Question

In halogenation of aromatic hydrocarbon, a halogen carrier is used which is generally a Lewis acid. The main function of this reagent is to generate the species (a) \(X\) (b) \(X^{-}\) (c) \(X^{+}\) (d) \(X^{*}\)

Short answer

\(X^{+}\) is the species generated when a Lewis acid is used as a halogen carrier in the halogenation of aromatic hydrocarbons.

Step-by-step solution

Understand the Role of the Lewis Acid

The Lewis acid in halogenation of aromatic hydrocarbons provides a function of accepting an electron pair. It reacts with the halogen to make it more electrophilic.

Determine the Species Generated by the Action of the Lewis Acid

When a Lewis acid reacts with a diatomic halogen molecule (e.g., Cl2, Br2), it pulls an electron pair towards itself from the halogen bond, thus creating a halonium ion and a halide anion. The species that is strongly electrophilic and attacks the aromatic ring to form the halogenated product is the halonium ion.

Identify the Correct Species

The correct species is the positively charged halogen ion, since it is the one that will act as an electrophile in the halogenation process. It is represented by the formula given in option (c) \(X^{+}\).

Key concepts

Lewis Acid

In the context of organic chemistry, Lewis acids play a crucial role in the halogenation of aromatic hydrocarbons. But what exactly are these substances? Simply put, a Lewis acid is a chemical compound that can accept an electron pair. This definition stems from the broader Lewis theory of acids and bases, where an acid is identified by its ability to accept electrons rather than donate them.

During halogenation, the Lewis acid (such as FeCl₃ or AlCl₃ for chlorination and bromination, respectively) serves as the halogen carrier. Its main job is to interact with a diatomic halogen molecule like Cl₂ or Br₂. This interaction creates a more reactive halogen species by accepting electrons and pulling them away from the halogen bond. As a result, you get a positively charged species ready to attack the aromatic ring - essentially setting the stage for the all-important electrophilic aromatic substitution to occur.

Electrophilic Aromatic Substitution

The electrophilic aromatic substitution (EAS) is a fundamental type of reaction in organic chemistry, particularly in the transformation of aromatic hydrocarbons. In this reaction, the stability of the aromatic system is momentarily disrupted when an electrophile, such as a positively charged halonium ion, forms a bond with one of the carbon atoms in the aromatic ring.

The allure of the EAS lies in its ability to replace a hydrogen atom on the benzene ring with another group, in this case, a halogen. Initially, the electrophile reacts with the delocalized electrons of the aromatic ring, creating an unstable intermediate. Subsequently, the ring regains its aromaticity by releasing a proton (H⁺) to a base, typically the halide anion formed alongside the electrophile during the Lewis acid interaction. This two-step dance is essential for the introduction of halogens onto the ring, making EAS a cornerstone of halogenation processes.

Halonium Ion

Diving deeper into the reaction mechanism, let's focus on the halonium ion, the true star of the halogenation show. Represented by option (c) in the exercise, the halonium ion (\(X^{+}\) ) is an intermediate species that carries a positive charge. You can think of it as an 'activated' halogen - it's a halogen atom that has been given a positive charge through its interaction with a Lewis acid.

Why is the halonium ion so reactive? The positive charge makes it 'hungry' for electrons. It desperately wants to fill its electron deficiency, and it does this by seeking out electron-rich areas, like the pi cloud of electrons in an aromatic ring. When the halonium ion collides with the ring, it forms a new bond with a carbon atom, temporarily breaking the delocalized pi system and leading to the formation of a sigma complex. This is the essential step where the halogen is incorporated into the aromatic compound, marking the success of the halogenation reaction.