr$ (addition-elimination), electron-withdrawing groups (EWGs) speed up the reaction. Methyl groups are electron-donating — they slow down the rate.

Correct logic:

• More methyl groups → more electron density on ring → less stable Meisenheimer complex → slower reaction
• Order: chlorobenzene > 1-chloro-4-methylbenzene > 1-chloro-2,4-dimethylbenzene

Mistake #3: Misreading "Increasing Order"

The error: Students arrange from fastest to slowest instead of slowest to fastest.

How to avoid: Always underline the phrase "increasing order" and write:

• Slowest first → Fastest last
• Here: (c) < (b) < (a) → Option (iii)

Mistake #4: Forgetting the Exemplar Note …