Electrophilic Substitution Reaction

What is an Electrophilic Substitution Reaction?

An electrophilic substitution reaction is a chemical reaction in which the functional group attached to a compound is replaced by an electrophile. The displaced functional group is typically a hydrogen atom. Electrophilic substitution reactions generally proceed via a three-step mechanism that involves the following steps.

  • The generation of an electrophile
  • The formation of a carbocation (which is an intermediate)
  • The removal of a proton from the intermediate

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Types of Electrophilic Substitution Reactions

The two primary types of electrophilic substitution reactions undergone by organic compounds are electrophilic aromatic substitution reactions and electrophilic aliphatic substitution reactions. An illustration describing the electrophilic substitution of a hydrogen atom (belonging to a benzene molecule) with a chlorine atom is provided below.

Electrophilic Substitution Reaction Example

Example of an Electrophilic Substitution Reaction

Here, the chlorine cation acts as an electrophile and replaces a hydrogen atom in the benzene ring. The products formed in this electrophilic substitution reaction include a proton and a chlorobenzene molecule.

Electrophilic Aromatic Substitution Reaction

In electrophilic aromatic substitution reactions, an atom attached to an aromatic ring is replaced with an electrophile. Examples of such reactions include aromatic nitrations, aromatic sulfonations, and Friedel-Crafts reactions.

It is important to note that the aromaticity of the aromatic compound is preserved in electrophilic aromatic substitutions. Therefore, these reactions can be used to obtain aryl halides from aromatic rings and iodine, bromine, or chlorine.

Electrophilic Aliphatic Substitution Reaction

In electrophilic aliphatic substitution reactions, an electrophile replaces the functional group (generally hydrogen) in an aliphatic compound. These reactions can be classified into the following five types.

  • Halogenation of ketones
  • Nitrosation
  • Keto-Enol tautomerism
  • Insertion of a carbene into a carbon-hydrogen bond
  • Diazonium coupling (aliphatic)

These electrophilic substitution reactions can result in an inversion of configuration if the electrophilic attack occurs at an angle of 180o to the leaving group (attack from the rear).

Mechanism of Electrophilic Substitution Reaction

The electrophilic substitution reaction mechanism involves three steps.

Step 1: Generation of Electrophile

Anhydrous aluminium chloride is a very useful Lewis acid in the generation of electrophile from the chlorination, alkylation, and acylation of an aromatic ring. The resulting electrophiles (from the combination of anhydrous aluminium chloride and the attacking reagent) are Cl+, R+, and RC+O respectively as shown below:

Electrophilic Substitution Reaction Mechanism Step 1

Electrophilic Substitution Reaction Mechanism Step 1

Step 2: Formation of Carbocation

The electrophile attacks the aromatic ring, forming a sigma complex or an arenium ion. One of the carbons in this arenium ion is sp3 hybridized.

Electrophilic Substitution Reaction Mechanism Step 2a

Electrophilic Substitution Reaction Mechanism Step 2a

This arenium ion finds stability in a resonance structure. Since the delocalization of electrons stops at the sp3 hybridized carbon, the sigma complex or the arenium ion loses its aromatic character.

Electrophilic Substitution Reaction Mechanism Step 2b

Electrophilic Substitution Reaction Mechanism Step 2b

Step 3: Removal of Proton

In order to restore the aromatic character, the sigma complex releases a proton from the sp3 hybridized carbon when it is attacked by the [AlCl4]. The reaction describing the removal of a proton from the sigma complex is given below:

Electrophilic Substitution Reaction Mechanism Step 3

Electrophilic Substitution Reaction Mechanism Step 3

Thus, the electrophile replaces the hydrogen atom in the benzene ring. The electrophilic substitution reaction is a very important reaction in organic chemistry as the concept is used in many organic name reactions.

Frequently Asked Questions on Electrophilic Substitution Reaction

What are the Two Types of Electrophilic Substitution Reactions?

The two primary types of electrophilic substitutions include electrophilic aliphatic substitution and electrophilic aromatic substitution.

What Catalysts are used in the Chlorination and Bromination of Aromatic Rings?

For the chlorination of an aromatic ring, Lewis acid catalysts such as AlCl3 or FeCl3 greatly increase the rate of the reaction. This is because the Lewis acids form a complex with the chlorine molecule, liberating a highly electrophilic Cl+ species. When an aromatic ring must be brominated, catalysts such as AlBr3 or FeBr3 can be used instead.

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