Question

Write the reaction for the preparation of : (i) acetaldehyde by Rosenmund reaction. (ii) benzaldehyde by Gatterman-Koch formylation.

Hint:

Named reactions are important shortcuts in organic synthesis. Rosenmund reduction is a classic way to get aldehydes from acid chlorides, while Gatterman-Koch is a key method for formylating aromatic rings.

Answer 1

(i) Rosenmund Reaction for Acetaldehyde This reaction involves the catalytic hydrogenation of an acid chloride to an aldehyde. The catalyst is poisoned to prevent further reduction of the aldehyde to an alcohol.
\[ \underset{\text{Acetyl chloride}}{\text{CH}_3\text{COCl}} + \text{H}_2 \xrightarrow{\text{Pd/BaSO}_4, \text{S or quinoline}} \underset{\text{Acetaldehyde}}{\text{CH}_3\text{CHO}} + \text{HCl} \]
(ii) Gatterman-Koch Formylation for Benzaldehyde This reaction introduces a formyl group (-CHO) onto an aromatic ring. Benzene is treated with carbon monoxide and hydrogen chloride in the presence of a catalyst mixture of anhydrous aluminum chloride and cuprous chloride.
\[ \underset{\text{Benzene}}{\text{C}_6\text{H}_6} + \text{CO} + \text{HCl} \xrightarrow[\text{High Pressure}]{\text{Anhydrous AlCl}_3 / \text{CuCl}} \underset{\text{Benzaldehyde}}{\text{C}_6\text{H}_5\text{CHO}} + \text{HCl} \]