Question

Explain the lower reactivity of haloarenes as compared to haloalkanes on the basis of hybridisation state of carbon of C−X bond.

Answer 1

The lower reactivity of haloarenes compared to haloalkanes can be explained based on the following factors:

1. Resonance Stabilization: In haloarenes (aryl halides), the halogen atom is attached to a benzene ring, where the lone pairs on the halogen can delocalize into the aromatic π-electron system through resonance. This resonance delocalization creates a partial double bond character between the carbon and halogen (C−X), making the bond stronger and less susceptible to cleavage in reactions such as nucleophilic substitution. In haloalkanes, the C−X bond is a typical single bond without resonance stabilization, making it more reactive and easier to break.
2. Hybridisation State of Carbon: The carbon atom bonded to the halogen in haloarenes is sp² hybridized (part of the aromatic ring), which holds the electrons closer to the nucleus and results in a shorter and stronger C−X bond. In haloalkanes, the carbon is sp³ hybridized, providing a more reactive environment due to the higher electron density availability for reactions.
3. Polarity of the C−X Bond: The polarity of the C−X bond in haloarenes is lower than that in haloalkanes because the resonance effect reduces the polarity. Chlorine (and other halogens) in haloarenes have their lone pairs partly involved in resonance, reducing the partial positive charge on the carbon atom and thus decreasing the electrophilic character of the carbon, making nucleophilic attack harder. In haloalkanes, the C−X bond is more polar, with a significant partial positive charge on carbon, making it more susceptible to nucleophilic attack.