Question

Give reasons: Haloarenes are less reactive than haloalkanes.

Answer 1

The lesser reactivity of haloarenes can be explained as follows :
(1) Stabilization by resonance : The electron pair of halogen atom in haloarene, is in conjugation with the $\pi$ electrons of the benzene ring. Thus, haloarene is a resonance hybrid of following structures :-
Thus, due to resonance, the contributing structures II, III and IV indicate that $C-X$ bond has partial double bond character. As a result of this, $C-X$ bond in haloarene is shorter and hence stronger than that of haloalkane. So it is difficult to break $C-X$ bond in haloarenes compared to haloalkanes, therefore, it is less reactive
(2) Polarity of carbon-halogen bond : The dipole moment $C-X$ bond in chlorobenzene is $1.7D$ while that in chloroalkanes it is in the range $2.0-2.2D$. Greater the dipole moment, greater is the polarity and more is the reactivity. Thus, $C-X$ bond of haloalkane is more polar than the $C-X$ bond of haloarene. Hence, haloarenes are less reactive than haloalkanes.