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.

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.,

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).

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 -NO_2, -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.

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.

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

Reaction with lithium

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

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.

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

Reaction with copper powder

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

Reduction

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.

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).

Halogenation

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

Nitration

Haloarenes with concentrated HNO₃ in the presence of concentrated H₂SO₄.

Sulphonation

Sulphonation occurs when haloarene is treated with concentrated H₂SO₄.

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,

Problems

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

Solution

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

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?

Solution

(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.