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

Year 2026 2025 2024 Total
Questions 14 21 11 46
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Identify A and B in the following sequence of reaction
Preparation of Aldehydes diagram for Q69 - JEE Main 2024 Morning
The image shows a reaction scheme starting with toluene reacting with Cl2/hv to give A, followed by H2O at 373 K to give B.

Solution & Explanation

Core Logic

Step 1: Free radical side-chain halogenation of toluene. Toluene reacts with Cl₂ in the presence of light (hν) to undergo substitution on the methyl group. Under typical conditions intended to yield an aldehyde later, di-chlorination occurs forming benzal chloride (A).

Step 2: Hydrolysis. Benzal chloride upon hydrolysis with H₂O at 373 K yields a gem-diol intermediate which is unstable and loses water to form Benzaldehyde (B).

C₆H₅CH₃ Cl₂ / hν C₆H₅CHCl₂ H₂O, 373K C₆H₅CHO
Step 1: Identify Structures

(A) = Benzal chloride (C₆H₅CHCl₂) (B) = Benzaldehyde (C₆H₅CHO)

Preparation of Aldehydes diagram for Q69 - JEE Main 2024 Morning
The image shows a reaction scheme starting with toluene reacting with Cl2/hv to give A, followed by H2O at 373 K to give B.

Pattern Recognition

Toluene Cl₂, hν targets the side chain. If the next step is hydrolysis to an aldehyde, you must have stopped at the gem-dihalide stage (CHCl₂).

Chapter Mix

Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids Class 11 Chemistry: Hydrocarbons

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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JEE Physics: Waves (+15.5%) | Electrostatics: Concentric Shells (-29.7%) | Modern Physics: Photoelectric Clones (+34.2%) | Mathematics: Definite Integrals (+18.1%) | Chemistry: Coordination Splitting (-11.4%) | JEE Physics: Waves (+15.5%) | Electrostatics: Concentric Shells (-29.7%) | Modern Physics: Photoelectric Clones (+34.2%) | Mathematics: Definite Integrals (+18.1%) | Chemistry: Coordination Splitting (-11.4%)