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Why are aryl halides less reactivetowards nucleophilic substitution reactions than alkyl halides? How can we enhance the reactivity of aryl halides? |
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Answer» Solution :Aryl halides are less reactive towards nucleophilic substitution reaction due to the following reasons () In haloarenes the lone pair of electron on HALOGEN are in resonance with benzene ring So, C-Clbond acquires partial DOUBLE bond character which strengthen C-Cl bond Therefore, they are less reactive towords nucleophilic substitution reaction.  (i) In haloarenes, the carbon atom attached to halogen is `sp^(2)` hybridised. The `sp^(2)` hybridised carbon is more electronegative than `sp^(3)` hybridised carbon. This `sp^(2)`-hybridised carbon in haloarenes can hold the electron pair of C-X bond more tightly and make this C-CI bond shorter than C-Cl bond of haloalkanes Since, it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloarenes In haloarenes, the phenyl cation will not be stabilised by resonance therefore `S_(N)1` mechanism ruled out (iv) Because of the repulsion between the nucleaphile and electron rich arenes, aryl halides are less reactive than alkyl halides The reactivity of aryl halides can be increased by the presence of an electron withdrawing group `(-(NO_(2))` at ortho and para positions. However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing group at meta-position. Mechanism of the reaction is as depicted with `""^(-)OH` ion.   From the above resonance, it is very CLEAR that electron DENSITY is rich at orto and para positions. So, presence of EWG will fascilitate nucleophilic at ortho and para positions not on meta position. |
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