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Introduction
An electrophile is a species that seeks electrons. An electrophilic substitution reaction is a chemical process in which electrophile substitutes for a group linked to a compound in a compound’s structure. Typically, a hydrogen atom is used to represent the displaced group. Electrophilic substitution occurs in the case of haloarenes.
Electrophilic substitution reactions
An electrophilic substitution process occurs when an electrophile (an electron acceptor) replaces a functional group on a molecule. An electrophilic substitution often involves a hydrogen atom as the displaced functional group. This is referred to as electrophilic aromatic substitution in several arene reactions. Optically active electrophilic aliphatic substitution reactions are another kind. Electrophilic substitution is a three-stage process. To begin, make an electrophile. It would help if you first constructed a carbocation then removed a proton from the medium for electrophilic substitution reactions example, nitration, halogenation, sulphonation, and Friedel-Crafts reactions. Let us look at some more electrophilic substitution reaction examples.
Halogenation
Haloarenes are halogenated when they react with chlorine in the presence of a Lewis acid, and this reaction is known as chlorine reaction (also known as chlor arenes reaction) (Ferric chloride). The chlorine molecule behaves as an electrophile, and it will attack the compound’s ortho and para positions, which are both rich in electrons.
The reaction results in creating ortho and para molecules in equal amounts. On the other hand, the para isomer will be the major product of the reaction, while the ortho isomer will be the minor product.
Nitration
The ion NO2⁺ functions as an electrophile in the nitration of haloarenes. This electrophile is formed due to the interaction between nitric acid and sulphuric acid. The electron-rich centers attack the electrophile at both the ortho and para positions, resulting in the creation of both ortho and para compounds due to the reaction. On the other hand, the para isomer will be the major product of the reaction, while the ortho isomer will be the minor product.
Sulphonation
SO3, when it comes to the sulphonation of haloarenes, is the electrophile. The electron-rich compound attacks the ortho and para locations of the haloarene. As a result of the reaction, para and ortho chlorobenzenesulfonic acids are formed, with the para isomer being the more abundant product and the ortho isomer being the less great product.
Friedel-crafts reaction
Friedel-crafts reactions are divided into two categories:
- Friedel-crafts alkylation reaction
- Friedel-crafts acylation reactions
Friedel-crafts alkylation reaction
In Friedel-crafts alkylation, the alkyl group acts as the electrophile. The ortho and para positions of the haloarene attack the alkyl group, causing it to react—the reaction results in creating ortho and para compounds in equal quantities. On the other hand, the para isomer will be the predominant product of the reaction, while the ortho isomer will be a minor byproduct.
Friedel-crafts acylation reactions
During Friedel-crafts acylation, the acyl group acts as the electrophile, while the ortho and para positions of the haloarene attack the acyl group, causing it to react. The reaction results in creating ortho and para compounds in equal quantities. On the other hand, the para isomer will be the predominant product of the reaction, while the ortho isomer will be a minor byproduct.
Reaction mechanism of electrophilic substitution
The electrophilic substitution reaction mechanism consists of 3 steps i.e.
- Electrophile generation:
The chlorination, alkylation, and acylation of an aromatic ring, which takes place in the presence of anhydrous chloride, results in electrophiles. When anhydrous aluminium chloride is combined with the attacking reagent, it produces electrophiles with the following names: Cl⁺, R⁺, and RCO⁺.
- Carbocation formation:
Afterwards, the electrophile penetrates the aromatic ring and creates an arenium ion, also known as a premium complex; when one of the carbon atoms is hybridized with an sp3 atom, the sigma complex results.
The electrophilic substitution process contains parts a and b. The arenium ion or the sigma complex achieves stability inside the resonance structure. However, the aromatic feature of the sigma complex is lost since the delocalisation of electrons ends at the carbon that has been sp³ hybridized.
- Proton Removal:
When AlCl4– attacks the carbon that has been sp3 hybridized, the sigma complex or the arenium ion releases a proton, which is required to restore the aromatic nature of the carbon. In this third step, the electrophile is substituted for the hydrogen in the benzene ring.
Types
Electrophilic substitution reactions involving aromatic compounds and electrophilic substitution reactions involving aliphatic compounds are the two most prevalent types of electrophilic substitution reactions that organic compounds encounter. The chlorine cation acts as an electrophile, taking the place of a hydrogen atom in the benzene ring structure. When an electrophilic substitution process takes place, it creates two products: a proton and the chlorobenzene molecule, which are both useful compounds.
- Electrophilic aromatic substitution reaction
An electrophilic substitution reaction occurs when an atom that is chemically linked to an aromatic ring is substituted with an electrophile in an aromatic substitution process involving electrophilic substitution. Aromatic nitrations, aromatic sulfonation, and Friedel-crafts reactions are only a few examples of reactions within this reactivity area.
- Aliphatic electrophilic substitution reaction
The replacement of an electrophile for a functional group in an aliphatic molecule occurs when an electrophile replaces the functional group (often hydrogen) in the aliphatic molecule.
With these electrophilic substitution reactions, an inversion of configuration is feasible if the electrophilic reaction occurs at an angle of 180o relative to the leaving group (attack from the rear resulting in an aliphatic electrophilic substitution reaction.
Conclusion
A chemical reaction in which the functional group linked to a molecule is replaced by an electrophile is called an electrophilic substitution reaction. A hydrogen atom is commonly used to represent the displaced functional group. It is commonly accepted that electrophilic substitution reactions proceed through a three-step process, which includes the steps listed below.
