In aryl halides, lone pair is not available to halogen atom attached to benzene ring,i.e., it remains delocalised in the ring because its lone pair are involved in resonance with benzene ring. To leaving group, therefore it becomes difficult to leave its place.

Whereas this is not the case with R-X compounds, haloalkanes. Therefore, they act as better leaving group.

Substitution of nucleophile takes place either through SN1 mech. or SN2 mech.

In SN1 mechanism, heterolytic bond fission of R-X bond takes place and we get carbocation as the intermediate. Then we do the rearrangement of carbocation(if possible and required) and finally in 2nd step incoming nucleophile replaces the former one.

In SN2 mechanism, no carbocation is formed. In place of it we get transition state(we get PENTAVALENT carbon in transition state). Since no carbocation is formed, therefore there is no meaning of rearrangement. Finally, incoming nucleophile replaces the former one

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