Electrophilic Substitution Reaction

The process in which an electrophile (an electron pair acceptor) replaces the functional group attached to a compound is called an electrophilic substitution reaction. In an electrophilic substitution, the displaced functional group is usually a hydrogen atom. Electrophilic substitution occurs in many reactions of arenes (compounds containing benzene rings), and it is called electrophilic aromatic substitution reactions. The electrophilic aliphatic substitution reaction is another primary type of electrophilic substitution. The three steps involved in the electrophilic substitution reaction are the generation of an electrophile, then the formation of carbocation that acts as an intermediate, and the removal of a proton from the medium.

Basic examples of electrophilic substitution reactions of benzene are alkylation, acylation, halogenations, nitration, sulphonation, etc.

Electrophilic Substitution Reaction Mechanism

The electrophilic Substitution reaction mechanism consists of three steps, and we will discuss further them,

  1. Electrophile Generation:  

The anhydrous chloride is beneficial for the generation of electrophiles through the process of chlorination, alkylation, and acylation of an aromatic ring. The electrophiles produced by the combination of anhydrous aluminum chloride with the attacking reagent are Cl+, R+, and RC+O, respectively.

  1. Carbocation Formation:

The aromatic ring then is attacked by the electrophile, which forms an arenium ion or sigma complex. In the sigma complex, one of the carbon will be sp3 hybridized.

Part a of the electrophilic substitution reaction step 2: In the resonance structure, the arenium ion or the sigma complex finds stability. But the sigma complex loses its aromatic character because the delocalization of electrons stops at the carbon that is sp3 hybridized. 

  1. Proton Removal:

The sigma complex or the arenium ion releases a proton from the carbon that is sp3 hybridized when AlCl4 attacks it, and this step is necessary to restore the aromatic character. In this third step, the hydrogen is replaced by the electrophile in the benzene ring. 

Reaction of Amines

Firstly to answer what are amines- Amines are the organic derivatives of ammonia (NH3), in which the hydrogen is replaced by the alkyl, cycloalkyl, or aromatic groups to bond with the Nitrogen atom. The simplest case of aromatic amines is aniline, where amine-type nitrogen bound to an aromatic ring.

Some examples of the reaction of amines are,

  • Amine reacts as a base with acid to form a salt.

  • Amine reacts as a nucleophile with alkyl halide through the substitution reaction of SN2 is called alkylation.

  • Primary aliphatic amines, when oxidized by KMNO4, produce ethanol.

Types of Electrophilic Substitution Reactions of Anilines

Aniline is an organic compound that consists of a phenyl group attached to an amino group, and it has the molecular formula of C6H5NH2. In aniline, the functional group (NH2) is very active towards electrophilic substitution reaction because it is an electron-donating type group. And Aniline having several resonating structures also has an excess of negative charge or electron in the ortho and para positions of a benzene ring than that of the meta position; hence the o and p positions are directive towards the electrophilic substitution reaction of an aniline. The types of electrophilic substitution reactions that aniline can perform are,  

Nitration of Aniline   

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In the nitration of aniline reaction, meta isomer is also observed along with para because the aniline molecule gets protonated in an acidic medium to become an Anilion ion. Though the NH2 group is an o and p indicator, when the reaction takes place with acid, it produces meta-Nitroaniline.

Sulphonation of Aniline

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Aniline reacts vigorously with Sulphuric Acid to form anilinium hydrogen sulfate, which on heating gives 4-amino benzene sulphonic acid, which also has a resonating structure with zwitterion. This reaction is called the sulphonation of aniline.

Halogenation of Aniline

Aniline, when reacted with bromine water, then at room temperature, produces a white-colored precipitate named the 2, 4, 6 – tribromoaniline. This happens because polarity is developed within the bromine molecule, and bromine acts as an electrophile for having a slightly positive charge. It attacks the electron-rich ortho and para position of the aniline.

Solved Examples

In the Chlorination and Bromination of Aromatic Rings, what catalysts are used?

Lewis acid catalysts like ALCl3 or FeCl3 are used for the chlorination of an aromatic ring. The catalysts used for the Bromination of Aromatic Rings are AlBr3 or FeBr3.  

Fun Fact

One of the only electrophiles that will only accept electrons is H+ because it has no electrons.  

Water molecule is both an electrophile and a nucleophile for the oxygen molecule in water is more electronegative (because the oxygen molecule has two lone pairs and a d- charge, which makes it nucleophilic) and each of the hydrogen molecules behaves as an electrophile for it bears a d+ charge.

FAQ (Frequently Asked Questions)

Q. What are the Two Primary Types of Electrophilic Substitution Reactions?

The two types of electrophilic substitution reaction are,

Electrophilic aromatic substitution reaction: When an electrophile replaces an atom attached to an aromatic ring is called an electrophilic aromatic substitution reaction. In this reaction, the aromaticity of the aromatic compound is not lost. This reaction can be used to attain aryl-halides from aromatic rings, chlorine, bromine, and iodine. Examples of such reactions are aromatic nitrations, aromatic sulfonation, and Friedel- Crafts reactions.

Electrophilic aliphatic substitution reaction: In this reaction, the functional group usually hydrogen is replaced by an electrophile is an aliphatic compound. This reaction has five different types, and if the electrophilic attack occurs at an angle of 180 degrees to the leaving group, then the electrophilic substitution reaction can also result in the inversion of configuration.

Q. What are the Differences Between the Electrophilic Substitution Reaction and Nucleophilic Substitution Reaction?

Substitution involves the replacement of an atom or group being replaced by another. The one that is forced to leave the bond or the one being replaced is called the leaving group, and the other that replaces is the one that is attacking to occupy the leaving groups/atoms position.

Electrophiles are positively charged or neutral species that are attracted to electrons. Electrophiles are also called electron acceptors. The electrophilic substitution reactions include the displacement of a functional group (usually hydrogen) by an electrophile.

A nucleophile is a chemical species that is also called electron donors because of the nature of donating an electron pair of electrons or one pi bond. The attack of a positively or partially-positively charged atom or groups by a nucleophile is called Nucleophilic substitution reaction.