Haloarenes

Lesson

Halogens on a benzene ring

Part 1 — how a halogen bonded directly to a benzene ring differs from one in a haloalkane.

In this lesson you will

Explain why the C–Cl bond in chlorobenzene is shorter and stronger than in chloroalkanes
Write equations for preparing haloarenes by electrophilic substitution and from benzenediazonium chloride
Explain the physical properties of haloarenes, including the high melting point of the para-dichloro isomer

Table of haloarenes and their uses
Haloarene	Use
Chlorobenzene	Solvent; intermediate for phenol, aniline, DDT
1,4-Dichlorobenzene	Moth balls, toilet deodoriser blocks
Bromobenzene	Making phenylmagnesium bromide
DDT	Insecticide (now restricted)

Concept

In a haloarene (aryl halide), X is bonded to an sp² carbon of an aromatic ring: C₆H₅X. In a benzylic halide (C₆H₅CH₂X), X is on an sp³ carbon next to the ring and behaves like a reactive haloalkane.

Table distinguishing aryl and benzylic halides
Compound	IUPAC name	Type
C₆H₅Cl	Chlorobenzene	Aryl halide
C₆H₄Cl₂ (1,4)	1,4-Dichlorobenzene	Aryl dihalide
CH₃C₆H₄Br (para)	1-Bromo-4-methylbenzene	Aryl halide
C₆H₅CH₂Cl	(Chloromethyl)benzene	Benzylic halide (not aryl)

Diagram

Nature of the C–X bond

In chlorobenzene a lone pair on Cl is delocalised into the ring (+R effect), so the C–Cl bond has partial double-bond character. In addition, the carbon is sp² (more s character, more electronegative). Both make the C–Cl bond shorter and stronger than in chloroalkanes.

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