Electrophilic Substitution

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What Is 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.

FAQ (Frequently Asked Questions)

1. What is the Acetylation of Aniline?

Ans. Acetanilide is an analgesic that was also once formally known as Antifebrin. It is similar in structure to acetaminophen or Tylenol. However, acetanilide is very toxic, unlike acetaminophen. Using an acetylation reaction, acetanilide is prepared from the aniline. Acetamides are crystalline solids that can be used in purification by recrystallization.


To characterize and identify the corresponding compounds, the melting points are used. Using an acetylation reaction, acetanilide is formed from the aniline. To reduce the reactivity towards the oxidizing agent or the electrophiles, acetylation is usually used to place an acetyl protecting group on the primary or the secondary amines.

2. Is Halogenation an Electrophilic Substitution?

Ans. Yes, halogenation is an example of electrophilic aromatic substitution. When benzene is attacked by an electrophile it substitutes the hydrogen in an electrophilic aromatic substitution.


Halogens are not electrophilic enough to break the aromaticity of benzenes which requires a catalyst to activate. To become a very strong electrophile, the halogens need the help as well as the aid of the Lewis Acidic Catalysts to activate. Ferric Halides (FeX3) and Aluminum Halides (AIX3) are some of the examples of the activated halogens where X=Br or Cl.

3. How to Make an Aniline Acetanilide?

Ans. Acetanilide is an organic compound. From aniline, when Acetanilide reacts with the acetic anhydride or glacial acetic acid in the presence of the zinc dust. Under the anhydrous condition, a mixture of the aniline, zinc dust, glacial acetic acid, and acetic anhydride is refluxed. Then the mixture is poured into the ice-cold water which gives the precipitation of acetic anhydride.


The precipitation of the acetic anhydride is in the crude form. This is recrystallized to get the pure acetanilide crystals. Some amount of zinc is used to prevent aniline to oxidize during a chemical reaction. Acetanilide has medicinal importance and is also used as ‘febrifuge’.


Along with the presence of very concentrated hydrochloric acid, Acetanilide can also be prepared by acetylating with acetic anhydride. Stir well the mixture of dissolved aniline in hydrochloric acid and acetic anhydride. Now, pour the mixture into the sodium acetate in water. Acetanilide is formed, which can be separated and recrystallized by ethyl alcohol.