Glucose reacts with Tollen's reagent (ammoniacal silver nitrate) due to the presence of
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Aldehyde
Tollens’ reagent (ammoniacal silver nitrate) is a mild oxidising agent. It oxidises aldehydes to carboxylate ions and is itself reduced to metallic silver, which deposits as a silver mirror; simple ketones do not react with it:
\( \displaystyle \ce{RCHO + 2[Ag(NH3)2]+ + 3OH- -> RCOO- + 2Ag v + 2H2O + 4NH3} \)
Glucose, \( \ce{CHO-(CHOH)4-CH2OH} \), is an aldohexose: its open-chain form has an aldehyde group at C-1. This group is oxidised to the carboxylate of gluconic acid, so glucose gives a positive Tollens’ test.
Why the other options are incorrect
Option (1) Ester — Incorrect. Glucose has no ester group, and an ester is not oxidised by Tollens’ reagent.
Option (3) Ketone — Incorrect. Glucose has no keto group, and a simple ketone does not reduce Tollens’ reagent; this test is used to tell aldehydes from ketones.
Option (4) Alcoholic \(AgNO_3\) — Incorrect. This is a reagent, not a group present in glucose; Tollens’ reagent is ammoniacal, not alcoholic, silver nitrate.
NCERT (2026-27 edition):
Oxidation – “… (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.”