Mechanism of Electrophilic Substitution

Electrophilic substitution reaction is a chemical reaction wherein an electrophile reacts with a given compound to produce a new compound through substitution. A hydrogen atom is usually the displaced functional group. Electrophilic substitution reactions typically follow a three-step process, including the stages below.

• The process of making an electrophile
• The development of a carbocation is a complex process
• A proton is removed from the intermediate

Electrophilic aromatic substitution reactions and electrophilic aliphatic substitution reactions are the two main types of electrophilic substitution reactions that organic compounds go through.

Mechanism of Electrophilic Substitution Reaction

Three stages are involved in the mechanism of electrophilic substitution reaction:

• Electrophile production

Anhydrous aluminium chloride is a valuable Lewis acid for generating electrophiles from aromatic ring chlorination, alkylation and acylation.

• Carbocation formation

The electrophile assaults the aromatic ring and forms a sigma complex or an arenium ion. In this arenium ion, one of the carbons is sp3 hybridised.

In a resonance structure, this arenium ion achieves stability. The sigma complex or the arenium ion loses its aromatic feature because electron delocalization stops at the sp3 hybridised carbon.

• Proton Extraction

When the [AlCl4]– attacks the sigma complex, it releases a proton from the sp3 hybridised carbon to restore the aromatic property. In the benzene ring, the electrophile, therefore, takes the role of the hydrogen atom. The mechanism of electrophilic substitution reaction is a crucial step in organic chemistry since it is employed in various organic name reactions.

Electrophilic Aromatic Substitution Mechanism

The electrophilic aromatic substitution mechanism is the procedure of aromatic compounds of hydrocarbons substitution reaction. Aromatic compounds, organic molecules and hydrocarbons are all examples of this process. A benzene reaction occurs when one atom of benzene reacts with an electrophile. It also acts as a substitute for that atom. The electrophile fills the role of the aromatic ring’s hydrogen atom in some processes. The aromatic chemical’s aromaticity is preserved via this aromatic reaction. Aromatic stability is a type of reaction between chlorine and a benzene ring to produce hydrochloride and iron chloride. When sulphur trioxide reacts with benzene, it produces sulphuric acid. 

Electrophilic Aromatic Substitution Reaction Forms 

Here are some instances of electrophilic aromatic substitution mechanism

Nitration

The nitro (NO2) group is involved in aromatic nitration processes. To substitute the hydrogen atom, the nitro group works as an electrophile. A catalyst such as sulfuric acid (H2SO4) is also used in this process. Nitric acid is also used, which loses a proton to generate the nitronium ion. The electrophilic aromatic substitution approach can be used to deal with this nitronium ion. TNT, a widely-used explosive, is a good illustration of an electrophilic substitution reaction using the nitro group.

Halogenation

Aromatic halogenation methods use halogen group elements, primarily bromine and chlorine. In a replacement procedure, the hydrogen atoms in benzene are replaced with chlorine or bromine. Aluminium or iron bromide, for example, donates an electron pair to their atoms, permitting them to form permanent bonds (Cl-Cl or Br-Br).

The benzene ring loses its aromaticity and generates activation energy in this process. Br or Cl employ their electrophilic strength to overcome that energy because of their positive charge. But because they lack the strength to do it independently, Lewis acids as a catalyst to speed up or complete the process.

Sulphonation

As the name suggests, sulphonic acid (SO3) is utilised in the aromatic sulfonation reaction with sulfuric acid as a catalyst. Because of this, sulphonic acid can gain a proton and become a potent electrophile. After that, the electrophile interacts with benzene and replaces the hydrogen atom. The reaction is subsequently completed via the electrophilic aromatic substitution mechanism. 

Friedel Crafts Alkylation

An alkyl group is used in the Friedel Crafts alkylation reaction (R). We saw several molecules react with benzene’s carbon in the preceding steps, but a carbon-carbon bond can also occur. Alkyl halides must react with benzene in the presence of a catalyst, such as Lewis acids. An electrophilic substitution reaction occurs when chloromethane interacts with aluminium chloride or iron chloride to form benzene. Despite the reaction’s great nucleophilic intensity, the Lewis acids make it easier for the chlorine atom to escape by weakening the link.

Friedel Crafts Acylation

This reaction is similar to Friedel Crafts alkylation, except that an acyl group (RC=O) is utilised instead of an alkyl group. One pair contains chlorine, and the other contains an aluminium octet. When Lewis acids are present, the reaction is expedited. A proton is added to acyl chlorides in the presence of Lewis acids, and they become acyl ions. This ion weakens the carbon-chlorine bond by acting as an electrophile. The most common end product of this reaction is aryl ketone. 

Mechanism of Electrophilic Substitution Reaction of Benzene

The delocalized electron spans efficiently throughout the carbon atoms in the benzene ring due to resonance in the ring. It also helps to stabilise the arenium ion. Because of the partial stability of the arenium ion, benzene is especially susceptible to electrophilic substitution reactions. When an electrophile substitutes the hydrogen atom in benzene, it is known as electrophilic replacement.

Such reactions are highly spontaneous because the aromaticity of benzene stays disrupted during the process. Nitration, sulphonation, halogenation, Friedel Crafts alkylation and acylation are all examples of mechanisms of electrophilic substitution reaction of benzene.

For these electrophilic substitution reactions, a two-step mechanism has been proposed. The electrophile establishes a sigma-bond to the benzene ring in the first, slow or rate-determining phase, resulting in a positively charged uranium intermediate. A proton is eliminated from this intermediate in the quicker second step, producing a substituted benzene ring.

• The electrophile establishes a sigma-bond with the benzene ring, resulting in a positively charged benzene intermediate
• This intermediate is deprotonated, resulting in a substituted benzene ring

This method for electrophilic aromatic substitution should be viewed in the context of other carbocation intermediate-based techniques. 

Conclusion

We learned that aromatic compounds are essential for electrophilic aromatic substitution processes that frequently add functional groups within benzene rings.