Electrophilic Substitution

We are surrounded by chemical reactions. Our daily life is full of chemical reactions. These chemical reactions are of different types and it depends on the nature of reactants. In this article we are going to discuss one of these types of reactions.

A chemical reaction is a process by which one or more substances (reactants) are converted into one or more other substances (products). The substance is either a chemical element or a compound. The chemical reaction rearranges the constituent atoms of the reactants to produce various substances as products.

Definition of Electrophilic Substitution

A chemical reaction where a functional group from a compound is substituted with an electrophilic species is called Electrophilic substitution. A proton or any other electrofuge can be replaced by electrophilic substitution. It is also the most commonly used process to functionalize the aromatic rings.

The nucleophilic substitution is very complicated. It may also require special substituents or conditions for the reaction. There are two major types of electrophilic substitution reaction. The first one is called electrophilic aromatic substitution and the second one is called electrophilic aliphatic substitution.

Examples of the Electrophilic Substitution Reaction

The formation of bromobenzene due to the bromination of benzene is an example of an electrophilic aromatic substitution reaction. Also, the chlorination of the acetone is an example of an electrophilic aliphatic compound.

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Electrophilic Substitution of the Anilines

An organic compound where an amine group is attached directly to a benzene ring is known as aniline. The pair of electrons which remains unshared on the nitrogen atom is obtainable for straight combination with the ring. Due to this, the aniline can be represented as a resonance hybrid of five structures.

The maximum electron density can be found in the ortho positions and para positions of the amine group. An electrophile can easily attack the ring with high electron density at the para positions and the ortho positions. This leads the aniline to undergo an electrophilic substitution reaction which is very highly activating.

In the electrophilic substitution reaction of aniline, the aniline is often called para directing and ortho directing because the electrophile usually adds with the ortho position and para position. Bromination, sulphonation, and nitration are the common electrophilic substitution reaction of anilines.

Halogenation

Aniline will undergo bromination because of its reaction with bromine water that too at room temperature. Aniline reacts to give a white precipitate of 2, 4, 6- tri-Bromo-aniline. The –NH2 group must be protected to get a monosubstituted aniline derivative. It is guarded by acetylation with acetic anhydride.

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Nitration

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In the above-given reaction, with the para isomer, meta isomer is observed too. This happens because of the aniline group getting protonated in the acidic medium to become an aniline ion, which is meta directing.

Sulphonation

The vigorous reaction of sulphuric acid with aniline gives the formation of aniline hydrogen sulfate. After heating anilinium hydrogen sulfate, it gives sulphanilic acid. Sulphanilic acid also has a resonating structure with the zwitterion. The dipolar ion where the molecule as a whole is neutral and there is an existence of both positive, as well as the negative charge, is referred to as Zwitterion. Sometimes it is also referred to as inner salt. It has negative and positive charges both simultaneously; therefore it varies from the amphoteric ion.

The reason why aniline does not undergo Friedel crafts alkylation and acylation reaction is that they react with the ferric chloride of the reaction mixture. Ferric chloride of the reaction mixture acts as a catalyst for the reaction.

Further Application of the Nitration and the Sulphonation

Nitration is used to add nitrogen to the benzene ring, which can also be used further in the substitution reactions. The nitro group always acts as a ring deactivator. To have a presence of nitrogen in the ring is very essential as this can be used as a directing group as well as a masked amino group. In industrial chemistry, the products of the aromatic nitrations are very pivotal.

Sulphonation is a reversible reaction. Sulphonation can be used for further substitution reaction in the form of a blocking group as it can be removed easily. To prevent the attack of the carbon by the other substituents, the sulphonation group blocks the carbon.

After the completion of the reaction, it can be easily removed by reversing sulphonation. Benzenesulfonic acids are used in the synthesis of detergents, sulfa drugs, and dyes. Benzensulfonyl chloride is used in chemotherapy because it is a precursor to sulphonamides.

Do You Know?

• Henry Armstrong was the first person to discover electrophilic substitution reaction in 1891.

• Electrophilic Substitution reactions preferentially occur in the aryl group.

• If an atom is replaced during electrophilic reaction then it is called electrophilic substitution but if only an atom is added, no atom removed from the existing molecule is known as electrophilic addition. This is the only difference between addition and substitution.

Summary

Finally we can summarize that the formation of bromobenzene due to the bromination of benzene is an example of an electrophilic aromatic substitution reaction. An electrophile can easily attack the ring with high electron density at the para positions and the ortho positions. This leads the aniline to undergo an electrophilic substitution reaction which is very highly activating. Bromination, sulphonation, and nitration are the common electrophilic substitution reaction of anilines. Nitration is used to add nitrogen to the benzene ring, which can also be used further in the substitution reactions. Sulphonation can be used for further substitution reaction in the form of a blocking group as it can be removed easily.