Answer the question on the basis of passage given below:
Strong oxidizing agents, oxidize toluene and its derivatives to benzoic acid. However, it is possible to stop the oxidation at the aldehyde stage with suitable reagents that convert methyl group to an intermediate that is difficult to oxidize further.
Reagents which do not have any action on ketone are:
A. Schiff's reagent
B. \(KMnO_4\)
C. Tollens' reagent
D. Fehling's solution
E. \(LiAlH_4\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A, C and D only
Ketones have no hydrogen on the carbonyl carbon, so they are not oxidised by mild oxidising agents; tests that work only through such easy oxidation (or through the aldehyde group) give no result with ketones.
(A) Schiff’s reagent — No action. It gives a pink colour with aldehydes only.
(B) KMnO4 — Acts. Ketones are oxidised under vigorous conditions, with cleavage of a C–C bond to a mixture of carboxylic acids.
(C) Tollens’ reagent — No action. Only aldehydes give the silver mirror:
\( \ce{RCHO + 2[Ag(NH3)2]+ + 3OH- -> RCOO- + 2Ag + 2H2O + 4NH3} \)
(D) Fehling’s solution — No action. Only aliphatic aldehydes give the red-brown precipitate:
\( \ce{RCHO + 2Cu^2+ + 5OH- -> RCOO- + Cu2O + 3H2O} \)
(E) LiAlH4 — Acts. It reduces ketones to secondary alcohols:
\( \ce{R2C=O} \xrightarrow[\text{(ii) }\ce{H3O+}]{\text{(i) }\ce{LiAlH4}} \ce{R2CHOH} \)
NCERT (2026-27 edition):
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. Even mild oxidising agents, mainly Tollens’ reagent and Fehlings’ reagent also oxidise aldehydes. … Ketones are generally oxidised under vigorous conditions, i.e., strong oxidising agents and at elevated temperatures. Their oxidation involves carbon-carbon bond cleavage to afford a mixture of carboxylic acids having lesser number of carbon atoms than the parent ketone.”