Electrophilic Aromatic Substitution

Introduction

Substitution reactions take place when 1 functional group in a chemical particle is substituted by another functional group. It is also referred to as a single displacement & perhaps a single substitution reaction. These reactions are critical in inorganic chemistry. This reaction occurs in organic compounds or aromatic hydrocarbons. These reactions aid in the preservation of the aromaticity of aromatic compounds. In these reactions, the hydrogen (H) atom of a benzene ring with an electron-accepting reagent is substituted.

Electrophilic Aromatic Substitution Reaction

These reactions take place when an electron-accepting reagent substitutes an atom attached to an aromatic ring. In these reactions, a hydrogen (H) atom from a benzene ring is frequently substituted with an electrophile (electron accepting reagent).

The aromaticity of the aromatic system is preserved in an electrophilic aromatic substitution (EAS) reaction. E.g., when bromobenzene is formed from the reaction of benzene & bromine, the aromatic ring's stability is not jeopardised.

Aromatic halogenation reactions with iodine (I), chlorine (Cl), or bromine (Br) can produce aryl halides or haloarenes. In these procedures, aluminium trihalides are frequently utilised as catalysts.

Different Types of Electrophilic Aromatic Substitution Reaction

There are several sorts of such reactions which are: Friedel-Crafts acylation, Aromatic sulfonation, Electrophilic aromatic halogenation, Friedel-Crafts alkylation as well as Aromatic nitration reactions.

Nitration

• This process involves the nitro (NO_2) group. The nitro group acts as an electron-accepting reagent to substitute the hydrogen (H) atom.

• In this procedure, a catalyst in the form of sulfuric acid is also utilised. Some other acid utilised is nitric acid, which ends up losing a proton to produce the nitronium ion. The electrophilic aromatic substitution (EAS) procedure can be utilised to deal with this nitronium ion.

• TNT & other high explosives are great examples of electrophilic substitution processes that use the nitro group. Trinitrotoluene is created by passing toluene, also known as methylbenzene, through this process.

Halogenation

• In these processes, halogen group components, mainly bromine & chlorine, are used.

• Substitution replaces the hydrogen (H) atoms in benzene with chlorine (Cl) or bromine (Br).

• But since they have little strength to do it on their own, acids like Lewis’s acids are utilised as a catalyst to accelerate or complete the reaction.

• These acids, such as aluminium or iron bromide, move an electron (e^-) pair to their atoms, enabling them to form permanent bonds.

• In this procedure, the benzene ring ends up losing its aromaticity & produces activation energy. Bromine or Chlorine utilises their electrophilic effort to face that energy because of their positive (+ve) charge.

Sulfonation

• The process of substituting hydrogen (H) sulfonic acid is known as sulfonation (SO³).

• This reaction is very comparable to nitration, which results in the formation of an electron-accepting reagent by protonation of SO_3 with H² SO⁴.

• This promotes the development of a strong electrophile (electron-accepting reagent). After the product is acquired, the reaction proceeds through the electrophilic aromatic substitution (EAS) process.

Friedel Crafts Alkylation

• An alkyl group is utilised in the Friedel Crafts alkylation reaction. In the earlier steps, various molecules react with the carbon (C) of benzene, but carbon-carbon (C-C) bonds can also be established.

• Alkyl halides must react with benzene in the existence of a catalyst, such as Lewis’s acids. An electrophilic substitution reaction (ESR) occurs when chloromethane reacts with benzene in the existence of aluminium chloride or iron chloride.

• The Lewis acids make it a lot easier for the chlorine (Cl) atom to break free by decimating the connection. Even though the reaction's outcome has high nucleophilic strength.

Friedel Crafts Acylation

• In this procedure, hydrogen (H) is typically substituted by an acyl group (RC=O).

• In this process, reagents such as carboxylic acid halides & perhaps acyl chlorides are commonly used.

• Catalysts for Lewis acids are also utilised.

• To produce the electrophile, a single pair of chlorine from the H_3 C(C=O)Cl is utilised.

• Finally, the chlorine carbon link dissolves. As an outcome of the reaction, an aryl ketone is usually formed.

What is the Mechanism for Electrophilic Aromatic Substitution?

An electrophilic aromatic substitution (EAS) is made up of 3 basic elements −

• A new bond is created during the reaction from a C=C in the arene nucleophile.

• The splitting of the C-H bond removes the proton.

• The aromaticity is restored by reforming the C=C.

The mechanism of the reaction typically consists of 2 major steps.

Step 1

• The electron-accepting reagent (electrophile) initiates the reaction by targeting the pi electrons (e^-) in the aromatic benzene ring.

• As an outcome, an arenium ion is produced, which is a positively (+vely) charged as well as delocalized cyclohexadienyl cation or a resonance-stabilised carbocation.

• This ion consists primarily of 3 resonance donors. Overall, an electrophilic attack on the aromatic ring requires patience & is a time-consuming process.

• It is also endergonic as well as has greater activation energy due to aromaticity loss.

• The electrophile's attack is determined by several factors, including resonance, probability, & steric hindrance.

Step 2

• The established carbocation intermediate is targeted by a base, resulting in the loss of a proton.

• The electrons (e^-) are then utilised to restructure a pi (π) bond, restoring aromaticity once more.

• That's a very rapid & typically exergonic process. The carbocation ends up losing a proton as an outcome of the electron-accepting reagent targeting the benzene ring.

Conclusion

The electrophilic aromatic substitution (EAS) mechanism relates to the procedure of substitution reaction in aromatic hydrocarbons or complexes. When 1 functional group in a chemical substance is substituted by some other functional group, a substitution reaction occurs. Substitution reactions are critical in inorganic chemistry. Aromatic substitution reactions are required for the synthesis of important intermediates. These chemicals are found in a wide range of industries, including agriculture, industrial products, & pharmaceuticals. Aromaticity is a characteristic of cyclic, planar formations with pi (π) bonds in resonance that confers stability to other geometric forms with a similar no. of carbon (C).

FAQs

1. Explain the substitution reaction.

This process takes place when one functional group in a chemical particle is substituted by another functional group. They are crucially significant in inorganic chemistry.

2. What exactly is aromaticity?

It is a feature of cyclic, planar forms with pi (π) bonds in resonance that makes them more stable than other associative arrangements with the same no. of atoms. Aromatic rings are extremely stable as well as thus do not disintegrate easily.

3. Define the term electrophile.

It is a chemical substance that accepts an electron (e^-) pair as well as forms bonds with nucleophiles. Since they accept electrons, electrophiles are Lewis’s acids.

4. What Is the Purpose of Aromatic Substitution?

Aromatic substitution reactions are required for the synthesis of important intermediates. These chemicals are found in a wide range of industries, such as agriculture, industrial products, & pharmaceuticals. We use antiquated methods to bring out this reaction, which can result in the formation of Regio isomer combinations. New methods are being created to improve the system's efficiency.

5. How do you calculate the reactivity of the Electrophilic aromatic substitution (EAS) reaction?

An electron-accepting reagent attacks the benzene ring & displaces its substituents with itself. Therefore, substituent groups have a direct effect on the reactivity of the reaction. If the substituent is a donor, the reaction will be fast, but if the substituent receives electrons, the reaction will be slow.