Electrophilic Substitution Reaction

Electrophilic Substitution is a type of organic reaction in which an electrophile replaces a functional group in a molecule. The functional group is generally a hydrogen atom. This reaction is commonly observed in aromatic compounds, where the aromaticity of the ring is maintained. Some of the common examples are Nitration, Halogenation, Sulfonation, etc.

In this article, we will learn about Electrophilic substitution reactions, its example, its mechanisms, their types, and the difference between electrophilic and nucleophilic substitution reactions.

What is Electrophilic Substitution Reaction?

Electrophilic substitution reactions are among the most fundamental and interesting reactions in organic chemistry. These reactions play an important role in the formation of a large number of organic compounds.

Electrophilic substitution reaction is a type of organic reaction in which an atom or group of atoms in a molecule is replaced by an electrophile. An electrophile is a species that seeks electrons and is electron-deficient.

This reaction is particularly common in aromatic compounds, where the aromaticity of the ring is preserved during the substitution process.

In an electrophilic substitution, the functional group which gets displaced is usually a hydrogen atom. These reactions are of paramount importance in organic synthesis, allowing chemists to selectively modify aromatic compounds and create a diverse range of organic products with various applications, including the synthesis of pharmaceuticals, dyes, and fragrances.

Steps in Electrophilic Substitution Reaction

Following steps happens when any compound goes through electrophilic substitution

• Generation of an Electrophile
• Formation of an Intermediate (which is a Carbocation)
• Removal of a proton from Intermediate

Examples of Electrophilic Substitution Reaction

Electrophilic substitution reactions are a class of organic reactions where an electrophile (an electron-seeking species) replaces a functional group or atom in a molecule. Examples of electrophilic substitution reactions include:

• Electrophilic aromatic nitration
• Electrophilic aromatic sulphonation reactions
• Friedel Craft Alkylation and Acylation
• Halogenation of Aromatic ring
• Halogenation of Alkanes
• Aliphatic Diazonium Coupling, etc.

Types of Electrophilic Substitution Reactions

There are two types of electrophilic substitution reactions that take place in organic compounds.

• Electrophilic Aromatic Substitution Reactions
• Electrophilic Aliphatic Substitution Reactions

Electrophilic Aromatic Substitution Reaction

Electrophilic aromatic substitution reactions are the organic reactions in which the electrophile replaces the atom attached to the aromatic ring. In these reactions, the hydrogen atom from the benzene ring is replaced by an electrophile.

Examples of Electrophilic Aromatic substitution are electrophilic aromatic nitration, electrophilic aromatic sulphonation reactions, Friedel Craft Alkylation, and acetylation.

An electrophilic aromatic system substitution reaction preserves the aromaticity of an aromatic system. For example, when benzene is reacted with bromine, the stability of benzene is not lost when the aromatic ring is replaced with brominated benzene.

Electrophilic Aliphatic Substitution Reaction

Electrophilic aliphatic substitution involves the substitution of a hydrogen atom in an aliphatic (non-aromatic) compound with an electrophile. An electrophile substitutes the functional group (usually hydrogen) in an aliphatic molecule in electrophilic aliphatic substitution processes.

Unlike electrophilic aromatic substitution, which mainly occurs on aromatic rings, electrophilic aliphatic substitution occurs on aliphatic carbon chains. Electrophilic aliphatic substitution reactions are generally less common and less studied than electrophilic aromatic substitution reactions.

Electrophilic aliphatic substitution often involves free radical mechanisms, and the conditions for these reactions can vary depending on the specific reaction and reactants involved. Electrophilic aliphatic substitution is classified into 5 types:

• Halogenation in Ketone
• Nitrosation Reaction
• Keto- Enol tautomerism
• Insertion of a carbene into a carbon-hydrogen bond
• Aliphatic Diazonium Coupling

Mechanism of Electrophilic Substitution Reaction

The mechanism of electrophilic substitution reactions involves several steps that are discussed below:

• Generation of Electrophile: An electrophile is generated, by the interaction of a reagent or a catalyst with a precursor molecule. Common electrophiles are positively charged ions or species with partially positive charges.

• Attack of Electrophile: The electrophile attacks the electron-rich site in the substrate, a molecule containing a π – electron system, such as an aromatic ring. The attack occurs at a position with high electron density.

• Formation of a Sigma Complex (Intermediate): The attack of electrophile results in the formation of a sigma complex or an intermediate. This intermediate is often a positively charged species (carbocation), which may be stabilized by resonance or other factors.

• Rearrangement and Stabilization: Intermediate formed may undergo rearrangement to a more stable form depending on the specific reaction conditions and the nature of the substrate. Stabilization can occur through resonance, delocalization of charge, or other electronic effects.

• Loss of a Proton (or any group): The intermediate loses a proton (or any other group) to regain aromaticity or to achieve a more stable configuration. This step is essential for completing the substitution and maintaining the overall stability of the molecule.

Electrophilic Substitution Reaction Order

The order of an electrophilic substitution reaction refers to the number of steps involved in the reaction mechanism. Electrophilic substitution reactions on aromatic compounds typically follow a two-step mechanism known as the electrophilic aromatic substitution mechanism. The general steps are:

Formation of the Electrophile (Activation Step): In this step, an electrophile is generated or activated. This often involves the reaction of a strong Lewis acid catalyst with a reactant to form an electrophile.

Attack of the Electrophile on the Aromatic Ring (Attack Step): The activated electrophile then attacks the electron-rich aromatic ring, leading to the substitution of a hydrogen atom with the electrophile. This step forms the final substituted aromatic compound.

The order of the reaction is determined by the rate-determining step, which is usually the slower of the two steps. For many electrophilic aromatic substitution reactions, the attack step is the slower step, and thus the overall reaction is considered to be second-order.

However it is important to know that these reactions are not always strictly second-order, and the specific conditions and nature of the reactants can influence the kinetics of the reaction. However, the general two-step mechanism is a common feature of electrophilic aromatic substitution reactions.

Electrophilic Substitution Reaction of Various Compounds

The Electrophilic Substitution Reaction differs with distinct compound. Some of the most common Electrophilic Substitution Reaction are mentioned below:

• Electrophilic Substitution Reaction of Benzene
• Electrophilic Substitution Reaction of Aniline
• Electrophilic Substitution Reaction of Phenol

Electrophilic Substitution Reaction of Benzene

In Electrophilic substitution of benzene, an electrophile substitutes the hydrogen atom of benzene. This reaction occurs when benzene reacts with nitric acid and sulfuric acid to form nitrobenzene. Here, the benzene and sulfuric acid forms an electrophile, which creates a resonance-stabilized structure when reacted with nitric acid.

Electrophilic substitution reaction of benzene is an example of an electrophilic aromatic substitution reaction. There are different types of electrophilic aromatic substitution reactions but the most important of these reactions are:

• Aromatic Nitration reactions
• Electrophilic aromatic halogenation reactions
• Aromatic Sulfonation reactions
• Friedel-Crafts Alkylation reaction
• Friedel-Crafts Acylation reaction

The aromatic system which is used as reactant in these reactions is Benzene.

Electrophilic Substitution Reaction of Aniline

Electrophilic reaction that anilines experience are sulphonation, nitration, and halogenation. Because aniline’s functional group (-NH₂) donates electrons, it is highly activating for the electrophilic substitution process. The benzene ring’s ortho- and para-positions have more electrons or negative charge than the meta-position because of the ring’s different resonating structures. Therefore, in an electrophilic substitution process, anilines are o- and p-directing.

Halogenation of Aniline

When aniline comes in contact with bromine water, the bromine molecule polarizes itself. With a little positive charge, bromine functions as an electrophile and targets the electron-rich ortho and para locations of aniline. At room temperature, aniline reacts with bromine water and produces a white precipitate named, 2,4,6-tri-bromoaniline.

Nitration of Aniline

Aniline produces tarry oxidation products in addition to nitro derivatives when it is directly nitrated. Aniline is protonated to create the meta-directing anilinium ion in a highly acidic media. For this reason, very less quantity of meta derivative is also generated in addition to ortho and para derivatives.

Sulphonation of Aniline

Reaction of aniline with concentrated sulfuric acid forms aniline hydrogen sulfate which when heated with sulfuric acid at a temperature of 453–473 K, gives p-amino benzenesulfonic acid, which is known as sulfanilic acid.

Electrophilic Substitution Reaction of Phenol

Due to rich electron density, phenols are highly prone to electrophilic substitution reaction.

The hydroxyl group which is attached to the aromatic ring in phenol provide the delocalization of the charge in the aromatic ring. Thus, it stabilizes the arenium ion through resonance. Thus, hydroxyl group acts as ortho para directors as electron density is increased at ortho and para position due to delocalization of charge in the ring.

Difference Between Electrophilic and Nucleophilic Substitution Reaction

The key difference between electrophilic and nucleophilic substitution reaction is as follows:

Electrophilic Substitution Reaction	Nucleophilic Substitution Reaction
It involves the attack of an electron-rich atom or group on an electrophile generated during the process.	It involves the attack of a nucleophile on an electron-deficient atom in the substrate.
The mechanism typically proceeds through the formation of a positively charged intermediate (arenium ion or sigma complex) followed by the regain of aromaticity.	The mechanism has two pathways to follow: SN1 (unimolecular) or SN2 (bimolecular), depending on the structure of the substrate and the nature of the nucleophile.
Electrophilic substitution reactions are more common in aromatic compounds.	Nucleophilic substitution reactions are more common in aliphatic compounds.
Examples of electrophilic substitution reactions are: aromatic substitution nitration, halogenation, sulphonation, and Friedel-Crafts reactions.	Examples of Nucleophilic substitution reactions are: SN1 and SN2 reactions in alkyl halides.

Related Reads

• Electrophiles and Nucleophile
• Chemical Reaction of Amine
• Reaction of Haloarenes

Electrophilic Substitution Reaction FAQs

What is Electrophilic and Nucleophilic Substitution Reaction?

Electrophilic Substitution Reaction involves the attack an electron-rich atom or group on an electrophile generated during the process whereas Nucleophilic substitution reaction involves the attack of a nucleophile on an electron-deficient atom in the substrate.

What are Four Electrophilic Aromatic Substitution Reactions?

Aromatic nitration, Aromatic halogenation, Aromatic sulfonation, Acylation Friedel–Crafts reaction are the four examples of electrophilic aromatic substitution reaction.

What is Electrophilic Substitution Condition?

In a electrophilic substitution reaction, a pair of pi-bonded electrons attacks the electrophile and a proton is abstracted from an adjacent carbon to reestablish the double bond.

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

Catalysts such as AlCl₃ or FeBr₃ also known as Lewis Acid catalyst are used which speeds up the chlorination and bromination reaction of an aromatic ring.

What are Types of Electrophilic Substitution Reactions?

It is of two types i.e. Electrophilic Aliphatic Substitution and Electrophilic Aromatic Substitution reactions.

What is Electrophilic Substitution Reaction?

Electrophilic substitution reaction is a type of organic reaction in which an atom or group of atoms in a molecule is replaced by an electrophile. In an electrophilic substitution, the functional group which get displaced is usually a hydrogen atom.

What are Electrophiles and Nucleophiles?

An Electrophile is a species that is electron-deficient in nature and tends to gain electrons during a chemical reaction. Electrophiles are attracted towards electron-rich sites in other molecules.

A nucleophile is a species that is electron-rich and tends to donate its electrons during a chemical reaction. Nucleophiles are attracted towards electron-deficient sites in other molecules.

What are Examples of Electrophilic Substitution Reaction?

Examples of electrophilic substitution reaction are : Nitration of Benzene , Bromination of Toluene, Halogenation of Alkanes (Free Radical Substitution) , Friedel-Crafts Alkylation etc.

What are Applications of Electrophilic Substitution Reaction?

These reactions are important part of organic synthesis, allowing chemists to selectively modify the aromatic compounds and forms a wide range of organic products with various applications, including the synthesis of pharmaceuticals, dyes, and fragrances. It is mainly use in Pharmaceutical Industry, Polymer Chemistry, Materials Science, Functionalization of Aromatic Compounds etc.