A student is given one compound among the following compounds that gives positive test with Tollen's reagent.
Four cyclic ether derivatives labeled A, B, C, and D are shown.
The compound is :
A.D$\text{D}$
B.A$\text{A}$
C.B$\text{B}$
D.C$\text{C}$
Solution & Explanation
Core Logic
Tollen's reagent ([Ag(NH₃)₂]^+$[Ag(NH_3)_2]^+$ in basic medium) is a mild oxidizing agent that oxidizes aldehydes (and α$\alpha$-hydroxy ketones) to carboxylate ions, giving a silver mirror.
Acetals are stable in basic medium and will not react with Tollen's reagent.
Hemiacetals, however, are in equilibrium with their open-chain aldehyde forms in aqueous basic solutions. Therefore, hemiacetals will give a positive Tollen's test.
Analyzing the options:
(A) is an acetal (two -OCH₃$-OCH_3$ groups on the same carbon).
(B) is a cyclic acetal (a ring oxygen and an -OCH₃$-OCH_3$ group on the same carbon).
(C) is a cyclic hemiacetal (a ring oxygen and an -OH$-OH$ group on the same carbon).
(D) is a di-ether (no acetal/hemiacetal linkage).
Step 1: Mechanism
Under basic conditions, the hemiacetal (C) undergoes ring opening:
Four cyclic ether derivatives labeled A, B, C, and D are shown.
The base deprotonates the hydroxyl group, kicking off the ring oxygen to form a free aldehyde. The resulting aldehyde then easily reacts with Tollen's reagent to produce the Silver mirror.
Step 2: Final Conclusion
Compound C is the only hemiacetal and thus gives a positive test.
Pattern Recognition
Always scan cyclic sugar-like structures for the anomeric carbon (a carbon bonded to two oxygens). If one is an -OH, it is a hemiacetal (reducing). If both are -OR, it is an acetal (non-reducing in base).
Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
Class 12 Chemistry: Biomolecules
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More Aldehydes Ketones and Carboxylic Acids Previous-Year Questions — Page 9
Qjee_main_2024_30_jan_morningPreparation of Aldehydes
This reduction reaction is known as:
The image shows the catalytic hydrogenation of benzoyl chloride to benzaldehyde in the presence of Pd-BaSO4.
The reaction depicts the partial reduction of an acid chloride (benzoyl chloride) to an aldehyde (benzaldehyde) using hydrogen gas in the presence of a poisoned palladium catalyst (Pd supported on BaSO₄$BaSO_4$).
The image shows the catalytic hydrogenation of benzoyl chloride to benzaldehyde in the presence of Pd-BaSO4.
This specific reaction is known as the Rosenmund reduction.
Pattern Recognition
Acid Chloride + H₂, Pd/BaSO₄$H_2, Pd/BaSO_4$arrow$\rightarrow$ Aldehyde is strictly the Rosenmund reduction. The BaSO₄$BaSO_4$ poisons the catalyst to prevent over-reduction to an alcohol.
Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
Pathway 1 (Upper): Toluene is treated with Reagent 'A' and acetic anhydride (CH₃CO)₂O$(CH_3CO)_2O$ to form an intermediate (benzylidene diacetate), which on hydrolysis gives benzaldehyde. The reagent used here is Chromic oxide (CrO₃$CrO_3$). Thus, A is CrO₃$CrO_3$.
Pathway 2 (Lower): Toluene is treated with Reagent 'B' in CS₂$CS_2$ to form a chromium complex intermediate, which on hydrolysis yields benzaldehyde. This is the Etard reaction, and the reagent used is Chromyl chloride (CrO₂Cl₂$CrO_2Cl_2$). Thus, B is CrO₂Cl₂$CrO_2Cl_2$.
The image shows the oxidation of toluene to benzaldehyde via two different pathways requiring reagents A and B.
Step 1: Selection
Therefore, A = CrO₃$CrO_3$ and B = CrO₂Cl₂$CrO_2Cl_2$.
Pattern Recognition
Etard reaction always uses Chromyl chloride (CrO₂Cl₂$CrO_2Cl_2$). Oxidation of toluene with acetic anhydride uses Chromic acid (CrO₃$CrO_3$). Both stop the oxidation at the aldehyde stage via intermediate formation.
Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
Ethanal (CH₃CHO$CH_3CHO$) reacts with semicarbazide (H₂N-NH-CO-NH₂$H_2N-NH-CO-NH_2$) via nucleophilic addition followed by elimination of water to form a semicarbazone.
Step 1: Product Analysis
The product is Ethanal semicarbazone: CH₃-CH=N-NH-CO-NH₂$CH_3-CH=N-NH-CO-NH_2$.
Counting the nitrogen atoms in this structure:
The imine nitrogen (=N-$=N-$)
The amine nitrogen (-NH-$-NH-$)
The amide nitrogen (-NH₂$-NH_2$)
Total = 3 Nitrogen atoms.
Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
Qjee_main_2024_31_jan_eveningPreparation of Aldehydes and Ketones
Identify the name reaction.
The image shows the conversion of benzene to benzaldehyde using CO, HCl, and Anhydrous AlCl3/CuCl.
The reaction of benzene with carbon monoxide (CO$CO$) and hydrogen chloride (HCl$HCl$) in the presence of anhydrous aluminium chloride (AlCl₃$AlCl_3$) and cuprous chloride (CuCl$CuCl$) to give benzaldehyde is known as the Gatterman-Koch reaction.
The image shows the conversion of benzene to benzaldehyde using CO, HCl, and Anhydrous AlCl3/CuCl.
Pattern Recognition
CO + HCl arrow$CO + HCl \rightarrow$ Formyl chloride intermediate (in situ) with Lewis acid arrow$\rightarrow$ formylation of benzene. This is definitively the Gatterman-Koch formylation.
Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
More Aldehydes Ketones and Carboxylic Acids Questions — jee_main_2026_24_january_morning
Practice past-year questions one chapter at a time. Pick an exam → subject → chapter and get every PYQ for that topic — pulled together from all past papers — with the chapter's key formulas alongside.
