What Is 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.
FAQs on Electrophilic Substitution
1. What is an electrophilic substitution reaction and what are some common examples?
An electrophilic substitution reaction is a type of chemical reaction where an atom or functional group in a compound, typically a hydrogen atom on an aromatic ring, is replaced by an electrophile (an electron-seeking species). The overall aromaticity of the ring is preserved. Common examples seen in the CBSE syllabus include:
- Nitration of Benzene: A hydrogen atom on the benzene ring is replaced by a nitro group (-NO₂).
- Halogenation of Benzene: A hydrogen atom is replaced by a halogen like chlorine (-Cl) or bromine (-Br) in the presence of a Lewis acid catalyst.
- Friedel-Crafts Alkylation: An alkyl group (-R) replaces a hydrogen atom.
- Sulphonation of Benzene: A hydrogen atom is replaced by a sulphonic acid group (-SO₃H).
2. How does an electrophilic substitution reaction differ from a nucleophilic substitution reaction?
The primary difference lies in the attacking reagent and the substrate. In an electrophilic substitution, an electron-deficient species (electrophile) attacks an electron-rich substrate, such as a benzene ring. In contrast, a nucleophilic substitution involves an electron-rich species (nucleophile) attacking an electron-deficient substrate, like a carbon atom in an alkyl halide.
3. What is the general mechanism for an electrophilic aromatic substitution reaction?
The mechanism for electrophilic aromatic substitution typically follows three main steps:
- Generation of a strong electrophile: A reactive electrophile (E⁺) is generated, often with the help of a catalyst. For example, AlCl₃ helps generate the carbocation in a Friedel-Crafts reaction.
- Formation of a carbocation intermediate: The electron-rich aromatic ring attacks the electrophile, breaking its aromaticity and forming a resonance-stabilised carbocation known as an arenium ion or sigma complex.
- Removal of a proton: A base removes the proton from the carbon atom bonded to the electrophile, restoring the ring's aromaticity and forming the final substituted product.
4. Why is a Lewis acid catalyst, like FeBr₃, required for the halogenation of benzene but not for the halogenation of phenol?
A catalyst like FeBr₃ is necessary for the halogenation of benzene because benzene is very stable due to its aromaticity. The non-polar bromine molecule (Br₂) is not a strong enough electrophile to attack the stable ring. The Lewis acid polarises the Br-Br bond, creating a powerful Br⁺ electrophile. However, in phenol, the -OH group is a strong activating group that donates electron density to the ring, making it highly electron-rich and reactive enough to be attacked directly by Br₂ without a catalyst.
5. How do activating and deactivating groups influence electrophilic substitution on a benzene ring?
Substituents already present on a benzene ring significantly affect the rate and position of further electrophilic attack.
- Activating Groups: These groups (e.g., -OH, -NH₂, -OCH₃) donate electron density to the ring, making it more nucleophilic and increasing the rate of reaction. They direct the incoming electrophile to the ortho and para positions.
- Deactivating Groups: These groups (e.g., -NO₂, -CN, -COOH) withdraw electron density from the ring, making it less reactive and slowing down the reaction. They direct the incoming electrophile to the meta position. Halogens are an exception; they are deactivating but still ortho, para-directing.
6. Why does the nitration of aniline yield a significant amount of meta-nitroaniline, even though the -NH₂ group is ortho, para-directing?
This is a classic exception based on reaction conditions. While the amino (-NH₂) group is a strong activating and ortho, para-director, nitration is carried out in a strongly acidic medium (a mixture of concentrated HNO₃ and H₂SO₄). In this acidic environment, the basic -NH₂ group of aniline gets protonated to form the anilinium ion (-NH₃⁺). This anilinium ion is a powerful deactivating group and is meta-directing. As a result, a substantial amount of the meta-isomer is formed along with the ortho and para products.
7. Why does aniline not undergo Friedel-Crafts alkylation or acylation reactions?
Aniline fails to undergo Friedel-Crafts reactions because the nitrogen atom of the amino group (-NH₂) acts as a Lewis base. It reacts with the Lewis acid catalyst (like AlCl₃) that is required for the reaction. This acid-base reaction forms a salt, which places a positive charge on the nitrogen atom. This positively charged group strongly deactivates the benzene ring, making it resistant to further electrophilic attack and thus preventing the Friedel-Crafts reaction from occurring.
8. What is the fundamental difference between an electrophilic substitution and an electrophilic addition reaction?
The key difference is what happens to the substrate molecule. In an electrophilic substitution, an electrophile replaces an atom (usually hydrogen) on the substrate, and the overall saturation level of the molecule remains unchanged; for example, an aromatic ring stays aromatic. In an electrophilic addition, the electrophile adds across a double or triple bond. The pi bond is broken, and two new sigma bonds are formed, leading to a more saturated product. No atom is lost from the substrate in an addition reaction.
