Electrophilic Substitution Reaction - Practice Questions & MCQ

According to experimental evidences, electrophilic substituion reactions are supposed to proceed via the following three steps:

(a) Generation of the eletrophile

During chlorination, alkylation and acylation of benzene, anhydrous AlCl₃, being a Lewis acid helps in generation of the electrophile by extracting a lone pair donor and forming the respective electrophile

(b) Formation of carbocation intermediate

The electrophile generated in the first step attacks the Benzene ring and forms the Arenium ion or the -complex

It is to be noted that the fromation of Arenium ion leads to a loss of aromaticity.There is resonance stabilisation of the arenium ion

(c) Removal of proton

 To restore the aromaticity of the Benzene ring, there is a removal of proton from the Arenium ion by the conjugate base of the Lewis acid

Directive influence of a functional group in monosubstituted benzene

When the substituent is Electron Donating in nature

• The groups like  etc are electron donating and they create a partial negative gharge at their ortho and para positions
• The ortho, para positions of these substituted benzenes are electron-rich in nature.
• Electrophile is electron deficient in nature. Thus, it tries to attack the position which is electron-rich.
• Thus, these groups are called as are called ortho-para directing.

When the substituent is Electron Withdrawing in nature

• The groups like Nitro, acyl, sulpho groups are electron withdrawing  and they create a partial postiive charge at their ortho and para postions
• Thus, relatively the meta position is more electron rich in such cases and the electrophile attacks there
• These groups are hence known as Meta-directing groups because they sent the electrophile to the meta position.

Case of Halogens

• Halogens exert a +M and -I effect and generally the effect of -I dominates
• Halogens are hence deactivating in nature
• However, the +M effect stabilises the Arenium ion which is formed by the attachment of electrophile at the Ortho and para positions
• Halogens are thus deactivating and yet Ortho-Para directing in nature as far as the electrophilic aromatic substitution is concerned