The aldehyde group is converted into carboxylic acid in the presence of
(A) aqueous copper sulfate and alkaline sodium potassium tartrate.
(B) Freshly prepared ammoniacal silver nitrate solution.
(C) Lithium aluminium hydride.
(D) H2 and Palladium on barium sulphate.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) and (B) only
Converting an aldehyde group (–CHO) into a carboxylic acid group (–COOH) is an oxidation. Aldehydes are easily oxidised, even by mild oxidising agents, because the carbonyl carbon carries a hydrogen atom. Reducing agents move the other way and give alcohols or stop at aldehydes.
(A) aqueous copper sulfate and alkaline sodium potassium tartrate. — Correct. This is Fehling’s reagent. The aldehyde is oxidised to the carboxylate ion and a red-brown precipitate of \( \ce{Cu2O} \) forms.
\( \displaystyle \ce{RCHO + 2Cu^2+ + 5OH^- -> RCOO^- + Cu2O v + 3H2O} \)
(B) Freshly prepared ammoniacal silver nitrate solution. — Correct. This is Tollens’ reagent. The aldehyde is oxidised to the carboxylate ion and silver is deposited as a bright silver mirror.
\( \displaystyle \ce{RCHO + 2[Ag(NH3)2]^+ + 3OH^- -> RCOO^- + 2Ag + 2H2O + 4NH3} \)
(C) Lithium aluminium hydride. — Incorrect. \( \ce{LiAlH4} \) is a reducing agent. It reduces an aldehyde to a primary alcohol.
\( \displaystyle \ce{RCHO} \xrightarrow[\text{(ii) }\ce{H2O}]{\text{(i) }\ce{LiAlH4}} \ce{RCH2OH} \)
(D) H2 and Palladium on barium sulphate. — Incorrect. This is the Rosenmund catalyst, which reduces acyl chlorides to aldehydes. Hydrogen cannot oxidise an aldehyde.
\( \displaystyle \ce{RCOCl + H2} \xrightarrow{\ce{Pd-BaSO4}} \ce{RCHO + HCl} \)
In both tests the acid is present as its anion because the reagent is alkaline; acidifying the mixture gives the free carboxylic acid.
NCERT (2026-27 edition):
Oxidation – …
Reduction – “… (i) Reduction to alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4) as well as by catalytic hydrogenation (Unit 7, Class XII).”
Preparation of Aldehydes – “… Acyl chloride (acid chloride) is hydrogenated over catalyst, palladium on barium sulphate. This reaction is called Rosenmund reduction.”