Why Haloarenes are indeed less reactive than haloalkanes..

 

Haloarenes are indeed less reactive than haloalkanes, particularly in nucleophilic substitution reactions. Here's a breakdown of the key reasons:

1. Resonance Effect

 * In haloarenes, the halogen atom's lone pair of electrons interacts with the pi electron system of the aromatic ring. This leads to resonance, where the electrons are delocalized across the ring.

 * This resonance gives the carbon-halogen bond a partial double bond character, making it stronger and more difficult to break compared to the single bond in haloalkanes.

2. Hybridization of Carbon

 * The carbon atom attached to the halogen in haloarenes is sp² hybridized, while in haloalkanes, it's sp³ hybridized.

 * sp² hybridized carbons have a higher s-character, making them more electronegative. This means they hold onto the electrons in the C-X bond more tightly, making it less polarized and less susceptible to nucleophilic attack.

3. Instability of Phenyl Cation

 * If a nucleophilic substitution reaction were to occur via an SN1 mechanism, it would involve the formation of a phenyl cation.

 * Phenyl cations are highly unstable due to the lack of electron delocalization to stabilize the positive charge. This makes SN1 reactions less favorable in haloarenes.

4. Repulsion between Nucleophile and Aromatic Ring

 * The aromatic ring in haloarenes is electron-rich due to the delocalized pi electrons.

 * This can create repulsion between the nucleophile (which is also electron-rich) and the haloarene, making it more difficult for the nucleophile to approach and attack the carbon atom bonded to the halogen.