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Aldehydes, Ketones and Carboxylic Acids appeared 46 times across 3 years — 5.3% of Chemistry. This question is from Tollen's Reagent Test.

Year 2026 2025 2024 Total
Questions 14 21 11 46
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A student is given one compound among the following compounds that gives positive test with Tollen's reagent.
Four organic structures A, B, C, D
Four cyclic ether derivatives labeled A, B, C, and D are shown.
The compound is :

Solution & Explanation

Core Logic

Tollen's reagent ([Ag(NH₃)₂]^+ in basic medium) is a mild oxidizing agent that oxidizes aldehydes (and α-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 groups on the same carbon). (B) is a cyclic acetal (a ring oxygen and an -OCH₃ group on the same carbon). (C) is a cyclic hemiacetal (a ring oxygen and an -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:

Ring opening of hemiacetal to aldehyde
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

Reference Study Guides

Aldehydes Ketones And Carboxylic Acids

More Aldehydes Ketones and Carboxylic Acids Previous-Year Questions — Page 10

Q jee_main_2024_31_jan_morning Reactions with Grignard Reagent
The product of the following reaction is P.
Reactions with Grignard Reagent diagram for Q84 - JEE Main 2024 Morning
The image shows p-hydroxybenzaldehyde reacting with one equivalent of PhMgBr followed by aqueous ammonium chloride.
The number of hydroxyl groups present in the product P is
Numerical Answer. Answer: 0 to 0

Solution

Core Logic

The given reactant is p-hydroxybenzaldehyde, which contains both a phenolic -OH group (acidic) and an aldehyde group (electrophilic).

When one equivalent of Grignard reagent (PhMgBr) is added, it behaves primarily as a strong base due to the presence of an acidic proton. Acid-base reactions are extremely fast compared to nucleophilic additions.

The acidic phenolic -OH reacts with PhMgBr:

PhMgBr + HO-C₆H₄-CHO arrow Ph-H (Benzene) + BrMg-O-C₆H₄-CHO

Upon workup with aq. NH₄Cl, the phenoxide ion simply regenerates the starting p-hydroxybenzaldehyde. However, the question asks for the number of hydroxyl groups present in the formed product (Benzene).

Reactions with Grignard Reagent diagram for Q84 - JEE Main 2024 Morning
The image shows p-hydroxybenzaldehyde reacting with one equivalent of PhMgBr followed by aqueous ammonium chloride.

The distinct product formed in the reaction is Benzene. Benzene has 0 hydroxyl groups.

Pattern Recognition

Whenever a Grignard reagent encounters a molecule with an acidic hydrogen (alcohol, phenol, amine, alkyne), it will invariably act as a base first. If only 1 equivalent is used, nucleophilic addition to carbonyls will NOT happen.

Chapter Mix

Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids Class 12 Chemistry: Alcohols, Phenols and Ethers

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