Electrophilic Substitution Reaction
Benzene undergoes Electrophilic Susbtitution Reaction. In these reactions, an electron deficit species that need an electron also called electrophile replaces an atom or group of atoms in benzene to form another compound. Electrophilic Substitution Reaction includes the following reactions let’s discuss them one by one.
Nitration of Benzene
The nitration of benzene is an electrophilic substitution reaction between benzene and a mixture of concentrated nitric acid and concentrated sulfuric acid.
The reaction of nitration of benzene is shown mentioned below:
C6H6 —-Conc. H2SO4 and Conc. HNO3—-> C6H5NO2 + H2O
The mechanism for the nitration of benzene can be summarized as follows:
- Nitric acid takes a proton from sulfuric acid to form the nitronium ion.
- The nitronium ion acts as an electrophile and reacts with benzene to form an arenium ion.
- The arenium ion loses its proton to a Lewis base, forming nitrobenzene.
Sulfonation of Benzene
The sulfonation of benzene proceeds via an electrophilic aromatic substitution mechanism, specifically utilizing sulfur trioxide (SO3) or fuming sulfuric acid (oleum, H2S2O7) containing SO3 as the electrophile.
Here is a step-by-step description of the mechanism:
- Generation of the electrophile: In the case of fuming sulfuric acid, SO3 is already present; however, if concentrated sulfuric acid is used, SO₃ is formed through the dissociation of some H2SO4 into SO3 and H₂O.
H2SO4 ⇌ H2O + SO3
- Attack by the electrophile: The electrophilic sulfur atom attracts the electron density within the benzene ring.
- Formation of the sigma complex (arenium ion): An intermediate called the arenium ion is formed when the sulfur atom bonds to a carbon atom in the benzene ring.
- Proton transfer: A proton from the benzene ring moves to the negatively charged oxygen atom of the attached SO3, resulting in the formation of benzenesulfonic acid.
Here is a simplified representation of these steps:
C6H6 + SO3 → C6H5SO3H
Halogenation of Benzene
The halogenation of benzene involves the substitution of a hydrogen atom on the benzene ring with a chlorine or bromine atom. This reaction occurs in the presence of a catalyst such as aluminum chloride or iron. The catalyst helps activate the halogen to become a strong electrophile.
For example, chlorobenzene is formed when benzene reacts with chlorine in the presence of aluminum chloride.
The specific reactions for chlorination and bromination of benzene are as follows:
C6H6 + Cl2 → C6H5Cl + HCl
The mechanism of electrophilic aromatic substitution between benzene and chlorine or bromine involves several steps:
- The halogen molecule approaches the benzene ring, and the delocalized electrons in the ring repel electrons in the halogen-halogen bond.
- The slightly positive end of the halogen molecule acts as the electrophile, facilitated by the presence of the catalyst.
- The electrophilic substitution occurs, leading to the formation of chlorobenzene or bromobenzene.
Benzene Reactions
Benzene is aromatic compound which act as precursor to derive other compounds. Reactions of benzene involve the substitution of a proton by other groups. Electrophilic aromatic substitution is a method of derivatizing benzene. The most common example of this reaction is the ethylation of benzene. Different important reactions of benzene include sulfonation, chlorination, nitration, and hydrogenation. The activating or deactivating effect of substituents on the benzene ring determines the reaction’s direction and the ring’s reactivity.
In this article, we will learn about the different reactions of benzene, along with basic introduction of benzene and its structure.
Table of Content
- What is Benzene?
- Reactions of Benzene
- Electrophilic Substitution Reaction
- Electrophilic Addition Reaction
- Benzene Reduction
- Benzene Ring Opening Reaction
- Nucleophilic Aromatic Substitution