Electrophilic Substitution Reaction Mechanism - Definition, Examples, FAQs

Electrophilic Substitution Reactions

Electrophilic substitution reactions are a class of chemical reactions where some electrophile replaces a hydrogen atom in the aromatic ring. Such reactions are typical for the class of aromatic compounds, among which the most famous are benzene and its derivatives. Commonly, this process contains two major steps: the formation of a sigma complex, also referred to as an arenium ion, and the subsequent loss of the proton to recover the aromaticity of the ring.

It is attracted to the electron-deficient electrophile by the electron-rich aromatic ring. This mechanism can be described as an electrophile's attack on the aromatic ring in the first step, in which, for a very short time, a stable aromatic structure gets disrupted and a non-aromatic sigma complex is formed. In the final step, this complex loses a proton, re-forming the aromatic system and undergoing substitution by the electrophile of one hydrogen atom.

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According to experimental evidence, electrophilic substitution reactions are supposed to proceed via the following three steps:

(a) Generation of the electrophile

During chlorination, alkylation and acylation of benzene, anhydrous AlCl₃, being a Lewis acid helps in the 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 $\sigma$-complex

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

(c) Removal of proton

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

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Types of Electrophilic Substitution Reactions

Electrophilic substitution reactions are several in type, characterized by the kind of electrophile involved. The typical ones include nitration, halogenation, sulfonation, and Friedel-Crafts alkylation and acylation.

Directive influence of a functional group in monosubstituted benzene

When the substituent is Electron Donating in nature

• The groups like $\mathrm{OH},{\mathrm{NH}}_2,{\mathrm{CH}}_2$, etc are electron donating and they create a partial negative charge at their ortho and para positions
• The ortho, and para positions of these substituted benzenes are electron-rich.
• Electrophiles are deficient. 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 positive charge at their ortho and para positions
• 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 dominate
• 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

1. Nitration: In this process, an aromatic ring is introduced with a nitro group (-NO₂) from nitric acid and sulfuric acid. This reaction is utilized in the large-scale production of such explosives as TNT, and trinitrotoluene.

2. Halogenation: A hydrogen atom is replaced by a halogen—chlorine or bromine—with the use of a halogen molecule and a catalyst like iron(III) chloride. This reaction is relevant in the synthesis of a number of halogenated aromatic compounds that find application as pesticides and pharmaceuticals.

3. Sulfonation: A process by which the sulfonic acid group$(-\mathrm{SO}3\mathrm{H})$ is introduced into the ring; sulfuric acid is used. Crucial intermediates in the manufacture of detergents and dyes are formed, which are sulfonated aromatic compounds.

4. Friedel-Crafts Alkylation and Acylation: This is an electrophilic substitution, whereby an alkyl or acyl group from alkyl halides or acyl chlorides, respectively, is introduced into the ring of an aromatic compound using a catalyst such as aluminum chloride. Indeed, such processes are of appreciable importance in the preparation of different types of aromatic hydrocarbons and ketones used in the chemical industry.

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Applications and Importance

Electrophilic substitution reactions have broad applicability in industry and the academic sense. These reactions are much more of a concern in the pharmaceutical industry for the synthesis of a vast array of drugs. Aspirin is one such example: an extremely common analgesic whose synthesis includes an electrophilic substitution reaction to introduce a nitro group onto the benzene ring. The same case applies in the manufacture of dyes and pigments; electrophilic substitution is necessary for the introduction of some targeted functional groups that end up imparting color and other properties in the final products.Electrophilic substitution reactions illustrate a vast portion of organic chemistry in the sphere of academic research, allowing students and researchers to know in detail the behavior of aromatic compounds. A core of explaining much more complicated chemical processes and developing new synthetic methodologies is discussed herein.

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Some Solved Examples

Example 1

Question:The most common reactions of benzene and its derivatives are:
1) electrophilic addition reactions
2) electrophilic substitution reactions
3) nucleophilic addition reactions
4) nucleophilic substitution reactions

Solution:
Benzene and its derivatives undergo electrophilic substitution reactions commonly. This is because the aromaticity of the benzene ring is retained after the reaction. If benzene were to undergo electrophilic addition, the aromaticity would be lost. Nucleophilic addition or substitution on the benzene ring is quite difficult due to the negatively charged electron cloud which is delocalized over the whole ring.

Hence, the answer is option (2).

Example 2

Question:The correct order of reactivity towards electrophilic substitution of the compounds aniline (A), benzene (B), and nitrobenzene (C) is:

1) A > B > C
2) C>B>A
3) B > C >A
4) A<B>C

Solution:
Electron-releasing groups activate the benzene ring towards electrophilic substitution reactions, whereas electron-withdrawing groups deactivate the benzene ring. The \(\mathrm{NH_2}\) group is electron-releasing while the \(\mathrm{NO_2}\) group is electron-withdrawing. Thus, the order of reactivity is:
(A) Aniline > (B) Benzene > (C) Nitrobenzene.

Hence, the answer is option (1).

Example 3

Question: Benzene on nitration gives nitrobenzene in the presence of a mixture, where :

1)both${\mathrm{H}}_2{\mathrm{SO}}_4$ and ${\mathrm{HNO}}_3$ act as bases

2)${\mathrm{HNO}}_3$ acts as an acid and ${\mathrm{H}}_2{\mathrm{SO}}_4$ acts as a base

3)both ${\mathrm{H}}_2{\mathrm{SO}}_4$ and ${\mathrm{HNO}}_3$act as acids

4) (correct)${\mathrm{HNO}}_3$ acts as a base and ${\mathrm{H}}_2{\mathrm{SO}}_4$ acts as an acid

Solution:

During nitration, a mixture of conc. and conc.H₂SO₄ is taken as the nitrating mixture which generates the electrophile NO₂⁺.

Here, HNO₃ acts as a base and is protonated to generate the leaving group H₂O.

H₂SO₄ acts as an acid.

Hence, the correct answer is option (4)

Summary

Electrophilic substitution is that area in organic chemistry, mainly in the case of aromatic compounds. This implies the replacement of hydrogen atoms in the ring of an aromatic compound with an electrophile through two steps: the formation of a sigma complex and the restoration of the condition of aromaticity. Such electrophilic substitution reactions include nitration, halogenation, sulfonation, and Friedel-Crafts reactions.

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