Which of the following reagents can be used to convert Ethanol to Ethanal?
(A) Chromium trioxide in Pyridine and \( HCl \)
(B) Heated Copper at 573 K
(C) Anhydrous Chromium trioxide
(D) Acidic \( KMnO_4 \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A), (B) and (C) only
Ethanol, a primary alcohol, is oxidised first to ethanal and then further to ethanoic acid. To stop at the aldehyde, a mild or anhydrous oxidising agent, or catalytic dehydrogenation, must be used; a strong oxidising agent in water carries the oxidation on to the carboxylic acid.
(A) Chromium trioxide in Pyridine and \(HCl\) — Correct. This complex is pyridinium chlorochromate (PCC), which oxidises primary alcohols to aldehydes in good yield.
\( \displaystyle \ce{CH3CH2OH} \xrightarrow{\text{PCC}} \ce{CH3CHO} \)
(B) Heated Copper at 573 K — Correct. The vapour of a primary alcohol passed over hot copper loses hydrogen (dehydrogenation) and gives the aldehyde.
\( \displaystyle \ce{CH3CH2OH} \xrightarrow[573\ \mathrm{K}]{\ce{Cu}} \ce{CH3CHO + H2} \)
(C) Anhydrous Chromium trioxide — Correct. \( \ce{CrO3} \) in an anhydrous medium is used to isolate aldehydes from primary alcohols, because without water the aldehyde is not oxidised further.
(D) Acidic \(KMnO_4\) — Incorrect. Acidified permanganate is a strong oxidising agent; it oxidises ethanol directly to ethanoic acid.
\( \displaystyle \ce{CH3CH2OH} \xrightarrow{\ce{KMnO4}/\ce{H+}} \ce{CH3COOH} \)
NCERT (2026-27 edition):
Oxidation – …
Oxidation – “… When the vapours of a primary or a secondary alcohol are passed over heated copper at 573 K, dehydrogenation takes place and an aldehyde or a ketone is formed while tertiary alcohols undergo dehydration.”