Explanation of Mechanism Of Electrophilic Substitution

The electrophilic substitution reaction is a reaction in which functional groups and electrophiles interact. In the mechanism of electrophilic substitution reaction, the electrophilic groups replace the hydrogen or functional group from the atom. After replacement, they sit there and make a bond with another atom. Three main steps are involved in the mechanism of electrophilic substitution reaction. In the first, the Electrophile is generated; in the second, the carbocation is formed; and in the last step, the proton is removed, and the Electrophile replaces the functional group. So, in this whole mechanism, the carbocation formation is intermediate, which is not seen in the reaction results. 

Mechanism of electrophilic substitution reaction 

As discussed above, the mechanism of electrophilic substitution reaction contains three main steps. Let us discuss them in detail. 

Step 1: Generation of Electrophile

In the first step of the electrophilic substitution reaction, the electrophiles are generated by reacting the two chemicals: anhydrous aluminium chloride and any reagent. When both reagent and anhydrous aluminium chloride integrate, they release positive ions. Mainly three types of Electrophile form from these reactions. These are R+, Cl+, and RC+O. These electrophiles are formed by the aromatic rings’ Alkylation, Chlorination, and Acylation. 

Step 2: Formation of carbocation 

It is the second step of the mechanism of electrophilic substitution reaction. Electrophile attacks the rings, resulting in the formation of arenium ions and sigma complex anions. These arenium ions are of sp³ hybridisation. Due to their sp³ hybridisation, these ions tend to be stable and start searching for stability in the resonance structure of aromatic rings. However, the formation of Electrophiles does not allow this stabilisation and stops the formation of sp³ hybridisation under the resonance structure. Due to this, the arenium ions and sigma complex ions release their aromatic characteristics. So, the arenium ions and sigma complexions are the carbocations, intermediate in the reaction. The form then gets destroyed. The whole electrophilic substitution reaction does not get affected by the formation of carbocations.

Step 3: Removal of proton 

The removal of the proton is the last step in the mechanism of electrophilic substitution reaction. In this step, the sigma complex releases positive ions from the carbon, which is sp3³ hybridised. The sigma complex releases protons to regain its aromatic nature. So, when the proton gets released from the sigma complex, the electrophiles take their place. The replacement of hydrogen by the Electrophile in the benzene ring is an example of an electrophilic substitution reaction.

Types of Electrophilic substitution reactions 

Electrophilic substitution reactions are used widely in the various reactions of organic chemistry. This reaction is mainly of two types:

• Electrophilic Aromatic Substitution Reaction
• Electrophilic Aliphatic Substitution Reaction

Electrophilic Aromatic Substitution Reaction

In this, electrophiles take the atom’s place in this electrophilic substitution reaction attached to the aromatic rings. Instead of removing the hydrogen, any atom with a positive charge gets removed in this reaction. This type of reaction forms iodine, chlorine, bromine, and aryl halides. So, they are formed when the Electrophile replaces the attached iodine, chlorine, bromine, and aryl halides. 

Electrophilic Aliphatic Substitution Reaction

Electrophiles replace the functional groups in this electrophilic substitution reaction. These types of reactions generally occur in the Aliphatic compounds. Electrophilic Aliphatic Substitution Reaction is subdivided into five types:

• Nitrosation
• Halogenation of ketones
• Diazonium coupling
• Keto-Enol tautomerism
• Combination of carbene and carbon-hydrogen bond. 

Example of electrophilic substitution reaction

In organic chemistry, electrophilic substitution reactions take place under many reactions. Let us understand some examples of electrophilic substitution reactions. 

• Bromination and Chlorination 

Bromination and Chlorination take place on the aromatic rings. The electrophiles remove the atom attached to the aromatic rings in this reaction. When the Electrophile replaces the Chlorine from aromatic rings, it is called Chlorination. Although, when the Electrophile replaces the bromine from aromatic rings, it is called Bromination. 

• Sulfonation and nitration 

In this reaction, the atom of Sulphur and nitrogen is replaced by the electrophiles. In Sulfonation, the

electrophiles replace Sulphur from the SO₃H group. On the other hand, the electrophiles replace nitrogen from the NO₂ group in nitration. 

• Alkylation and Acylation

In this reaction, the electrophiles replace alkyl halides and acyl halides. In Alkylation, alkyl halides get replaced, and in Acylation, acyl halides get replaced. 

Conclusion 

The mechanism of the electrophilic substitution reaction is straightforward, although the whole is completed at high-speed rates. In organic chemistry, electrophilic substitution reactions are used widely. These reactions generally occur with aromatic rings like benzene and halides. An electrophilic substitution reaction is also used to extract chlorine, bromine, iodine, sulphur, etc. The mechanism of electrophilic substitution reaction contains only three major steps. The whole reaction will not get affected by the formation of carbocations, as they disappear just after the formation.