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Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides? How can we enhance the reactivity of aryl halides?

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1. Resonance effect: In aryl halides, the electron pair of halogen atom is in conjugation with ? electrons of benzene ring.  Halobenzene is a resonance hybrid so there is delocalization of electrons on C-X bond which acquires a partial double bond character. Hence the bond cleavage in haloarenes is difficult than haloalkanes since there is no double bond involved in them.  

2. Polarity: The sp2 hybridised carbon atom involved in C-X bond in haloarene molecule is more electronegative than the sp3 hybrid carbon atom in alkyl halide so the sp2 hybridised carbon has less tendency to release electrons to the halogen atom.   

3. Bond length: The sp2 hybridised carbon atom with a greater s-character is more electronegative and hence the C-X bond is short. This shortening of bond length imparts stability to aryl halides and so bond cleavage becomes difficult.   

4. Because of electron rich arenes, electron rich nucleophile will not approach closely for the attack because of repulsion.

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