Aryl Halide - Reactions

Introduction

Haloarene or aryl halide is a chemical compound containing arenes, where one or more hydrogen atoms are bonded to the aromatic ring are replaced with halogens. They are generally denoted by “Ar-X” where ‘X’ denotes the halogen atom attached and ‘Ar’ stands for the aryl group. Some reactions of aryl halides are discussed below.

Reaction of Aryl halide – Electrophilic Substitution Reaction

Electrophile refers to an electron seeking species. Thus, electrophilic substitution reaction refers to the reaction in which an electrophile substitutes another electrophile in an organic compound. Aryl halide undergoes the usual electrophilic reactions of the benzene ring such as halogenation, nitration, sulphonation and Friedel-Crafts reactions.  We will discuss them one-by-one, but let’s first understand the behaviour of aryl halide towards the attack of an electrophile:

1. Due to –I effect (electron withdrawing nature) of halogen, benzene ring gets somewhat deactivated towards electrophilic substitution reaction.
2. Due to its various resonating structures, there’s an excess of an electron or negative charge over ortho- and para- positions of the ring than the meta- position. Thus, aryl halide is o- and p- directive towards electrophilic substitution reaction.

Due to the above reasons, an aryl halide is somewhat deactivated than normal benzene ring towards the electrophilic substitution reaction. Thus, these reactions occur slowly and require more drastic conditions as compared to those in benzene.

Halogenation

In the above reaction when an aryl halide comes in the vicinity of chlorine in the presence of ferric chloride as solvent, then the chlorine molecule develops a polarity within itself and the chlorine with a slightly positive charge acts as an electrophile (electron seeking) and attacks the electron-rich ortho and para positions of aryl halide. Both ortho and para compounds are formed out of which para isomer is the major product and ortho isomer is the minor product.

Aryl Halide Halogenation

Nitration

In this reaction, the first NO₂ is formed from nitric acid which is initiated by the presence of a sulphuric acid. NO₂ has an electrophilic center over N due to the presence of two electronegative oxygen atoms in the molecule. NO₂ attacks the electron-rich ortho and para positions and we get para isomer as the main product and ortho isomer as the minor product.

Aryl Halide Nitration

Sulphonation

In this reaction, SO₃ formed from sulphuric acid acts as an electrophile. SO₃ attacks the electron-rich ortho and para positions of the aryl halide, out of which para isomer is obtained as the major product and ortho isomer as the minor product.

Aryl Halide Sulphonation

Friedel-Crafts reaction

In this reaction, alkyl or carbonyl group acts as an electrophile due to the presence of positive charge over the carbon atom. These groups attack the electron-rich ortho and para positions of the aryl halide. The para isomer of the product is obtained as the major product and ortho as the minor product.

Aryl Halide Friedel Crafts reaction

Reactions of Aryl halide with Metal

• Wurtz-Fittig reaction: When a mixture of alkyl halide and aryl halide is treated with sodium in dry ether alkylarene is formed. This reaction is known as a Wurtz-Fittig reaction.

Aryl Halide Wurtz Fittig reaction

• Fittig reaction: When haloarenes are treated with sodium in dry ether, two aryl groups get joined together. This reaction is known as Fittig reaction.

Aryl Halide Fittig reaction

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