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Explain why are aryl halides extremely less reactive towards nucleophilic substitution reactions? |
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Answer» Solution :Aryl halides are extremely less REACTIVE towards nucleo-philic substitution reaction due to the following reasons : (1) Reasonance effect : In haloarenes, the electron pairs on halogen atom are in conjugation with `pi`-electrons of the benzene ring. The DELOCALIZATION of these electrons `C-Cl` bond acquires partial double bond character.  Due to partial double bond character of `C-Cl` bond in aryl halides, the bond cleavage in haloarene is difficult and are less reactive towards nucleophilic substitution. (2) Different hybridization state of carbon atom in C-X bond : In aryl halides, the carbon of C-X bond is `sp^(2)` hybridized with more s-character and shorter bond length of 169 PM which requires more energy to break C-X bond. It is difficult to break a shorter bond than a longer bond, therefore, aryl halides are less reactive towards nucleophilic substitution reaction. (3) Instabillity of phenyl cation : In aryl halides, the phenyl cation formed due to self ionisation will not be stabilized by resonance. Thus cations are not formed and hence aryl halides do not UNDERGO nucleophilic substitution reaction easily. (4) As any halides are electron rich molecules due to the presence of `pi-` bond, they repel electron rich nucleophilic. attack. Hence, aryl halides are less reactive toward nucleophilic substitution reactions. |
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