Electrophilic Aromatic Substitution Reactions and their Mechanisms

What is Electrophilic Aromatic Substitution Reaction?

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Electrophilic aromatic substitution reactions take place when an electrophile, in an organic process, substitutes or replaces an atom that is connected to an aromatic ring. The substitution of a hydrogen atom from a benzene ring with an electrophile is common in these reactions.

In an electrophilic aromatic substitution reaction, the aromaticity of the aromatic system is retained. The stability of the aromatic ring is not compromised when bromobenzene is generated from the reaction of benzene and bromine, for example. The following diagram depicts this reaction.

Electrophilic aromatic halogenation reactions of aromatic rings with iodine, chlorine, or bromine can yield aryl halides or haloarenes. Aluminum trihalides are commonly used as catalysts in these processes.

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Mechanism of Electrophilic Aromatic Substitution

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The mechanism of an electrophilic aromatic substitution reaction contains three main components which are:

1. A new sigma bond from C=C is formed during the reaction in the arene nucleophile.
2. Due to the breaking of the C-H sigma bond, a proton is removed.
3. The C=C bond is reformed and it restores the aromaticity of the compound.

The mechanism can be understood as:

• The process of substitution reaction in aromatic hydrocarbons or compounds is known as the electrophilic aromatic substitution mechanism.
• This reaction is commonly seen in aromatic chemicals, hydrocarbons, and organic molecules.
• When an atom of a molecule like benzene combines with an electrophile, it's called a benzene reaction. The electrophile also takes the place of that atom (i.e. attaches to the aromatic ring).
• Three fundamentals are involved in this system. A new pi bond is formed from the carbon double bond, a proton is removed from the carbon-hydrogen connection, and the carbon double bond is reformated. 
• The two fundamental phases in the electrophilic aromatic substitution reaction process must be understood. The first step involves an electrophile attacking the benzene ring. Following that, by gaining a positive charge of protons, the initial attack aids in the production of the arenium ion. As a result of the electrophile's slow attack on the aromatic ring, the entire process is slow.
• As the aromatic ring loses its aromaticity, a large amount of activation energy is released. The electrophilic attack is influenced by several variables, including steric hindrance, likelihood, and resonance. A weak base is used to remove a proton from the ion in the second step. 
• The attack of a weak base on the generated carbocation causes this elimination. The aromaticity is then stored again by forming a pi bond with electrons. This entire procedure just takes a few minutes. It's important to note that carbocation loses a proton as a result of the electrophile's attack.
• In some reactions, such as benzene, the electrophile takes the place of the aromatic ring's hydrogen atom. This aromatic reaction aids in the preservation of an aromatic compound's aromaticity.
• The reaction of a benzene ring with chlorine to create iron chloride and hydrochloride is an example of aromatic stability. Sulfur trioxide forms sulphuric acid when it interacts with benzene. The electrophile in this case is sulfur trioxide.

Mechanism of Electrophilic Aromatic Substitution Reaction

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Types of Electrophilic Aromatic Substitution Reactions

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Electrophilic aromatic substitution reactions come in a variety of forms, the most common of which are:

• Aromatic sulfonation reactions 
• Aromatic nitration reactions
• Electrophilic aromatic halogenation reactions
• Friedel-Crafts acylation reaction
• Friedel-Crafts alkylation reaction

Aromatic Sulfonation Reactions 

• Sulfonation is the process of replacing hydrogen with sulfonic acid (SO₃).
• This reaction is very similar to nitration, which produces an electrophile by protonating SO₃ with H₂SO₄.
• This aids in the formation of a robust electrophile. The reaction follows the electrophilic aromatic substitution process after the item is acquired.

Aromatic Sulfonation Reaction

Aromatic Nitration Reactions

• The nitro (NO₂) group is involved in aromatic nitration processes. To replace the hydrogen atom, the nitro group works as an electrophile. 
• A catalyst in the form of sulfuric acid (H₂SO₄) is also used in this process. Another acid that is employed is nitric acid, which loses a proton to generate the nitronium ion. We can handle this nitronium ion by using the electrophilic aromatic substitution method.
• TNT or high explosives are an excellent example of an electrophilic substitution process using the nitro group. Trinitrotoluene is made from toluene, commonly known as methylbenzene, which goes through this procedure.

Aromatic Nitration Reactions

Electrophilic Aromatic Halogenation Reactions

• Halogen group elements, primarily bromine and chlorine, are used in aromatic halogenation processes. 
• The hydrogen atoms in benzene are replaced with chlorine or bromine in a substitution process. 
• We utilize acids such as lewis acids as a catalyst to speed up or finish the reaction because they lack the strength to do it on their own.
•  These acids, such as aluminum or iron bromide, transfer a pair of electrons to their atoms, allowing them to establish permanent connections (Cl-Cl or Br-Br). 
• The benzene ring loses its aromaticity and generates activation energy in this process. Due to their positive charge, Br or Cl use their electrophilic strength to overcome that energy.

Electrophilic Aromatic Halogenation

Friedel-Crafts Acylation Reaction

• Hydrogen is generally substituted by an acyl group (RC=O) in the Friedel-Crafts Acylation process.
• Carboxylic acid halides or acyl chlorides are popular reagents in this type of reaction. Lewis acid catalysts are also employed.
• A single pair from the chlorine of the H₃C(C=O)Cl, which is also employed in filling the open octet of the aluminum belonging to AlCl₃, is used to generate the electrophile (usually acylium ion).
• In the end, the chlorine carbon link breaks down, and Cl+-Al–Cl₃ is formed. Normally, an aryl ketone is generated as a result of the reaction.

Friedel-Crafts Acylation Reaction

Friedel-Crafts Alkylation Reaction

• The Friedel Crafts alkylation reaction uses an alkyl group (R). We saw different molecules react with the carbon of benzene in the previous processes, but a carbon-carbon bond can also be formed.
• In the presence of a catalyst, such as lewis acids, alkyl halides must react with benzene. Chloromethane reacts with benzene in the presence of aluminum chloride or iron chloride, which is an example of an electrophilic substitution reaction.
• By weakening the connection, the lewis acids make it easier for the chlorine atom to escape. Even though the reaction's result has high nucleophilic strength.

Friedel-Crafts Alkylation Reaction

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Things to Remember

• The process of substitution reaction in aromatic hydrocarbons or compounds is known as the electrophilic aromatic substitution mechanism.
• A substitution reaction (also known as a single displacement reaction or a single substitution reaction) occurs when one functional group in a chemical molecule is replaced by another functional group. Inorganic chemistry, substitution reactions are extremely important.
• Aromatic substitution reactions are necessary for the production of key intermediates. These chemicals are used in a variety of industries, including agriculture (chemical), industrial products, and pharmaceuticals.
• Aromaticity is a feature of cyclic, planar structures with pi bonds in resonance that provides them more stability than alternative geometric or connective arrangements with the same number of atoms in chemistry.

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