Aromatic formylation, also known as the Gattermann-Koch reaction, is a classic organic synthesis reaction involving converting an aromatic compound to an aldehyde by introducing a formyl (-CHO) group onto the ring. The reaction is named after its discoverers, the German chemists Ludwig Gattermann and Julius Arnold Koch.
Mechanism
The reaction proceeds through the following steps:
Step 1: Activation of the formylating reagent
The formylating reagent, usually a mixture of hydrogen cyanide (HCN) and hydrogen chloride (HCl), is activated by reacting with anhydrous aluminum chloride (AlCl3) to form an electrophilic species, known as a “cyanonium ion” or “formyl cation”:
This highly reactive species readily reacts with the aromatic compound in the next step.
Step 2: Attack the electrophilic species on the aromatic ring
The electrophilic formyl cation then attacks the aromatic ring, with the most nucleophilic site being the ortho and para positions concerning any substituents on the ring. This is because the substituents can either stabilize or destabilize the intermediate, leading to different products.
For example, if the aromatic ring contains an electron-donating group, such as a methoxy (-OCH3) group, the intermediate is stabilized by resonance, and the para product is favoured, and if the aromatic ring contains an electron-withdrawing group, such as a nitro (-NO2) group, the intermediate is destabilized by resonance, and the ortho product is favoured.
Step 3: Deprotonation and formation of the final product
The intermediate formed in step 2 is highly unstable and quickly loses a proton to form the final product, an aromatic aldehyde:
Overall reaction
The overall reaction can be summarized as follows:
Reaction conditions
The Gattermann-Koch reaction is typically carried out under anhydrous conditions, as the reaction is sensitive to water. The reaction can occur in various solvents, such as benzene, toluene, or chloroform. The reaction is also carried out at low temperatures, typically between 0-5°C, to control the reaction and minimize the formation of side products.
Conclusion
The aromatic formylation reaction, or Gattermann-Koch reaction, is an important organic synthesis reaction that converts an aromatic compound to an aldehyde by introducing a formyl group onto the ring. The reaction proceeds through a series of steps, starting with the activation of the formylating reagent, then the electrophilic species attack on the aromatic ring, and ending with the formation of the final product. The reaction is sensitive to water and is typically carried out under anhydrous conditions in a variety of solvents at low temperatures.
