Question
Why does chlorobenzene undergo electrophilic substitution but resist nucleophilic substitution, while methyl chloride readily undergoes nucleophilic substitution?
Solution — Step by Step
In chlorobenzene, the chlorine’s lone pair conjugates with the benzene ring (resonance). This means:
- The C-Cl bond gets partial double-bond character → shorter and stronger.
- The chlorine’s lone pair is delocalised into the ring → less available to leave as Cl⁻.
In CH₃Cl, the carbon is hybridised. The C-Cl bond is purely , with no resonance stabilisation. Cl⁻ leaves easily because:
- The carbon is electrophilic (δ+) due to electronegativity difference.
- No competing resonance to hold Cl in place.
The same lone-pair donation that resists nucleophilic substitution actually activates the ring (slightly) for electrophilic attack. Cl is a +M (mesomeric donor) but −I (inductive withdrawer) substituent. Net effect: weak deactivator but ortho/para director because the lone pair stabilises ortho/para intermediates.
So chlorobenzene undergoes EAS (slowly) at ortho/para positions.
Final answer: Resonance with the ring strengthens C-Cl in chlorobenzene (no SN), but the lone pair makes ortho/para positions electron-rich (favours EAS).
Why This Works
Two competing effects of chlorine:
- +M (resonance): donates lone pair into the ring, activating it for electrophiles.
- −I (inductive): pulls electron density via bond, deactivating the ring overall.
The −I dominates in terms of rate (chlorobenzene reacts slower than benzene), but +M dominates in terms of position (ortho/para preferred).
Alternative Method
Compare bond lengths and bond strengths:
- C-Cl in chlorobenzene: 169 pm, ~96 kcal/mol
- C-Cl in methyl chloride: 178 pm, ~84 kcal/mol
Shorter and stronger in chlorobenzene → harder to break → SN doesn’t happen under normal conditions.
Common Mistake
Students claim chlorobenzene cannot react at all. False — it undergoes EAS (ortho/para directed) and even SN under harsh conditions (Dow process: high temperature and pressure with NaOH).
Also: students sometimes call Cl an “activator” because it directs ortho/para. Wrong — it’s a deactivator (overall slows EAS) but still ortho/para directing because the resonance argument applies to position, not rate.
JEE Main 2023 asked about the order of reactivity of towards SN reactions. Allyl chloride is fastest (SN1 stabilised by allyl cation), methyl chloride is medium, chlorobenzene is essentially unreactive.