Q.Conversion of chlorobenzene to phenol by aqueous sodium hydroxide requires high temperture of about 623K and high pressure. Explain.

The halogen in a haloarene is attached to an aromatic ring carbon, which is always sp2 hybridized.

In a haloarene (e.g., chlorobenzene), the halogen is bonded to a carbon that is part of the benzene ring's planar, delocalised π system. Every ring carbon in benzene is sp2 hybridized (three sp2 orbitals form the three σ-bonds — two to adjacent ring carbons, one to the halogen — while the unhybridized p-orbital is part of the delocalised ring π system). This sp2 carbon, and partial conjugation of …

Resonance delocalisation of the halogen's lone pair into the aromatic ring strengthens and shortens the C–X bond, while the sp2 aromatic carbon binds the halogen more tightly than an sp3 alkyl carbon — both effects resist nucleophilic attack.

1. Resonance (C–X bond gets partial double-bond character): In a haloarene, one of the lone pairs on the halogen atom is delocalised into the aromatic π-system through resonance. This gives the C–X bond partial double-bond character, making it shorter and stronger than the pure single C–X bond in a haloalkane, and therefore much harder to break.

2. Difference in hybridisation of carbon: The carbon bearing the halogen in a haloarene is sp²-hybridised (part of the aromatic ring), whereas in a haloalkane it is sp³-hybridised. Since sp² carbon has greater s-character, it holds its bonding electrons (and hence the attached halogen) more tightly and at a shorter bond length than sp³ carbon does — again strengthening the C–X bond in haloarenes.

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