Reactions of Haloarenes - Electrophilic Substitution Reaction

Electrophiles

The species which are electron deficient are called electrophiles. An electrophile can be a positively charged or neutral species (having a vacant orbital).

Electrophilic Substitution Reaction

The replacement of an atom or group of atoms by an electrophile in a molecule is called an electrophilic substitution reaction. Haloarenes undergo the usual electrophilic reactions of the benzene ring such as halogenation, nitration, sulphonation and Friedel-Crafts reactions.

Behaviour of the Reaction Of Haloarenes towards the attack of an electrophile

We will discuss about them one-by-one, but let’s first understand the behaviour of Reaction Of Haloarenes 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 electron or negative charge over ortho- and para- positions of the ring than the meta- position. Thus, haloarenes are o- and p- directive towards electrophilic substitution reaction.

Due to the above reasons, haloarenes are 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 of Haloarenes when a haloarene comes in the vicinity of chlorine in the presence of ferric chloride, then the chlorine molecule cleaves homolytically to produce chloride ion and Chlorine with a slightly positive charge. The chlorine with a slightly positive charge acts as an electrophile (electron seeking) and attacks the electron-rich ortho and para position of haloarene. Both ortho and para compounds are formed out of which para isomer is the major product, and ortho isomer is the minor product.

Nitration

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

Sulphonation

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

Friedel-Crafts reaction

In this reaction, the alkyl and acetyl groups act as electrophiles due to the presence of a positive charge over the carbon atom. These groups attack the electron-rich ortho and para positions of the haloarene, in which the para isomer of the product is obtained as the major product and ortho as the minor product.

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