Increasing order of reactivity with Naoh solution
o- nitrochlorobenzene, chlorobenzene, benzylchloride
Answers
Answer:
Electron withdrawing groups activate the benzene ring to nucleophilic attack.
NaOH does react with chlorobenzene, but only under extreme conditions.
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Aryl halides cannot undergo an SN2 reaction. The C–Cl bond is in the plane of the ring and, to attack from the back, the nucleophile would have to appear inside the benzene ring. This is not possible.
An SN1 reaction is possible but unfavourable. It would involve the unaided loss of the leaving group and the formation of an aryl cation.
Electron withdrawing groups ortho and para to the leaving group activate the ring to nucleophilic attack. The more electron withdrawing groups you have, the faster the reaction becomes.
For example, the relative rates for reaction with NaOH are:
Chlorobenzene: 1
4-Nitrochlorobenzene: 7 × 10¹⁰
2,4-Dinitrochlorobenzene: 2.4 × 10¹⁵
2,4,6-Trinitrochlorobenzene: Too fast to measure
The reaction goes by a two-step SNAr (substitution, nucleophilic, aromatic) mechanism.
Step 1 is an addition. Step 2 is an elimination reaction. So this is an addition-elimination reaction