What is the final product X in the following reaction?
\( \displaystyle \ce{CH3-CH2-CH2-CHO} \xrightarrow[\ce{H3O+}]{\text{Ammoniacal }\ce{AgNO3}} \text{X} \)
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → CH3CH2CH2COOH
Tollens’ reagent is ammoniacal silver nitrate, which contains the complex ion \( \ce{[Ag(NH3)2]^+} \). It is a mild oxidising agent. It oxidises aldehydes but not ketones, because an aldehyde has a hydrogen on its carbonyl carbon. The aldehyde group –CHO becomes a carboxylate ion –COO− (the solution is alkaline), and Ag+ is reduced to silver metal, which forms a silver mirror on the glass.
The given compound, \( \ce{CH3CH2CH2CHO} \), is butanal. With Tollens’ reagent it gives the butanoate ion:
\( \displaystyle \ce{CH3CH2CH2CHO + 2[Ag(NH3)2]^+ + 3OH^- -> CH3CH2CH2COO^- + 2Ag + 2H2O + 4NH3} \)
The acid work-up (\( \ce{H3O+} \)) then protonates the carboxylate ion and gives the free acid:
\( \displaystyle \ce{CH3CH2CH2COO^- + H3O^+ -> CH3CH2CH2COOH + H2O} \)
The carbon chain does not change; only the –CHO group becomes –COOH. So X is butanoic acid, \( \ce{CH3CH2CH2COOH} \).
Why the other options are incorrect
Option (1) CH3CH2CH2CH2NH2 — Incorrect. This is butan-1-amine. Tollens’ reagent only oxidises, and it supplies no nitrogen to the product. This amine is made by reducing a nitrogen compound, for example butanenitrile:
\( \displaystyle \ce{CH3CH2CH2CN} \xrightarrow[\text{(ii) }\ce{H2O}]{\text{(i) }\ce{LiAlH4}} \ce{CH3CH2CH2CH2NH2} \)
Option (3) CH3CH2CH2CH2NO2 — Incorrect. This is 1-nitrobutane. The silver nitrate in Tollens’ reagent is only a source of Ag+; it does not nitrate the aldehyde, and the –CHO group is not replaced by –NO2.
Option (4) CH3CH2CH2CH3 — Incorrect. This is butane, the product of reducing the C=O group to CH2 (Clemmensen reduction with Zn-Hg/HCl, or Wolff–Kishner reduction). Tollens’ reagent does the opposite: it oxidises the aldehyde.
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. …
(i) Tollens’ test: On warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent), a bright silver mirror is produced due to the formation of silver metal. The aldehydes are oxidised to corresponding carboxylate anion. The reaction occurs in alkaline medium.”