Electrophilic Substitution of Benzene

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What is an Electrophilic Substitution Reaction?

Electrophilic replacement reactions are chemical reactions in which an electrophile displaces in a compound a functional group, which is usually, but not always, an atom of hydrogen. Aromatic compounds are typical of electrophilic aromatic substitution reactions and are important ways of adding functional benzene ring groups. An electrophilic aliphatic substitution reaction is the other primary form of electrophilic replacement reaction.

In several of the reactions of compounds containing benzene rings – the arenas, electrophilic substitution occurs. Aromatic nitration, aromatic halogenation, aromatic sulfonation, and Friedel-Crafts reaction alkylation and acylation are some of the most important electrophilic aromatic substitutions.

Generally, electrophilic substitution reactions proceed through a three-step process involving the following steps.

  1. The appearance of an electrophile.

  2. The appearance of a carbocation (which is an intermediate).

  3. The elimination from the intermediate of a proton charge.

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What is the Electrophilic Substitution Reaction of Benzene?

In the electrophilic substitution of benzene, the hydrogen atom of benzene is substituted by an electrophile. These reactions are extremely random in nature as the aromaticity of benzene is not disrupted in the reaction. Nitration, halogenation, sulfonation, Friedel Craft’s alkylation, and acylation, etc., are simple examples of electrophilic benzene replacement reactions.

As per the chemical reactivity of benzene compared to that of alkenes in the preference order to addition reactions, substitution reactions occur. These reactions are generally referred to as Electrophilic Aromatic Substitution because the reagents and conditions used in these reactions are electrophilic. The catalysts and co-reagents are used to produce the powerful electrophilic species required to perform the initial substitution step.

Experiments have shown that substituents on a benzene ring may have a profound effect on reactivity. As determined by molecular dipole moments, this activation or deactivation of the benzene ring against electrophilic substitution can be associated with the electron-donating or electron-withdrawing effect of the substituents.

The second element that becomes important in substituted benzene reactions concerns the position at which electrophilic substitution takes place.

What is the General Mechanism of the Electrophilic Substitution Reaction of Benzene?

A two-step process-the addition of the electrophile, followed by deprotonation-is the mechanism of electrophilic aromatic substitution.

A significant feature of this process is that if we know the product since it is the atom or group that replaces the H+, we can define the electrophile. Conversely, we can predict the product's structure if we know the electrophile. The catalyst's job is to bond with the leaving group and make it a better group to leave. A more thorough study includes substitution reactions of compounds having an antagonistic orientation of substituents. The symmetry of the molecule would again simplify the decision if the substituents are similar. If a substituent has a pair of non-bonding electrons usable for adjacent charge stabilization, the product deciding power would usually be exercised.

Three steps are involved in the electrophilic substitution reaction mechanism.

Step 1: Electrophile Generation

In the generation of electrophiles from the chlorination, alkylation, and acylation of an aromatic ring, anhydrous aluminum chloride is a very helpful Lewis acid. Electrophile production takes place due to the presence of Lewis acid. The electron pair from the attacking reagent is accepted by the Lewis acid. The resulting electrophiles are Cl+, R+, and RC+O respectively (from the combination of anhydrous aluminum chloride and the attacking reagent).

Step 2: Formation of carbocation

The electrophile, forming a sigma complex or an arenium ion, attacks the aromatic ring. One of the hybridized carbons in this ion of uranium is sp3. This arenium ion, in a resonance structure, finds stability. The sigma complex or the arenium ion loses its aromatic character since the delocalization of electrons stops at the sp3 hybridized carbon.

Step 3: Deprotonation

Deprotonation is the third step of electrophilic substitution. Deprotonation is the reaction's driving force, making it energetically possible to proceed. This step's activation energy is much lower, and the reaction happens very rapidly.

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Examples of Electrophilic Substitution Reaction

Some examples of electrophilic aromatic substitution include nitration and halogenation of benzene. The electrophiles are nitronium ion (NO2+) and Sulphur trioxide (SO3) and react with benzene individually to provide nitrobenzene and benzene sulfonic acid, respectively.

  1. Benzene Sulfonation

Benzene sulfonation is a method of fuming sulphuric acid (H2SO4 + SO3) to heat benzene to create benzene-sulfonic acid. In nature, the reaction is reversible.

  1. Benzene Nitration

Via the protonation of nitric acid by sulfuric acid, the source of the nitronium ion induces the loss of a water molecule and the creation of a nitronium ion.

  1. Benzene Halogenation

In the presence of Lewis acid, such as FeCl3, FeBr3, Benzene reacts with halogens to form aryl halides. This reaction is known as benzene halogenation.

  1. Sulfuric Acid Activation of Nitric Acid

The first step in benzene nitration is to activate HNO3 with sulfuric acid to create a nitronium ion, a stronger electrophile.

FAQ (Frequently Asked Questions)

Question 1. What are the Two Variants of Electrophilic Substitution Reaction?

Answer: The two main kinds of electrophilic substitutions are aliphatic electrophilic substitution and aromatic electrophilic substitution.

An electrophile displaces a functional group during electrophilic substitution in aliphatic compounds. This reaction is similar to aliphatic nucleophilic substitution, where a nucleophile rather than an electrophile is the reactant. An atom appended to the aromatic ring, normally hydrogen, is substituted by an electrophile in electrophilic substitution in aromatic compounds.

Question 2. What are the Significant Differences Between the Electrophile Substitution Reaction and Nucleophilic Substitution Reaction?

Answer: In Organic and Inorganic chemistry, both nucleophilic and electrophilic substitution reactions are observed. In the synthesis of certain compounds, these substitution reactions are very important. Electrophilic substitutions include displacement by an electrophile of a functional group (generally a hydrogen atom). Species that are attracted to electrons are electrophiles.

Nucleophilic replacements include the attack by a nucleophile of a positively charged (or partially positively charged) atom or community. Species that can donate an electron pair are nucleophiles.

The key difference between nucleophilic and electrophilic substitution reactions is that nucleophilic substitution reactions require the displacement by a nucleophile of a leaving group, whereas electrophilic substitution reactions involve the displacement by an electrophile of a functional group.