What is formed when an aldehyde is oxidised with potassium dichromate or potassium permanganate?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A carboxylic acid
The aldehyde group, –CHO, has a hydrogen on the carbonyl carbon. Oxidising agents such as potassium dichromate and potassium permanganate turn this C–H into C–OH, so –CHO becomes –COOH, for example:
\( \displaystyle \ce{RCHO} \xrightarrow{[\ce{O}]} \ce{RCOOH} \)
Why the other options are incorrect
Option (2) An alcohol — Incorrect. An aldehyde gives a primary alcohol on reduction (for example with NaBH4 or LiAlH4), not on oxidation.
Option (3) A ketone — Incorrect. A ketone has two carbon groups on the carbonyl carbon; oxidation does not put a carbon group in place of the aldehyde hydrogen.
Option (4) An ester — Incorrect. An ester forms when a carboxylic acid reacts with an alcohol (esterification), not by oxidising an aldehyde.
NCERT (2026-27 edition):
3. Oxidation – “Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate, etc.”