Question

Which Reagent is used in Rosenmund reduction?
$(A)H_2/Pd-BaS{O}_4$
$(B)LiAl{H}_4$
$(C)N{H}_2-N{H}_2/KOH/C{H}_{2}OH-C{H}_{2}OH$
$(D)Zn-Hg/HCl$

Answer 1

Hint: Rosenmund reaction is very popular organic chemical reaction. It is an example of Hydro-genation reaction. It is a very selective reaction in which Acyl or acid chloride is reduced to Aldehyde. This reaction is named after Karl Wilhelm Rosenmund, who first reported it in 1918.The application of Rosenmund reaction is in preparation of Saturated Aldehydes.

Complete answer:
The Rosenmund reaction is shown as follows-
$RCOCl+H_2\underset{BaS{O}_4}{\overset{H_2/Pd}{\to }}RCHO+HCl$
Where R is either Alkyl group (methyl, ethyl etc.) or Aryl group like phenyl).This reaction occurs in low pressure and low temperature conditions.
Hydrogen is highly reactive in nature. Hydrogen has a high bond dissociation energy so we need a catalyst. In this reaction there is a replacement of Chlorine by Hyrogen.In this reaction we use the palladium catalyst $(Pd)$ poisoned with Barium Sulphate $(BaS{O}_4)$as the reagent. We add Barium sulphate to the Palladium surface in order to poison it and stop the further reduction of Acyl chloride because if further reduction takes place it converts into primary alcohol which on reaction with Acid chloride produces Ester. The catalyst does not cause any chemical change in the reaction. The combination of palladium with barium sulphate is also called Lindlar's catalyst.
Hence option (A) is correct.

Note:
This reaction is an example of partial reduction of Acyl chloride. Formaldehyde $(HCHO)$ cannot be produced by this particular reaction since formyl chloride $(HCOCl)$ is very unstable. The production of acid chloride which is used as the reactant in this particular reaction is done from Carboxylic acid on reaction with $SOC{l}_2/PC{l}_5/PC{l}_3$ etc.