Question

Aromatic substitution reactions occur by addition of an electrophile such as \(\mathrm{Br}^{+}\) to an aromatic ring to yield an allylic carbocation intermediate, followed by loss of \(\mathrm{H}^{+} .\) Show the structure of the intermediate formed by reaction of benzene with \(\mathrm{Br}^{+}\)

Short answer

The intermediate is a resonance-stabilized bromonium ion with a positive charge on one carbon in a benzene ring.

Step-by-step solution

Understanding Electrophilic Aromatic Substitution

In electrophilic aromatic substitution, the aromatic ring initially reacts with an electrophile, which in this case is \( \text{Br}^+ \). This step results in the formation of a carbocation intermediate on the benzene ring.

Electrophile Addition to Benzene

When \( \text{Br}^+ \) approaches the benzene ring, it forms a bond with one of the carbon atoms, temporarily disrupting the aromaticity. This leads to the formation of an arenium ion (the cyclic carbocation), specifically a bromonium ion.

Visualization of the Intermediate

The intermediate structure is represented as a benzene ring where one carbon is bonded to a Br atom and carries a positive charge. This disrupts the conjugation and creates a non-aromatic, resonance-stabilized carbocation.

Resonance Structure Considerations

This carbocation can be stabilized by resonance. Draw the resonance structures by moving the positive charge around the ring to show delocalization of the charge over the ring.

Key concepts

Benzene

Benzene is a fascinating and unique chemical compound with a ring structure of six carbon atoms bonded together. It is the simplest type of aromatic ring and is often a starting material for many reactions in organic chemistry.

In benzene, each carbon atom shares electrons with two neighboring carbon atoms and a hydrogen atom, forming a structure known as an "aromatic ring." This specific arrangement allows the electrons to be shared across all the carbon atoms evenly, creating what is known as a conjugated system.

This conjugation imparts a special stability to benzene, known as aromaticity. The electrons in benzene participate in resonance, meaning they can be depicted as moving around the ring, which contributes to its stability. This stability makes benzene less reactive to addition reactions that would otherwise disturb its electron clouds. However, it is highly favorable for electrophilic substitution reactions, where it can maintain its aromatic structure even after reacting.

Carbocation Intermediate

A carbocation intermediate plays a crucial role in electrophilic aromatic substitution reactions. When benzene reacts with an electrophile like \( \text{Br}^+ \), the aromatic system is temporarily disrupted to form a highly reactive carbocation intermediate.

In this process, the benzene ring donates a pair of electrons to bond with \( \text{Br}^+ \), which forms a temporary positive charge on one of the carbons. This structure is specifically called a 'sigma complex' or an 'arenium ion', which momentarily loses its aromatic nature.

The positive charge in the carbocation makes it highly unstable, and therefore it must quickly regain stability. It can do this by losing \( \text{H}^+ \) from the carbon bonded to bromine, restoring the aromaticity to the benzene ring. The intermediate is crucial because it represents a potential pathway for the reaction and highlights the importance of having stable intermediates in complex reactions.

Resonance Structures

Resonance structures are a concept used by chemists to depict the delocalization of electrons in molecules, especially those with a conjugated system like benzene. When a carbocation intermediate forms during an electrophilic aromatic substitution, it can be stabilized through resonance.

In the intermediate where \( \text{Br}^+ \) bonds with benzene, the positive charge can shift across adjacent carbon atoms in the ring. This movement is depicted through different resonance forms. Each of these forms shows the positive charge on a different carbon atom, illustrating how the charge is spread out across the entire molecule.

Drawing these structures helps chemists understand the potential energy distribution and how molecules stabilize themselves despite having temporary disruptions in their usual arrangement. The more resonance structures a molecule can form, the more stable the carbocation intermediate typically is. This delocalization of charge minimizes the electron deficiency, which ultimately contributes to the overall stability of the aromatic compound during reaction.