Why haloarenes are much less reactive than haloalkanes towards nucleophilic reactions?
Haloarenes are much less reactive than haloalkanes towards nucleophilic substitution reactions due to the following reasons:
In haloarenes, the benzene ring undergoes resonance and as a result, the C−X bond acquires a partial double bond character. Therefore, it becomes difficult to break the C−X bond.
In haloalkanes, the halogen atom is attached to an sp3 hybridised carbon atom while in haloarenes, it is attached to an sp2 hybridised carbon atom. Since an sp2 hybridised carbon has more s-character than sp3 hybridised carbon, the former is more electronegative than the latter. As a result, the electron pair of C−X bond is held by carbon atom more tightly in haloarenes than haloalkanes. Therefore, the C−X bond becomes shorter in haloarenes and hence, becomes stronger.
The phenyl cation formed by the self ionisation is unstable and hence, SN1 mechanism is avoided.
Electron-rich nucleophile cannot approach electron-rich arenes due to repulsion.