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Aldehydes, Ketones and Carboxylic Acids appeared 46 times across 3 years — 5.3% of Chemistry. This question is from Reactions of Carbonyl Compounds.

Year 2026 2025 2024 Total
Questions 14 21 11 46
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The final product A formed in the following multistep reaction sequence is
Reactions of Carbonyl Compounds diagram for Q77 - JEE Main 2024 Morning
A reaction sequence starting with styrene undergoing Markovnikov hydration, followed by oxidation with CrO3, and finally reduction with hydrazine and KOH.

Solution & Explanation

Core Logic

The reaction sequence proceeds in three distinct steps from the starting material, styrene (Ph-CH=CH₂).

Step 1: Acid-catalyzed Hydration Styrene reacts with H₂O, H^+ to undergo electrophilic addition. Protonation yields the more stable secondary benzylic carbocation. Attack by water followed by deprotonation gives 1-phenylethanol (Ph-CH(OH)-CH₃).

Step 2: Oxidation 1-phenylethanol is a secondary alcohol. Treatment with chromium trioxide (CrO₃, Jones reagent condition) oxidizes the secondary alcohol to a ketone. This yields acetophenone (Ph-CO-CH₃).

Step 3: Wolff-Kishner Reduction Acetophenone is treated with hydrazine (NH₂-NH₂) and a strong base (KOH) under heating. This is the classic Wolff-Kishner reduction, which completely reduces the carbonyl group (C=O) to a methylene group (-CH₂-). The final product is ethylbenzene (Ph-CH₂-CH₃).

Step 1: Overall Reaction Pathway

Reactions of Carbonyl Compounds diagram for Q77 - JEE Main 2024 Morning
A reaction sequence starting with styrene undergoing Markovnikov hydration, followed by oxidation with CrO3, and finally reduction with hydrazine and KOH.

The final product A is ethylbenzene.

Chapter Mix

Class 11 Chemistry: Hydrocarbons Class 12 Chemistry: Alcohols Phenols and Ethers Class 12 Chemistry: 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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