Problem 107 - Chemistry The Molecular Nature of Matter

Question

If one mole of benzene were to add one mole of \(\mathrm{Br}_{2}\), what would form? What forms, instead, when \(\mathrm{Br}_{2}\), benzene, and a \(\mathrm{FeBr}_{3}\) catalyst are heated together? (Write the structures.)

Step-by-Step Solution

Answer

Without a catalyst, there is no reaction between benzene and \(\mathrm{Br}_{2}\). With \(\mathrm{FeBr}_{3}\) as a catalyst, bromobenzene (\(\mathrm{C}_{6}\mathrm{H}_{5}Br\)) forms through electrophilic aromatic substitution.

Step 1: Identifying the Reaction without a Catalyst

Without a catalyst, benzene and \(\mathrm{Br}_{2}\) would not react under normal conditions, as benzene is quite unreactive due to its stable aromatic structure. Thus, without a catalyst, one mole of benzene adding one mole of \(\mathrm{Br}_{2}\) would theoretically not lead to a reaction.

Step 2: Reaction with a Catalyst

When \(\mathrm{Br}_{2}\), benzene, and a \(\mathrm{FeBr}_{3}\) catalyst are heated together, the \(\mathrm{FeBr}_{3}\) acts as a Lewis acid and facilitates the electrophilic aromatic substitution (EAS) mechanism. The bromine \(\mathrm{Br}_{2}\) reacts with the catalyst, forming a bromonium ion and \(\mathrm{FeBr}_{4}^-\), which can then electrophilically attack the benzene ring. The product of this reaction is bromobenzene, where a bromine atom has substituted one of the hydrogen atoms on the benzene ring. The resulting structure is \(\mathrm{C}_{6}\mathrm{H}_{5}Br\).

Key Concepts

Benzene Reactivity

Benzene, with its unique aromatic structure, presents both an interesting challenge and a fundamental topic in organic chemistry. The ring structure of benzene, characterized by a hexagon with three double bonds, is stabilized by resonance. This stabilization, often depicted as a circle inside the hexagon, results in a reluctance to react, because breaking any of the double bonds would disrupt the aromaticity.

Benzene's reactivity is not akin to that of alkenes, where double bonds are readily attacked by electrophiles. Instead, benzene undergoes a specific type of reaction known as electrophilic aromatic substitution (EAS). In EAS, an electrophile replaces one of the hydrogen atoms on the benzene ring. Because of the stabilization, these reactions require a more clever approach, such as the use of a catalyst, to proceed.

Bromination of Benzene

The bromination of benzene is a classic example of an electrophilic aromatic substitution reaction. Under ordinary conditions, benzene and molecular bromine (\( \text{Br}_2 \) don't noticeably react due to the stability of benzene's aromatic system. However, in the presence of a suitable catalyst, bromine becomes an attentive participant in the dance of EAS.

This affair begins with the formation of a more reactive electrophilic species, the bromonium ion, which is primed to interact with the benzene ring. When this ion approaches the benzene ring, it replaces a hydrogen atom and binds to the carbon. The final result is bromobenzene (\( \text{C}_6\text{H}_5\text{Br} \) ), a compound where a bromine atom assumes the position once occupied by a hydrogen atom—without disturbing the aromatic sanctity of the ring.

Lewis Acid Catalyst

In these orchestrations of chemical finesse, the Lewis acid catalyst plays a pivotal role. It's a substance that, due to its electron-deficient nature, accepts electron pairs. In the case of bromination of benzene, iron(III) bromide (\( \text{FeBr}_3 \) ) is one such maestro. It coordinates with the bromine, creating a more reactive entity that heightens the electrophilic character of bromine—a necessary step for the process to proceed.

Subsequently, the newly formed complex stratagem (\( \text{FeBr}_4^- \) ) leaves the scene, allowing the bromonium ion to sweep in and connect with benzene. This magic touch, facilitated by the Lewis acid, is what makes such an inert compound like benzene receptive to change, opening up pathways for numerous derivatives through EAS. Mastery of the Lewis acid catalyst is akin to holding the key that unlocks the true potential of many organic reactions.