Question

In the Gatterman-Koch reaction, a formyl group (-CHO) is introduced directly onto a benzene ring. For example, reaction of toluene with CO and \(\mathrm{HCl}\) in the presence of \(\mathrm{AlCl}_{3}\) gives \(p\) -methylbenzaldehyde. Propose a mechanism.

Short answer

A formyl cation is generated and attacks toluene, leading to p-methylbenzaldehyde formation.

Step-by-step solution

Formation of Formyl Cation

In the Gatterman-Koch reaction, the first step involves the generation of a formyl cation ( ext{CH}^+O) from carbon monoxide ( ext{CO}) and hydrochloric acid ( ext{HCl}) in the presence of aluminum chloride ( ext{AlCl}_3). This is possible due to the strong ext{AlCl}_3, which acts as a Lewis acid, facilitating the formation of the electrophilic species.

Electrophilic Attack on Toluene

The generated formyl cation is a powerful electrophile and attacks the electron-rich benzene ring of toluene. Toluene acts as the nucleophile, and this step leads to the formation of a sigma complex as the aromatic ring temporarily loses its aromaticity.

Rearrangement to Form Intermediate

The sigma-complex undergoes rearrangement in which the positive charge is stabilized. This step involves the relocation of electrons within the ring to regain stability.

Deprotonation and Restoration of Aromaticity

Finally, the sigma complex loses a proton, which restores the aromaticity of the benzene ring. This step leaves us with the substitution product, ext{p}-methylbenzaldehyde, as a result of the formyl group being directly introduced onto the benzene ring.

Key concepts

Formyl Cation

The formyl cation is a key player in the Gatterman-Koch reaction. In this reaction, it serves as the electrophile that will eventually become part of the aromatic ring. The formyl cation, represented as \(\text{CH}^+\text{O}\), is generated from carbon monoxide (CO) and hydrochloric acid (HCl) in the presence of aluminum chloride (AlCl₃). AlCl₃ acts as a Lewis acid, which means it can accept electron pairs and help in forming the positively charged formyl cation.
This generation process is crucial because the formyl cation is very reactive and needs careful conditions to form. Its formation sets the stage for it to interact with the aromatic compounds, leading to a substitution reaction.
The concept of a formyl cation is pivotal because it links carbon monoxide, a simple gas, to complex aromatic chemistry. This showcases the versatility and ingenuity of chemical reactions.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a fundamental reaction mechanism in organic chemistry. It involves the introduction of an electrophile to an aromatic system, such as benzene, leading to the substitution of one of its hydrogen atoms. In the Gatterman-Koch reaction, the formyl cation acts as the electrophile.
Toluene, which contains an electron-rich aromatic benzene ring, encounters the formyl cation. This interaction sets off a series of events where the highly reactive cation targets the benzene ring. Here, the electrons from the benzene's cloudy electron system temporarily form a bond with the electrophile, leading to the loss of its aromatic stability.
Understanding electrophilic aromatic substitution is vital as it forms the basis for many other reactions aimed at functionalizing aromatic compounds. It explains how electrophiles, despite their positive charges, can effectively bond with stable aromatic rings.

Sigma Complex

A sigma complex, also known as an arenium ion, forms as an intermediate during the electrophilic aromatic substitution process. When the benzene ring of toluene interacts with the formyl cation, a temporary disruption of the ring’s aromaticity occurs.
As the formyl cation bonds with the aromatic ring, a sigma complex is created. This complex is characterized by a carbocation, where a portion of the electron cloud from the benzene shifts to accommodate the new bond. This state is not stable because it disrupts the aromatic structure.
The sigma complex is important because it highlights the transient state that aromatic compounds must pass through during substitution reactions. While it is a high-energy, less stable state, understanding and identifying this complex helps chemists grasp how aromatic compounds react and regain stability.

Aromaticity Restoration

After forming a sigma complex, the benzene ring temporarily loses its aromatic nature. However, one of the appealing traits of aromatic compounds is their tendency to restore aromaticity, as this provides enhanced stability due to electron delocalization.
In the final steps of the Gatterman-Koch reaction, the sigma complex undergoes deprotonation, which means a hydrogen ion (H⁺) leaves the adjacent carbon. This elimination allows the electrons that were temporarily engaged in bonding with the formyl cation to rearrange and return to a delocalized state.
This step is crucial because it restores the aromaticity of the benzene ring. In doing so, not only does the molecule regain stability, but it also forms the final product, \(\text{p-methylbenzaldehyde}\). Aromaticity restoration ensures that the chemical integrity of the benzene ring is maintained even after undergoing a reaction, showcasing the resilience and stability of aromatic systems.