Haloarenes

1. Introduction

Haloarenes (aryl halides) are compounds in which a halogen atom is directly bonded to the carbon of an aromatic (benzene) ring, e.g. chlorobenzene $\ce{C6H5Cl}$. They differ sharply from haloalkanes because the $\ce{C-X}$ bond in a haloarene is much stronger and less reactive.

Why the $\ce{C-X}$ bond is unreactive in haloarenes:

1. Resonance: a lone pair on the halogen delocalises into the ring, giving the $\ce{C-X}$ bond partial double-bond character; it is shorter and stronger, so the halogen is hard to displace.
2. Hybridisation: the ring carbon is $sp^2$ (more s-character, holds electrons more tightly) versus $sp^3$ in haloalkanes.
3. There is no easy carbocation and the ring repels an incoming nucleophile.

This is why ordinary nucleophilic substitution, so easy for haloalkanes, is very difficult for haloarenes.

2. Nomenclature and Isomerism

Named as halobenzene; disubstituted rings use ortho (1,2), meta (1,3), para (1,4) prefixes.

Haloarenes show position isomerism (o-, m-, p-) in disubstituted rings.