The total number of compounds from below when treated with hot KMnO4 giving benzoic acid is:

Year	2026	2025	2024	Total
Questions	14	21	11	46

Year

2026

2025

2024

Total

Questions

Solution & Explanation

Related Formula

Side-chain oxidation criteria:

Ar-CH(R)₂ hot KMnO₄ Ar-COOH

Requires the presence of at least one benzylic hydrogen atom on the aromatic side chain structure.

Core Logic

Alkyl side chains on a benzene ring are oxidized entirely down to a carboxylic acid group (benzoic acid) by strong oxidizing agents like hot alkaline $KMnO₄$ $KMnO_4$, provided the benzylic carbon contains at least one hydrogen atom.

Evaluating the structures from the diagram:

Toluene (contains $3$ $3$ benzylic H) $arrow$ $\rightarrow$ Yields benzoic acid
Ethylbenzene (contains $2$ $2$ benzylic H) $arrow$ $\rightarrow$ Yields benzoic acid
Isopropylbenzene / Cumene (contains $1$ $1$ benzylic H) $arrow$ $\rightarrow$ Yields benzoic acid
tert-Butylbenzene (contains $0$ $0$ benzylic H) $arrow$ $\rightarrow$ Resists oxidation
Isobutylbenzene (contains $2$ $2$ benzylic H) $arrow$ $\rightarrow$ Yields benzoic acid
2-Phenylpropan-2-ol (contains $0$ $0$ benzylic H, tertiary alcohol center) $arrow$ $\rightarrow$ Resists oxidation
n-Propylbenzene (contains $2$ $2$ benzylic H) $arrow$ $\rightarrow$ Yields benzoic acid
The last biphenyl derivative undergoes complex disruption or ring cleavages and does not cleanly yield simple benzoic acid under standard monocyclic oxidation definitions.

Step 1: Counting Valid Targets

The compounds that undergo oxidation to form benzoic acid are toluene, ethylbenzene, isopropylbenzene, isobutylbenzene, and n-propylbenzene.

Total count = 5 compounds.

Structural evaluation summary for side-chain oxidation targets — The image lists seven distinct aromatic side-chain hydrocarbon structures to assess for side-chain oxidation.

Pattern Recognition

Look immediately at the benzylic carbon (the carbon bonded directly to the ring). If it is a quaternary center (like in tert-butylbenzene) or lacks a hydrogen atom entirely, mark it as unreactive to hot $KMnO₄$ $KMnO_4$ side-chain oxidation.

Chapter Mix

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 $-OH$ $-OH$ group (acidic) and an aldehyde group (electrophilic).

When one equivalent of Grignard reagent ( $PhMgBr$ $PhMgBr$) 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 $-OH$ $-OH$ reacts with $PhMgBr$ $PhMgBr$:

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

Upon workup with $aq. NH₄Cl$ $aq. NH_4Cl$, 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).

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

The total number of compounds from below when treated with hot $KMnO₄$ $KMnO_{4}$ giving benzoic acid is:
The image lists seven distinct aromatic side-chain hydrocarbon structures to assess for side-chain oxidation.