Chemical properties of haloarenes

Haloarenes are chemically less reactive than haloalkanes. They can undergo the following reactions:

Replacement by hydroxy group (formation of phenol)

When aryl halides are heated at 623 K and under pressure (300 atm), with aqueous solution of sodium hydroxide, the halogen atom is replaced by hydroxyl group forming phenol. Firstly sodium phenoxide is formed, which on acidification gives phenol.

manufacture of phenol by Dow s process

This reaction forms the basis for the manufacture of phenol by Dow's process.

Substitution by amino group (formation of aniline)

The halogen atom is replaced by amino group when aryl halides are heated with aqueous ammonia in the presence of cuprous oxide (catalyst) at 475 K and under a pressure of 60 atm.,

formation of aniline by substitution by amino group

Substitution by cyano group (formation of cyanobenzene)

The halogen atom is replaced by cyano group on heating with anhydrous cuprous cyanide in the presence of pyridine or dimethylformamide (DMF).

formation of cyanobenzene in the presence of pyridine

The cyanobenzene can be converted into other useful compounds like Benzamide, benzoic aid and benzylamine under different conditions.

Effect of substituents in haloarenes (aryl halides) on the reactivity

The reactivity of haloarenes is markedly affected by the presence of certain groups at certain positions of the ring. The presence of electron withdrawing groups such as -NO2, -CN, -COOH, etc. at 'o' and 'p' positions to the halogen atom, greatly activates the halogen towards nucleophilic substitution reactions. For example, chlorobenzene is converted into phenol by aqueous NaOH only at temperatures above 573 K, whereas 'p' chloronitrobenzene is converted into nitrophenol by aqueous NaOH at a lower temperature of 433K. As the number of ortho and para nitro groups on the ring is increased the reactivity increases.

formation of nitrophenol from chloronitrobenzene

formation of dinitrophenol from dinitrochlorobenzene

formation of trinitrophenol from trinitrochlorobenzene

Nitro group meta to the chlorine has no effect on the reactivity.

Reaction of magnesium

Aryl bromides and iodides react with magnesium in dry ether to form Grignard reagent.

formation of phenyl magnesium bromide from arylbromides

Chlorobenzene reacts with magnesium in the presence of tetrahydrofuran (THF) solvent.

formation of phenyl magnesium chloride from chlorobenzene

Reaction with lithium

Bromo and iodoarenes react with lithium metal in the presence of dry ether to form corresponding organo metallic compounds.

formation of phenyl lithium from bromoarens with lithium

Reaction with sodium

Aryl halide undergo 'Wurtz Fittig reaction' when heated with alkyl halide in the presence of sodium in anhydrous ether. Halogen atom is replaced by alkyl group.

aryl halide undergo Wurtz Fittig reaction

However, when haloarenes react with sodium in the presence of ether, diphenyl is formed. This reaction is called 'Fittig reaction'.

diphenyl formation

Reaction with copper powder

Iodobenzene gives diphenyl when heated with copper powder in a sealed tube. This reaction is called 'Ullmann reaction'.

diphenyl from iodobenzene


With LiAlH4 or nickel aluminium alloy (Ni-Al), haloarenes undergo reduction to hydrocarbons in the presence of an alkali. The reduction is brought about by nascent hydrogen.

reduction of chlorobenzene

Ring substitution or electrophilic substitution reactions

The halogen atom is ortho and para directing and therefore, substitution takes place at ortho and para positions giving electrophilic substitution reactions at the benzene ring.

In the resonance structures of chlorobenzene the ortho and para positions get negatively charged, i.e. the electron density is relatively more at ortho and para positions. The incoming electrophile is more likely to attack these positions. But, because of steric hindrance at the ortho position, the para-product usually predominates over the ortho product. Also, the halogen atom has -I effect because it is a electron withdrawing group. As a result, it tends to deactivate the benzene ring. Therefore, the electrophilic substitution reactions of chlorobenzene occur slowly and under drastic conditions compared to benzene. Thus,
  • Haloarenes undergo electrophilic substitution reactions slowly as compared to benzene.
  • Halogen group is ortho and para directing (para-product usually predominates over the ortho product).


Haloarenes react with halogens in the presence of ferric salt as catalyst to give ortho para isomers.

Haloarenes react with halogens  to give ortho para isomers


Haloarenes with concentrated HNO3 in the presence of concentrated H2SO4.

nitration of haloarenes


Sulphonation occurs when haloarene is treated with concentrated H2SO4.

sulphonation of haloarene

Alkylation and acylation

The alkylation and acylation reaction, known as 'Friedel-Craft reaction', is carried by treating haloarene with alkyl chloride or acyl chloride in the presence of a catalyst like anhydrous aluminium chloride. For example,

alkylation of chlorobenzene

acylation of chlorobenzene


4. Write the possible isomers of the aromatic compound having molecular formula C7H7Cl. Which of these have weakest C-Cl bond?


The possible isomers of the aromatic compound having molecular formula C7H7Cl are benzyl chloride and ortho chlorotoluene and para chlorotoluene.

 possible isomers  having molecular formula C7H7Cl

Benzyl chloride has the weaker bond because the chlorine atom is at the substituted side chain of the alkyl group. In o- and p- chlorotoluene the C-Cl bond is stronger because it is directly part of the benzene ring. This makes the C-Cl bond less polar due to electron releasing resonance effect. p-chloro toluene is more stable than o-chloro toluene.

5. Which of the following is an aryl halide?

structure of chlorobenzene benzyl chloride chlorocyclohexane


(c) is chlorobenzene, an aryl halide because the chlorine is directly attached to the benzene ring. (a), is benzyl chloride which is a substituted haloarene, while (b) is chlorocyclohexane which is a haloalkane.

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