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  4. Arenes on treatment with chlorine in presence of ferric chloride as a catalyst undergo what type of reaction ?

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Q. Arenes on treatment with chlorine in presence of ferric chloride as a catalyst undergo what type of reaction ?

MHT CETMHT CET 2018Hydrocarbons

Solution:

Arenes on treatment with chlorine in presence of Lewis acid catalyst, ferric chloride or aluminium chloride and in the absence of light undergoes halogenation. It involves electrophilic substitution reaction

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Mechanism of electrophilic substitution is as follows:

Step I Generation of an electrophile

$Cl— Cl+AlCl_{3} \to Cl^{+}+Cl^{5+} \dots Al^{5-} —Cl_{3}$

$\to Cl^{+}+[AlCl_{4}]^{-}$ Chloronium ion

Step II Formation of carbocation (arenium ion)

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The arenium ion gets stabilised by resonance

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Step III Removal of proton

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