Question
Why is chlorobenzene less reactive than methyl chloride toward nucleophilic substitution? Predict the products and conditions for the reaction of chlorobenzene with .
Solution — Step by Step
C-Cl in chlorobenzene: stronger (partial double-bond character due to resonance with benzene ring).
C-Cl in methyl chloride: simple sigma bond, weaker.
In chlorobenzene, the lone pair on Cl delocalises into the ring, giving the C-Cl bond partial double-bond character. The bond is shorter and stronger, resisting nucleophilic attack.
Also, the carbon in chlorobenzene is hybridised, holding the bond pair more tightly than the carbon in methyl chloride. Plus, the ring’s electrons electrostatically repel incoming nucleophiles.
Chlorobenzene resists ordinary . Forcing conditions: K, atm pressure, fused . Product: sodium phenoxide , which on acidification gives phenol.
This is the Dow process for industrial phenol synthesis.
Final answer: chlorobenzene + at K, atm → sodium phenoxide → phenol on acidification.
Why This Works
The aromatic ring stabilises the C-Cl bond through resonance. Three resonance structures place the negative charge on ortho and para carbons, making the C-Cl bond effectively “double-bond-like” with about bond order.
Methyl chloride has none of this — the bond is a clean single bond and is easily attacked by .
Alternative Method
Use Hammond’s postulate. The transition state for on chlorobenzene requires breaking aromaticity, which has high activation energy. The transition state for on methyl chloride preserves nothing aromatic, so it’s much lower in energy.
NEET 2021 asked exactly why chlorobenzene resists nucleophilic substitution. The four reasons (resonance, C, ring repulsion, instability of phenyl carbanion) earn one mark each.
Common Mistake
Saying “chlorobenzene is more reactive because of resonance”. Resonance stabilises the C-Cl bond, making it less reactive. Resonance increases reactivity only when the substrate has electron-withdrawing groups in ortho/para positions activating the ring toward nucleophilic attack.