Question

And what is the role of +M effect of Cl

Answer 1

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The mesomeric effect in chemistry is a property of substituents or functional groups in a chemical compound. ... The mesomeric effect is negative (–M) when the substituent is an electron-withdrawing group and the effect is positive (+M) when the substituent is an electron releasing group.

Answer 2

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Cl in chlorobenzene has a strong electron withdrawing tendency and shows a -I effect and therefore it removes the electron density from the ring . But the lone pair of electrons present on Cl also participates in resonance and shows a (+M) effect and increases the electron density at O/P position. During electrophilic substitution reaction inductive effect decides the reactivity while mesomeric or resonance effect controls the regioselectivity ( site for attack).

Inductive effect dominates over resonance effect considering the stability of intermediate during electrophilic reaction,

Because of electron withdrawing nature chlorobenzene is less reactive compared to benzene.

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