Explain the following with suitable example:
(a) Rosenmund reduction
(b) Gatterman-Koch reduction

Hint:The Rosenmund reduction reaction is a hydrogenation reaction in which acyl chloride is converted to aldehyde. The Gatterman-Koch reduction reaction is a reaction in which the benzene molecule is converted to benzaldehyde. One can write suitable examples with the reagents for the reaction.

Complete step by step answer:1) First of all we will learn what reduction means. The reduction is a process in which there is a gain of electrons of hydrogen atoms. If you observe in the given both reactions there is the gain of hydrogen atoms. Now let us learn the reactions given in the question,
(a) Rosenmund reduction reaction:
In this reaction, the acyl chlorides are converted to the aldehyde in the presence of hydrogen gas over palladium metal which is poisoned by barium sulfate. Now let us see an example for this reaction,
${C_6}{H_5}COCl\xrightarrow[{Pd - BaS{O_4}}]{{{H_{2(g)}}}}{C_6}{H_5} - CHO + HCl$
In the above reaction, the benzoyl chloride is treated with hydrogen and palladium metal which is poisoned by barium sulfate that gives benzaldehyde as a product.
(b) Gatterman-Koch reduction reaction:
In this reaction, the formation of reactive species is done by the reaction of the acid with carbon monoxide which will yield an aldehydic group. The lewis acid $AlC{l_3}$ activates the benzene ring and the formation of aldehyde happens. Now let us see an example for the reaction,
${C_6}{H_6} + CO + HCl\xrightarrow[{CuCl}]{{Anhyd.AlC{l_3}}}{C_6}{H_5} - CHO$
In the above reaction when benzene is reacted with carbon monoxide in presence of acid and anhydrous lewis acid $AlC{l_3}$ it will yield benzaldehyde as the product.

Note:The Gatterman-Koch reduction reaction is also called the formylation reaction as there is the addition of the formaldehyde group in the reactant molecule. This reaction is not applicable for the substrates such as phenols and phenol ethers. In the Rosenmund reduction reaction, barium sulfate has a very low surface area which further reduces the palladium metal activity which prevents over-reduction in the reaction.