Electrophilic Substitution Reaction

1.0Introduction

An electrophilic substitution reaction is a type of chemical reaction where an electrophile replaces a functional group—usually a hydrogen atom—in an organic compound, often an aromatic system like benzene.

These reactions are common in aromatic chemistry, where the electron-rich aromatic ring is targeted by an electron-deficient species (electrophile).

2.0Mechanism of Electrophilic Substitution Reaction

The reaction typically occurs in three main steps:

1. Generation of the Electrophile

An electrophile (electron-loving species) is produced, often by using a catalyst or an acid.

2. Formation of a Carbocation Intermediate (Arenium Ion)

The electrophile attacks the π-electron-rich aromatic ring, forming a positively charged intermediate known as a carbocation or arenium ion.

3. Deprotonation (Loss of a Proton)

The carbocation loses a proton (H⁺) to restore aromaticity, resulting in the substituted aromatic compound.

An example of electrophilic substitution is the replacement of a hydrogen atom in a benzene ring with a chlorine atom. In this reaction, the chlorine cation (Cl⁺) acts as the electrophile and substitutes a hydrogen atom on the benzene ring. The reaction yields chlorobenzene and a proton (H⁺) as products.

3.0Types of Electrophilic Substitution Reactions

Electrophilic substitution reactions are broadly classified into two main types:

1. Electrophilic Aromatic Substitution (EAS)
2. Electrophilic Aliphatic Substitution (EASu)

Electrophilic Aromatic Substitution (EAS)

In these reactions, an atom (typically hydrogen) on an aromatic ring is replaced by an electrophile. Common examples include:

A key feature of these reactions is that the aromaticity of the ring is preserved, making them particularly important for synthesizing aryl halides and other substituted aromatics.

Electrophilic Aliphatic Substitution

In these reactions, an electrophile replaces a functional group (commonly a hydrogen atom) in an aliphatic compound. These can be further classified into:

• Halogenation of ketones
• Nitrosation
• Keto-enol tautomerism
• Carbene insertion into a C–H bond
• Diazonium coupling (aliphatic compounds)

In cases where the electrophile attacks from the opposite side of the leaving group (180° angle), the reaction may proceed with inversion of configuration, similar to an SN2 mechanism.

4.0Why Benzene Undergoes Electrophilic Substitution and Not Addition

Benzene is particularly prone to attack by electrophiles because of its exposed π-electrons. In this way, benzene behaves somewhat like an alkene, which also undergoes reactions at its π-bond when attacked by electrophiles.

However, benzene differs from an alkene in an important way: it has a stable closed shell of six π-electrons, often referred to as an aromatic sextet, which makes it much more stable than an ordinary alkene. Because of this stability, benzene tends to undergo substitution reactions instead of addition reactions. In substitution, a hydrogen atom is replaced by an electrophile, but the aromatic ring's π-system is restored, preserving the molecule’s aromaticity.

5.0Mechanism of Electrophilic Aromatic Substitution:

Step 1: Formation of a π-Complex and the Arenium Ion

When the electrophile (E⁺) is generated, it first forms a weak interaction with the electron-rich π-cloud of benzene. This initial interaction forms a π-complex, a type of donor-acceptor complex where benzene donates electrons and the electrophile accepts them. These are also known as charge-transfer complexes.

For example, when benzene interacts with bromine, the Br₂ molecule aligns perpendicularly to the benzene ring and forms such a π-complex.

Then, two electrons from the π-system are used to form a σ-bond between the electrophile and one carbon atom of the benzene ring. This disrupts the aromatic π-system, as the involved carbon becomes sp³ hybridized and loses its p-orbital. The resulting intermediate is called the arenium ion (or sigma complex). Though aromaticity is temporarily lost, the system is stabilized by resonance, with the positive charge delocalized across the ring.

Step 2: Restoration of Aromaticity

Next, a proton (H⁺) is removed from the same carbon that bonded with the electrophile. The electrons from the C–H bond return to the ring, restoring the π-system. This converts the sp³ carbon back to sp² hybridization, and the ring regains its aromatic character.

The base that removes the proton can be any anion present in the reaction mixture (such as the one derived from the original electrophile).

6.0Kekulé Structures in Mechanisms

In reaction mechanisms like electrophilic aromatic substitution, using Kekulé structures (with alternating single and double bonds) is useful because it allows us to illustrate resonance and electron movement clearly. Although modern representations of benzene show a circle to represent delocalized electrons, the Kekulé form is better suited for mechanistic steps.

Kinetics of the Reaction

This reaction can be broken down into steps with rate constants:

• k₁ is the rate constant for the forward formation of the arenium ion.
• k₋₁ is for the reverse reaction.
• k₂ is the rate at which the proton is lost and aromaticity is restored.

Note:  Electrophilic aromatic substitution and electrophilic aliphatic substitution are both types of electrophilic substitution reactions where an electrophile replaces a functional group. However, they differ in the type of molecule involved: aromatic (containing a benzene ring) and aliphatic (a chain or ring of carbon atoms not part of a benzene ring).