Question

What are Étard Reaction and Gattermann Koch Reaction? Give one example of each.

Answer 1

In Etard Reaction, we carry out oxidation using chromyl chloride. It involves direct oxidation. In the Gattermann – Koch reaction, aromatic compounds are formulated using a Lewis acid.

Complete step by step solution:
Etard Reaction is named after the French chemist Alexandre Léon Étard and the Gattermann – Koch reaction is named after the two German chemists Ludwig Gattermann and Julius Arnold Koch. First let us discuss the Etard reaction.
Étard Reaction: At first chromyl chloride, which is a weak oxidizing agent, reacts with toluene in presence of non-polar solvent carbon tetrachloride. This leads to homolytic cleavage of bonds of chromyl chloride. In the similar manner, Étard complex is formed through the homolytic cleavage of C-H bonds of methyl group. Finally, the two molecules of $Cr{{(OH)}_{2}}C{{l}_{2}}$ are removed through hydrolysis of the Étard complex and hence, the formation of benzaldehyde (aldehyde). Étard Reaction offers a way to oxidize aromatic methyl groups.

Now, let us discuss the Gattermann-Koch reaction.
Gattermann–Koch reaction: The Gattermann-Koch Reaction gives the mechanism of formation of an aryl aldehyde with the help of carbon monoxide and hydrochloric acid as reactants. Aluminum trichloride acts as a catalyst in the presence of trace amounts of cuprous chloride. We can take an example where benzene is converted into benzaldehyde. Now, let us see the mechanism- First carbon monoxide reacts with HCl and forms formyl chloride.

Then the formyl chloride reacts with anhydrous aluminum chloride and forms an electrophile $H-\overset{\oplus }{\mathop{C}}\,=O$ .

Finally, $H-\overset{\oplus }{\mathop{C}}\,=O$ reacts with benzene and forms benzaldehyde. During the reaction in this step $AlC{{l}_{3}}$ and HCl is removed.

We can see the mechanism of the given reactions in the above discussion and that is the required answer.

Note: We should not be confused between Gattermann and Gattermann – Koch reaction. The Gattermann reaction uses a mixture of hydrogen cyanide and hydrochloric acid and Gattermann-Koch uses carbon monoxide instead of hydrogen cyanide. This is also a method of preparation of aromatic aldehydes like benzaldehyde salicylaldehyde etc. In this reaction benzene or phenols are treated with H-CN and HCl in presence of $AlC{{l}_{3}}$.