Question

Which is more reactive in nucleophilic substitution chlorobenzene and chloroethane and why?

Answer 1

In chlorobenzene, the electron pair of a chlorine atom is in conjugation with π electrons of the 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.