An organic compound (A) (molecular formula C8H16O2) was hydrolysed with dilute sulphuric acid to give a carboxylic acid (B) and an alcohol (C). Oxidation of (C) with chromic acid produced (B). (C) on dehydration gives a mixture of but-1-ene and but-2-ene, both of which on reduction give (D). (C) on oxidation with pyridinium chlorochromate (PCC) gives (E), which is an aldehyde.
In the passage, the alcohol (C) is oxidised to the aldehyde (E) with pyridinium chlorochromate. On an industrial scale, volatile alcohols are converted into aldehydes or ketones by dehydrogenation. Over which of the following are the alcohol vapours passed in this method?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Silver or copper
In dehydrogenation, a molecule of hydrogen is removed from the alcohol. The vapours of a volatile alcohol are passed over a heavy metal catalyst (Ag or Cu); the method is used in industry. A primary alcohol gives an aldehyde and a secondary alcohol gives a ketone:
\( \displaystyle \ce{RCH2OH} \xrightarrow{\ce{Cu}\text{ or }\ce{Ag}} \ce{RCHO + H2} \)
\( \displaystyle \ce{R2CHOH} \xrightarrow{\ce{Cu}\text{ or }\ce{Ag}} \ce{R2CO + H2} \)
Why the other options are incorrect
Option (1) Anhydrous aluminium chloride — Incorrect. Anhydrous AlCl3 is the Lewis acid catalyst of Friedel-Crafts acylation and of the Gattermann-Koch reaction, where a carbonyl compound is made from benzene, not from an alcohol.
Option (3) Palladium on barium sulphate — Incorrect. This catalyst is used to hydrogenate an acyl chloride to an aldehyde (Rosenmund reduction); it adds hydrogen rather than removing it.
Option (4) Heated alumina — Incorrect. Alumina removes a molecule of water from an alcohol (dehydration), so it gives an alkene, not a carbonyl compound.
NCERT (2026-27 edition):
8.2.1 Preparation of Aldehydes and Ketones – “2. By dehydrogenation of alcohols
This method is suitable for volatile alcohols and is of industrial application. In this method alcohol vapours are passed over heavy metal catalysts (Ag or Cu). Primary and secondary alcohols give aldehydes and ketones, respectively (Unit 7, Class XII).”