Electrophilic Substitution Reaction Mechanism

Electrophilic Substitution Reaction Mechanism - Definition, Examples, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 05:05 PM IST

Imagine the bright colors of fireworks lighting up the night sky during a festival. These nice visual effects are not just because of pyrotechnics but due to complicated chemical reactions as well. One essential category of reactions in the colorful display of fireworks is electrophilic substitution. Actually, it is the bedrock of Organic Chemistry in which the bulk of Aromatic Compounds are synthesized, including dyes, pharmaceuticals, and polymers.

This Story also Contains

Electrophilic Substitution Reactions
Types of Electrophilic Substitution Reactions
Applications and Importance
Some Solved Examples
Summary

Electrophilic Substitution Reaction Mechanism - Definition, Examples, FAQs

Electrophilic substitution reactions really do happen everywhere—from the drugs we use to the brightly colored fibers worn on our bodies. The list also includes such widely applied painkillers as aspirin syntheses all over the world. There are so many dyes used to color our garments and other materials synthesized via such a reaction. These reactions do not only have applications limited to everyday life but are also quite important industrially for the production of a wide range of chemical products.

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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(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

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 $OH, {NH}_{2}, {CH}_{2}$ , 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 $(- SO 3 H)$ 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.

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.

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 $H_{2} {SO}_{4}$ and ${HNO}_{3}$ act as bases

2) ${HNO}_{3}$ acts as an acid and $H_{2} {SO}_{4}$ acts as a base

3)both $H_{2} {SO}_{4}$ and ${HNO}_{3}$ act as acids

4) (correct) ${HNO}_{3}$ acts as a base and $H_{2} {SO}_{4}$ 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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NCERT Chemistry Notes:

Frequently Asked Questions (FAQs)

1. Give nucleophilic substitution reaction of chlorobenzene.

As we know that the chlorine is lack of an electron in order to achieve a stable electronic configuration due to that it is less reactive towards the nucleophilic substitution reaction and forms a partial chloride bond.

2. What is ligand substitution reaction?

Here as the name only clarifies that the ligand is substituted in it. Basically ligands are already present also but the more suitable ligand is substituted in the place of the other.

3. Give a short note on enantioselective reaction.

In an enantioselective reaction basically the formation of the enantiomers took place. The enantiomers so formed are optically active in nature that means they are chiral as well as achiral.

4. Give the reactivity of electrophilic substitution reaction.

The reactivity order we should know that in this the electron releasing groups are more powerful or activated as compared to the other groups as the result of this the benzene ring do not interfere in the substitution reaction. Phenol is the most reactive followed by benzene which is followed by chlorobenzene and benzoic acid.

Phenol>benzene>chlorobenzene>benzoic acid

5. Discuss about the sulphonation in aromatic electrophilic substitution reaction.

Sulphonation of the benzene in organic chemistry is one of the best examples of the electrophilic substitution reaction. In the process of the sulphonation what happens is that we will add the sulphur trioxide to the benzene ring, here the sulphur trioxide is the electrophile which will further bring about the electrophilic substitution reaction. Then the sulphur trioxide gets added to the benzene ring and will lead to the substitution and further the formation of the product called the benzene sulphonic acid.

6. What is the significance of the Friedel-Crafts reaction in electrophilic substitution?

The Friedel-Crafts reaction is a classic example of electrophilic aromatic substitution, used to introduce alkyl or acyl groups onto aromatic rings. It demonstrates the importance of generating strong electrophiles (using Lewis acid catalysts) and showcases how different substituents can be introduced through this mechanism.

7. What is the significance of the π-complex in electrophilic substitution?

The π-complex is a weak intermediate formed when the electrophile first approaches the aromatic ring. It represents the initial interaction between the π-electrons of the ring and the electrophile, preceding the formation of the σ-complex (arenium ion). Understanding this step helps explain the orientation and reactivity in these reactions.

8. What is the Hammond Postulate, and how does it apply to electrophilic substitution?

The Hammond Postulate states that the structure of a transition state resembles the species nearest to it in energy. In electrophilic substitution, this means the transition state for the rate-determining step (usually electrophile attack) resembles the arenium ion intermediate. This helps predict reactivity and orientation effects.

9. How do steric effects influence electrophilic substitution reactions?

Steric effects can significantly influence the outcome of electrophilic substitution reactions. Bulky substituents on the aromatic ring can hinder attack at ortho positions, leading to increased para substitution. This is particularly noticeable with large alkyl groups or when using bulky electrophiles.

10. How do solvents affect electrophilic substitution reactions?

Solvents can significantly influence electrophilic substitution reactions. Polar solvents can stabilize charged intermediates and transition states, potentially accelerating the reaction. Non-polar solvents may slow the reaction by not stabilizing these species. The choice of solvent can also affect the strength and reactivity of the electrophile.

11. What is an electrophilic substitution reaction?

An electrophilic substitution reaction is a type of organic reaction where an electrophile (electron-seeking species) replaces an atom or group in an aromatic compound. The incoming electrophile takes the place of a hydrogen atom or another substituent, maintaining the aromatic character of the compound.

12. What is the first step in an electrophilic aromatic substitution reaction?

The first step in an electrophilic aromatic substitution reaction is the attack of the electrophile on the aromatic ring. This forms a resonance-stabilized carbocation intermediate called an arenium ion or Wheland intermediate.

13. Why are aromatic compounds more likely to undergo electrophilic substitution than addition reactions?

Aromatic compounds prefer electrophilic substitution over addition reactions because it allows them to maintain their stable aromatic structure. Addition reactions would disrupt the delocalized π-electron system, which is energetically unfavorable. Substitution preserves aromaticity and the associated stability.

14. How does benzene's reactivity compare to other aromatic compounds in electrophilic substitution?

Benzene is less reactive than many substituted aromatic compounds in electrophilic substitution reactions. This is because benzene lacks activating groups that would increase electron density in the ring. However, it's more reactive than aromatics with strong deactivating groups.

15. What role does the leaving group play in electrophilic aromatic substitution?

In most electrophilic aromatic substitutions, the leaving group is a proton (H+). Its role is to depart from the ring after the electrophile has attached, restoring aromaticity. In some cases, like ipso substitution, a different group leaves instead, influencing the reaction's energetics and kinetics.

16. How does the arenium ion intermediate contribute to the reaction mechanism?

The arenium ion intermediate is crucial because it distributes the positive charge throughout the ring via resonance, stabilizing the structure. This allows the reaction to proceed with lower activation energy. The most stable carbocation intermediate determines the major product of the reaction.

17. What is the rate-determining step in electrophilic aromatic substitution?

The rate-determining step in electrophilic aromatic substitution is typically the initial attack of the electrophile on the aromatic ring. This step forms the arenium ion intermediate and is usually the slowest, thus controlling the overall reaction rate.

18. How do activating groups affect electrophilic substitution reactions?

Activating groups increase the electron density of the aromatic ring, making it more reactive towards electrophiles. They direct substitution to ortho and para positions by stabilizing the carbocation intermediate through resonance. Examples include -OH, -NH2, and -NHR groups.

19. What effect do deactivating groups have on electrophilic substitution?

Deactivating groups decrease the electron density of the aromatic ring, making it less reactive towards electrophiles. They direct substitution to the meta position because they cannot stabilize ortho or para carbocation intermediates. Examples include -NO2, -CN, and -COOH groups.

20. Why does chlorobenzene undergo electrophilic substitution more slowly than benzene?

Chlorobenzene reacts more slowly than benzene because the chlorine atom is slightly electron-withdrawing through induction, despite being electron-donating through resonance. This makes the ring less electron-rich overall, reducing its reactivity towards electrophiles.

21. What is ipso substitution in electrophilic aromatic substitution?

Ipso substitution is a type of electrophilic aromatic substitution where the incoming electrophile replaces an existing substituent on the aromatic ring, rather than a hydrogen atom. This occurs when the existing substituent is a better leaving group than hydrogen.

22. How does the size of a halogen affect its directing ability in electrophilic substitution?

The size of a halogen affects its directing ability through the balance of inductive and resonance effects. Larger halogens (I, Br) are better at donating electrons through resonance, making them stronger ortho-para directors compared to smaller halogens (F, Cl).

23. What is the difference between kinetic and thermodynamic control in electrophilic substitution?

Kinetic control favors the product formed most quickly (lowest activation energy), while thermodynamic control favors the most stable product (lowest overall energy). In electrophilic substitution, kinetic control often leads to ortho/para products, while thermodynamic control may favor meta products under certain conditions.

24. How does resonance stabilization affect the orientation of electrophilic substitution?

Resonance stabilization plays a crucial role in determining the orientation of electrophilic substitution. Substituents that can donate electrons through resonance (e.g., -OH, -NH2) stabilize ortho and para carbocation intermediates, directing the electrophile to these positions. Groups unable to do this (e.g., -NO2, -CN) lead to meta substitution.

25. How does the concept of hyperconjugation apply to electrophilic substitution reactions?

Hyperconjugation involves the interaction between σ-bonds and neighboring π-orbitals or empty p-orbitals. In electrophilic substitution, alkyl groups can stabilize carbocation intermediates through hyperconjugation, making them ortho-para directors. This effect explains why alkyl-substituted benzenes are more reactive than benzene itself.

26. What is the Brown-Okamoto equation, and how is it used in electrophilic substitution?

The Brown-Okamoto equation relates substituent effects to reaction rates in electrophilic aromatic substitution. It uses σ+ constants to quantify how substituents affect the reaction center's electron density. This equation helps predict relative reactivities and can be used to design more efficient synthetic routes.

27. How do isotope effects provide insight into the mechanism of electrophilic substitution?

Isotope effects, particularly kinetic isotope effects (KIEs), can reveal details about the reaction mechanism. In electrophilic substitution, a primary KIE (comparing H vs. D as the leaving group) indicates that C-H bond breaking is involved in the rate-determining step. The magnitude of the KIE can suggest whether this step is fully or partially rate-determining.

28. What is the role of π-complexes in determining regioselectivity of electrophilic substitution?

π-complexes, formed when the electrophile initially interacts with the aromatic π-system, can influence regioselectivity. The stability and formation of these complexes at different positions around the ring can guide the electrophile to specific sites, contributing to the observed orientation of substitution.

29. How does aromaticity change during the course of an electrophilic substitution reaction?

Aromaticity is temporarily disrupted during electrophilic substitution. The initial aromatic compound loses aromaticity when forming the arenium ion intermediate (a non-aromatic species). Aromaticity is then restored in the final step when a proton is lost, reforming the aromatic sextet. This cycle of losing and regaining aromaticity is key to understanding the reaction's energetics.

30. What is the significance of carbocation rearrangements in electrophilic substitution?

Carbocation rearrangements can occur in electrophilic substitution reactions, especially when they lead to more stable carbocation intermediates. These rearrangements can result in unexpected products or isomer distributions. Understanding potential rearrangements is crucial for predicting reaction outcomes and designing synthetic strategies.

31. How do electronic effects and steric effects compete in determining the product distribution of electrophilic substitution?

Electronic effects (resonance and inductive) generally dominate in determining the orientation of electrophilic substitution. However, steric effects can become significant with bulky substituents or electrophiles. The balance between these effects determines the final product distribution, with electronic effects favoring certain positions and steric hindrance potentially redirecting substitution.

32. What is the importance of the electrophilicity parameter in these reactions?

The electrophilicity parameter quantifies an electrophile's reactivity. It helps predict whether a particular electrophile will react with a given aromatic compound and how fast. Understanding this parameter is crucial for designing reactions, as it influences both the feasibility and rate of electrophilic substitution reactions.

33. How does ring strain in non-benzenoid aromatics affect their electrophilic substitution reactions?

Ring strain in non-benzenoid aromatics (e.g., cyclopropenyls, cycloheptatrienyls) can significantly alter their reactivity in electrophilic substitution. Strained rings may be more reactive due to relief of strain upon forming the arenium ion. Conversely, some strained systems might resist substitution if it would increase strain. This concept is important in understanding the reactivity of diverse aromatic systems.

34. What is the concept of "push-pull" systems in electrophilic aromatic substitution?

"Push-pull" systems in electrophilic aromatic substitution refer to aromatic compounds with both strongly activating (electron-donating) and strongly deactivating (electron-withdrawing) groups. These systems can exhibit unique reactivity and selectivity patterns due to the opposing electronic effects, often leading to interesting synthetic applications.

35. How do neighboring group effects influence electrophilic substitution reactions?

Neighboring group effects occur when a substituent on the aromatic ring directly participates in the reaction mechanism. This can lead to unexpected products or altered reaction rates. For example, a neighboring group might stabilize a particular carbocation intermediate, directing substitution to a specific position or facilitating intramolecular reactions.

36. What is the significance of charge transfer complexes in electrophilic substitution?

Charge transfer complexes can form between electron-rich aromatic compounds and electron-deficient species (electrophiles). These complexes represent an early stage of the electrophilic substitution process and can influence reaction rates and selectivity. Understanding these complexes helps explain the reactivity patterns of different aromatic-electrophile pairs.

37. How does the concept of aromaticity extend to heterocyclic compounds in electrophilic substitution?

Heterocyclic aromatic compounds (e.g., pyridine, furan, thiophene) undergo electrophilic substitution, but their reactivity and regioselectivity differ from benzene. The heteroatom's electronic effects and its position in the ring significantly influence the reaction. For instance, pyridine is less reactive than benzene due to its electron-deficient nature, while furan is more reactive due to its electron-rich character.

38. What is the role of Lewis acid catalysts in electrophilic aromatic substitution?

Lewis acid catalysts play a crucial role in many electrophilic substitution reactions by enhancing the electrophilicity of the attacking species. They do this by coordinating with the electrophile, making it more electron-deficient and thus more reactive. This is particularly important in reactions like Friedel-Crafts alkylation and acylation, where the Lewis acid (e.g., AlCl3) activates the alkyl halide or acyl halide.

39. How do polycyclic aromatic hydrocarbons (PAHs) behave in electrophilic substitution reactions?

Polycyclic aromatic hydrocarbons (PAHs) generally show enhanced reactivity in electrophilic substitution compared to benzene. This is due to their extended π-systems, which can better stabilize the positive charge in the arenium ion intermediate. The most reactive positions in PAHs are typically those that can form the most resonance structures in the carbocation intermediate.

40. What is the significance of ipso protonation in electrophilic aromatic substitution?

Ipso protonation occurs when a proton attaches to a carbon already bearing a substituent, rather than to an unsubstituted position. This can lead to unexpected substitution patterns or even elimination of the original substituent. Understanding ipso protonation is important for predicting reaction outcomes, especially in systems with electron-donating substituents.

41. How does the concept of aromatic stabilization energy relate to the reactivity in electrophilic substitution?

Aromatic stabilization energy (ASE) is the extra stability an aromatic compound has compared to a hypothetical non-aromatic analog. Compounds with higher ASE are generally less reactive in electrophilic substitution because more energy is required to disrupt their aromaticity. This concept helps explain why benzene is less reactive than many of its derivatives in these reactions.

42. What is the importance of frontier molecular orbital theory in understanding electrophilic substitution?

Frontier molecular orbital theory helps explain reactivity and selectivity in electrophilic substitution by focusing on the interactions between the highest occupied molecular orbital (HOMO) of the aromatic compound and the lowest unoccupied molecular orbital (LUMO) of the electrophile. This theory provides insights into why certain positions on the aromatic ring are more reactive and how substituents affect this reactivity.

43. How do solvent effects influence the formation and stability of arenium ion intermediates?

Solvent effects can significantly impact the formation and stability of arenium ion intermediates. Polar solvents can stabilize these charged intermediates through solvation, potentially accelerating the reaction. The degree of this stabilization can affect the energy profile of the reaction, influencing both rate and product distribution. Understanding these effects is crucial for optimizing reaction conditions.

44. What is the significance of the ortho-para ratio in electrophilic substitution reactions?

The ortho-para ratio in electrophilic substitution provides valuable information about the reaction mechanism and the nature of the directing groups. A high ortho-para ratio suggests strong electronic effects from the directing group, while a lower ratio might indicate significant steric hindrance. Analyzing this ratio helps in understanding the interplay between electronic and steric factors in these reactions.

45. How does the concept of aromaticity violation apply to electrophilic substitution in non-benzenoid systems?

Aromaticity violation in non-benzenoid systems during electrophilic substitution can lead to unique reactivity patterns. Some non-benzenoid aromatics may undergo substitution more readily if it leads to a more aromatic product. Conversely, highly stable aromatic systems might resist substitution if it significantly disrupts their aromaticity. This concept is important in predicting the behavior of diverse aromatic compounds.

46. What is the role of substituent effects in determining the activation energy for electrophilic substitution?

Substituent effects significantly influence the activation energy for electrophilic substitution. Electron-donating groups lower the activation energy by stabilizing the transition state and arenium ion intermediate. Electron-withdrawing groups increase the activation energy by destabilizing these species. Understanding these effects is crucial for predicting reaction rates and designing efficient synthetic routes.

47. How do ring currents in aromatic compounds affect their reactivity in electrophilic substitution?

Ring currents, which are induced magnetic fields in aromatic compounds, can influence reactivity in electrophilic substitution. Stronger ring currents generally indicate greater aromaticity and stability, which can