Question

Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides?

Why are aryl halides less reactive than alkyl halides towards nucleophilic substitution reactions?

Why are haloarenes less reactive towards nucleophilic substitution reactions as compared to haloalkanes?

Answer 1

Aryl halides are less reactive towards nucleophilic substitution reaction due to the following reasons.

1. In haloarenes, the lone pair of electron on halogen are in resonance with benzene ring. So, C – Cl bond acquires partial double bond character which strengthen C – Cl bond and difficult to be substituted by nucleophile.
   Therefore, they are less reactive towards nucleophilic substitution reaction.
   
2. In haloarenes, the carbon atom attached to halogen is sp² hybridised. The sp² hybridised carbon is more electronegative than sp³ hybridised carbon. This sp²-hybridised carbon in haloarenes can hold the electron pair of \[\ce{C - X}\] bond more tightly and make this \[\ce{C - Cl}\] bond shorter than \[\ce{C Cl}\] bond of haloalkanes.
   
   Since, it is difficult to break a shorter bond than a longer bond, therefore, halorenes are less reactive than haloalkanes.
3. In haloarenesm the phenyl cation is not stabilised by resonance therefore S_N1 mechanism cannot be followed.
   
4. Because of the repulsion between the nucleophile and electron-rich arenas, aryl halides are less reactive than alkyl halides.