why chlorobenzene is less reactive towards nucleophilic substitution reaction towards nucleophilic reaction????

In chlorobenzene, the electron pair of chlorine atom is in conjugation with π electrons of benzene ring. Thus, chlorobenzene is a resonance hybrid of the following structures:

The contributing structures III, IV and V indicate that C-Cl bond has partial double character.As a result, the C-Cl bond in chlorobenzene is shorter and hence, stronger.Thus, cleavage of C-Cl bond in benzene becomes difficult which makes it less reactive towards nucleophilic substitution.

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There is an error in the question. Aromatic compounds undergo ' electrophilic substituion' normally.

Chlorobenzene is less reactive towards electrophilic substitution than benzene. Halogen group on benzene is a weekly deactivating group. Halogens being more electronegative than carbon, they withdraw electrons through inductive effect thereby deactivating the ring towards electrophilic substitution.

 

You can get more idea about abnormal behaviour of halobenzenes in my other answer. https://www.meritnation.com/ask-answer/question/although-chlorine-is-an-electron-withdrawing-group-yet/chemistry/5374889

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