What is formed when a primary alcohol is oxidised?

The correct answer is Option (1) → An aldehyde, which on further oxidation gives a carboxylic acid

• Oxidation of an alcohol forms a carbon–oxygen double bond by removing H from the O–H bond and from the carbon carrying the –OH group. A primary alcohol, \( \ce{RCH2OH} \), has two H atoms on that carbon, so it is oxidised in two stages:

      \( \displaystyle \ce{RCH2OH} \xrightarrow{[\mathrm{O}]} \ce{RCHO + H2O} \)

      \( \displaystyle \ce{RCHO} \xrightarrow{[\mathrm{O}]} \ce{RCOOH} \)

• So the first product is an aldehyde, which is oxidised further to a carboxylic acid. A mild reagent such as PCC stops at the aldehyde; acidified \( \ce{KMnO4} \) goes on to the acid.

Why the other options are incorrect

Option (2) A ketone, which is not oxidised further under mild conditions — Incorrect. Ketones come from the oxidation of secondary alcohols, not primary alcohols.

Option (3) An alkene, by the loss of a water molecule — Incorrect. Loss of water gives an alkene in dehydration (heating with an acid), which is not an oxidation.

Option (4) A ketone, which on further oxidation gives a carboxylic acid — Incorrect. A primary alcohol gives an aldehyde, not a ketone; ketones are oxidised only under vigorous conditions, with breaking of carbon–carbon bonds.

NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “4. Oxidation: Oxidation of alcohols involves the formation of a carbon-oxygen double bond with cleavage of an O-H and C-H bonds. … Depending on the oxidising agent used, a primary alcohol is oxidised to an aldehyde which in turn is oxidised to a carboxylic acid.”