Organic synthesis and applications
In aldehydes, carbonyl group is bonded to carbon and hydrogen while in ketones, it is bonded to two carbon atoms. The carbonyl carbon atom is sp2-hybridised and a trigonal coplanar structure. The bond angles are approximately 120°. Methanal is gas at room temperature. Ethanal is a volatile liquid. Other aldehydes and ketones are liquid or solid at room temperature. The boiling points of aldehydes and ketones are higher than hydrocarbons and ether of comparable molecular masses. It is due to weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions. Their boiling points are lower than those of alcohols of similar molecular masses due to absence of intermolecular hydrogen bonding. The lower members of aldehydes and ketones are miscible with water because they form hydrogen bonding with water. However, the solubility of aldehydes and ketones decreases rapidly on increasing the length of alkyl chain.
Aldehydes are generally more reactive than ketones in the nucleophilic addition reactions due to steric and electronic reasons. An organic compound (A) with molecular formula C8H8O forms an orange-red precipitate with 2,4-DNP reagent and gives yellow precipitate on heating with iodine in the presence of sodium hydroxide. It neither reduces Tollens' or Fehling's reagent, nor does it decolourise bromine water or Baeyer's reagent. On drastic oxidation with chromic acid, it gives a carboxylic acid (B) having molecular formula C7H6O2.
Identify the catalyst used in Rosenmund reduction.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Pd-BaSO4
Aldehydes are prepared by several named reactions, each with its own starting material and reagent:
Rosenmund reduction: an acyl chloride is hydrogenated over palladium on barium sulphate.
Stephen reaction: a nitrile is reduced by SnCl2/HCl to an imine, which is hydrolysed to an aldehyde.
Etard reaction: toluene is oxidised by chromyl chloride (\( \ce{CrO2Cl2} \)) to a chromium complex, which is hydrolysed to benzaldehyde.
Gattermann–Koch reaction: benzene is treated with CO and HCl in the presence of anhydrous AlCl3 or CuCl to give benzaldehyde.
In the Rosenmund reduction the catalyst is Pd on BaSO4. \( \ce{BaSO4} \) (often with a little sulphur or quinoline) poisons the palladium, so that the aldehyde is not reduced further to an alcohol:
\( \displaystyle \ce{R-COCl + H2} \xrightarrow{\ce{Pd}-\ce{BaSO4}} \ce{R-CHO + HCl} \)
So the catalyst used in Rosenmund reduction is Pd-BaSO4.
Why the other options are incorrect
Option (1) AlCl3/CuCl — Incorrect. This is the catalyst of the Gattermann–Koch reaction.
\( \displaystyle \ce{C6H6 + CO + HCl} \xrightarrow{\text{anhyd. }\ce{AlCl3}/\ce{CuCl}} \ce{C6H5CHO} \)
Option (2) CrO2Cl2 — Incorrect. Chromyl chloride is the reagent of the Etard reaction.
\( \displaystyle \ce{C6H5CH3} \xrightarrow[\text{(ii) }\ce{H3O+}]{\text{(i) }\ce{CrO2Cl2},\ \ce{CS2}} \ce{C6H5CHO} \)
Option (3) SnCl2/HCl — Incorrect. This is the reducing agent of the Stephen reaction.
\( \displaystyle \ce{R-CN} \xrightarrow[\text{(ii) }\ce{H3O+}]{\text{(i) }\ce{SnCl2}/\ce{HCl}} \ce{R-CHO} \)
NCERT (2026-27 edition):
From Acyl Chloride (Acid Chloride)
