Haloarenes are less reactive than haloalkanes. - Vidyadip

Question

Haloarenes are less reactive than haloalkanes.

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Answer

Haloarenes (Aryl halides) are less reactive than (alkyl halides) haloalkanes due to the following reasons :
(1) Resonance effect : In haloarenes, the electron pairs on halogen atom are in conjugation with 7r-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. On the other hand, in alkyl halides, carbon is attached to chlorine by a single bond and it can be easily broken. (2) Aryl halides are stabilized by resonance but alkyl halides are not. Hence, the energy of activation for the displacement of halogen from aryl halides is much greater than that of alkyl halides.
(3) Different hybridization state of carbon atom in C-X bond :
(i) In alkyl halides, the carbon of C-X bond is sp3-hybridized with less 5-character and greater bond length of 178 pm, which requires less energy to break the C-X bond.
(ii) In aryl halides, the carbon of C-X bond is sp3-hybridized with more 5-character and shorter bond length which requires more energy to break C-X bond. Therefore, aryl halides are less reactive than alkyl halides.
(iii) Polarity of the C-X bond : In aryl halide C-X bond is less polar than in alkyl halides. Because sp3-hybrid carbon of C-X bond has less tendency to release electrons to the halogen than a sp3-hybrid carbon in alkyl halides. Thus halogen atom in aryl halides cannot be easily displaced by nucleophile.

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