Reactions due to:
(i) Resonance effect: C – X bond acquires a partial double bond character and it becomes difficult to break C – X bond.
(ii) In C – X bond, C atom attached to halogen is sp2 hybridised. 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 5- character.