why aryl halides are less reactive than alkyl halides?

Aryl halides are less reactive than alkyl halides towards nucleophilic substitution reactions because-(i) Resonance effect-The electron pairs on halogen atom are in conjugation with pi-electrons of the ring. This causes delocalisation of electrons on C-X bond (X= halogen) which acquires partical double bond character. As a result the bond cleavage in aryl halides is difficult than alkyl halides in which carbon is attached to halogen by a pure single bond.For resonating structures please refer the image posted by Miss Poetess.(ii) Difference in hybridisation states of carbon in C-X bond-In alkyl halides carbon atom of C-X bond is sp3 hybridised while in aryl halides, the carbon atom attached to halogen is sp2 hybridised . The sp2 hybridised carbon atom with greater s-character is more electronegative . It can hold the electron pair of the bond more tightly than sp3 hybridised carbon atom in alkyl halides with less s-character. This causes shortening of bond length in aryl halides than in alkyl halides. The shortening of length imparts to aryl halides and as result, the bond cleavage becomes rather difficult.(iii) Instability of phenyl cation-​The phenyl cation formed as a result of self ionisation will not be stabilized by resonance and therefore SN1 mechanism cannot occur at all.