The sp2 hybridised carbon with a greater s-character is more
electronegative and can hold the electron pair of C—X bond
more tightly than sp3-hybridised carbon in haloalkane with
less s-chararcter Thus, C—Cl bond length in haloalkane is
177pm while in haloarene is 169 pm Since it is difficult to
break a shorter bond than a longer bond, therefore, haloarenes
are less reactive than haloalkanes towards nucleophilic
substitution reaction (iii) Instability of phenyl cation: In case of haloarenes, the phenyl
cation formed as a result of self-ionisation will not be stabilised
by resonance and therefore, SN1 mechanism is ruled out