Haloarenes

Haloarenes are aromatic compounds with halogen substituents.

There are two types of haloarenes

(a) Nuclear substituted haloarenes

The aromatic compounds in which halogen atom is directly attached to carbon atom of of aromatic ring, are called as nuclear substituted haloarenes.

(b) Side chain substituted haloarenes

The aromatic compounds in which halogen atom is directly attached to carbon atom of alkyl side chain are called as side chain substituted haloarenes.

Preparation of haloarenes

(a) From aromatic primary amines via diazonium salts

When aromatic primary amine treated with dilute HCl and sodium nitrite at 0 to 5 ^oC gives arene diazonium salt.
For example-

When arene diazonium salt treated with appropriate metal halide gives haloarene. This reaction is known as Sandmeyer reaction.
For example-

For preparation of fluoroarenes, we have to use sodium fluoroborate or fluoroboric acid instead of metal halide.
For example-

(b) By direct halogenations of arenes

(i) Preparation of aryl chloride/bromide

When benzene treated with chlorine in presence of Lewis acid like anhydrous FeCl₃, FeBr₃, AlCl₃, AlBr₃, ZnCl₂, etc. undergoes aromatic electrophilic chlorination and gives chlorobenzene.

When benzene treated with bromine in presence of Lewis acid like anhydrous FeCl₃, FeBr₃, AlCl₃, AlBr₃, ZnCl₂, etc. undergoes aromatic electrophilic bromination and gives bromobenzene.

(ii) Preparation of aryl iodide

When benzene treated with iodine in presence of oxidising agent like AgClO₄ (silver perchlorate), HgO (mercury oxide), HNO₃ (nitric acid), HIO₃ (iodic acid), etc. undergoes aromatic electrophilic iodination and gives iodobenzene.

Here oxidising agent removes HI formed from the reaction mixture so that it will not reduce iodobenzene to benzene.

(iii) Preparation of substituted haloarenes

When toluene treated with chlorine in presence of anhydrous FeCl3 (Lewis acid) gives mixture of 2-chlorotoluene and 4-chlorotoluene.

Limitations of direct halogenations

1. Due to non-polar nature of halogen atoms in halogen molecule, they will not react with benzene directly. Halogens can be polarized in presence of Lewis acid. Thus Lewis acid is necessary in direct halogenations of arenes.

2. Direct halogenations even in presence of Lewis acid results in poor yield of haloarenes.

3. Iodine is least reactive towards direct halogenations of arenes. The reactivity increased by adding oxidising agent like AgClO₄, HgO, HNO₃, etc.

4. Fluorine is very reactive towards direct halogenations (exothermic reaction) at room temperature. Thus fluorination can be done at lower temperature such as -70 ^oC.

Lack of reactivity towards S_N1 and S_N2 reactions

Alkyl halides undergo nucleophilic substitution reactions.

For example-

But aryl halides do not undergo such nucleophilic substitution reactions at normal conditions.
For example-

Hence it it concluded that haloarenes lack reactivity towards S_N1 and S_N2 reactions. The reasons for lack of reactivity are as-

1. The carbon-halogen bond in aryl halide is short and strong as compared to that of alkyl halide.

The carbon-halogen bond in aryl halide has some double bond character due to resonance and hence it is shorter than that of alkyl halide.

2. In carbon-halogen bond-

Carbon atom is sp² hybridised in aryl halide and

Carbon atom is sp³ hybridised in alkyl halide.

The carbon-halogen bond in aryl halide has more s character as compared to carbon-halogen bond in alkyl halide. Hence C-X bond in aryl halide is stronger than C-X bond in alkyl halide.

3. In aryl halide aromatic ring is centre of high electron density because of π-electrons. Due to this nucleophile is discouraged from attacking on ring carbon atom bearing halogen atom.

Due to all these reasons, haloarenes lack reactivity towards S_N1 and S_N2 reactions.

Aromatic nucleophilic substitution reactions of haloarenes

Under drastic conditions (high temperature, high pressure or strong base) haloarenes undergo aromatic nucleophilic substitution reactions.

For example-

Effect of nitro substituents on the nucleophilic substitution reactions

Presence of nitro substituent at ortho or para position helps the haloarene towards nucleophilic substitution reactions.

Aromatic electrophilic substitution reactions

The presence of halogen substituent on benzene ring increases the electron density at ortho and para positions. The increase in electron density at ortho and para positions is due to + mesomeric effect (resonance effect). Here + M effect overcomes – Inductive effect of electronegative halogen atom.

During aromatic electrophilic substitution reactions, the electrophile goes to electron rich ortho or para position of haloarenes.

1. Halogenation

When haloarene treated with chlorine or bromine in presence of Lewis acid such as FeCl₃ or FeBr₃ undergoes aromatic electrophilic substitution reaction i. e. halogenation and gives mixture of ortho and para isomer.

For example-

2. Nitration

When chlorobenzene treated with concentrated nitric acid in presence of concentrated sulphuric acid undergoes aromatic electrophilic substitution reaction i. e. nitration reaction and gives mixture of o-nirtochlorobenzene and p- nirtochlorobenzene.

When nitration of chlorobenzene continued for longer time with excess concentrated nitric acid results in formation of dinitro product i. e. 2,4-dinitrochlorobenzene.

Ullmann reaction

When aryl iodides and aryl bromides heated with copper metal or copper-bronze alloy gives biaryl. This reaction is known as Ullmann reaction.
For example-

When aryl chlorides have nitro group at ortho and/or para positions undergo Ullmann reaction.

Grignard reagent formation

When alkyl halide or aryl halide treated with magnesium metal in ether under anhydrous condition alkyl magnesium halide or aryl magnesium halide. This reaction was discovered by Victor Grignard in 1920 and thus the product of reaction i. e. organomagnesium compound is known as Grignard reagent.

For example-