Aldehydes, Ketones and Carboxylic Acids Previous Year Questions — JEE Main Chemistry
46 past-year Aldehydes, Ketones and Carboxylic Acids questions from JEE Main (Chemistry) — page 4.
Q40 (2025)
Which of the following arrangements with respect to their reactivity in nucleophilic addition reaction is correct?
- benzaldehyde $<$ acetophenone $<$ p-nitrobenzaldehyde $<$ p-tolualdehyde
- acetophenone $<$ benzaldehyde $<$ p-tolualdehyde $<$ p-nitrobenzaldehyde
- acetophenone $<$ p-tolualdehyde $<$ benzaldehyde $<$ p-nitrobenzaldehyde
- p-nitrobenzaldehyde $<$ benzaldehyde $<$ p-tolualdehyde $<$ acetophenone
### Core Logic
Reactivity in nucleophilic addition reactions is governed by a combination of **steric hindrance** and **electronic effects** around the electrophilic carbonyl carbon:
1. Ketones are significantly less reactive than aldehydes due to the bulkiness and electron-donating inductive effect ($+I$) of their two alkyl/aryl groups. Thus, **acetophenone** has the lowest reactivity.
2. For substituted benzaldehydes, electron-withdrawing groups heighten the partial positive charge on the carbonyl carbon, accelerating nucleophilic attack. Conversely, electron-donating groups suppress reactivity.
Symmetry breakdown structures are shown below:
- Acetophenone: {{SOL_IMG1}}
- p-tolualdehyde: {{SOL_IMG2}}
- Benzaldehyde: {{SOL_IMG3}}
- p-nitrobenzaldehyde: {{SOL_IMG4}}
- Methoxy/methyl donors decrease reactivity: $\text{p-tolualdehyde} < \text{benzaldehyde}$
- Nitro group ($-\mathrm{NO}_2$) acts as a strong electron-withdrawing agent via both $-M$ and $-I$ pathways, maximizing the electrophilic nature of the carbonyl site. Therefore, **p-nitrobenzaldehyde** is the most reactive.
Thus, the correct order of reactivity is:
$$\text{acetophenone} < \text{p-tolualdehyde} < \text{benzaldehyde} < \text{p-nitrobenzaldehyde}$$
### Pattern Recognition
Aldehydes naturally exhibit higher reactivity than ketones. Electron-withdrawing groups ($-\mathrm{NO}_2$) accelerate addition pathways, whereas electron-donating groups ($-\mathrm{CH}_3$) impede them.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
Q45 (2025)
The product (A) formed in the following reaction sequence is :
$$\mathrm {C H} _ {3} - \mathrm {C} \equiv \mathrm {C H} \xrightarrow [ (\mathrm {i i}) \mathrm {H} _ {2} / \mathrm {N i} ]{(\mathrm {i}) \mathrm {H} g ^ {2 +}, \mathrm {H} _ {2} \mathrm {S O} _ {4}} (A)$$
- $\mathrm{CH}_3-\mathrm{C}(\mathrm{NH}_2)(\mathrm{CH}_3)-\mathrm{CH}_2-\mathrm{OH}$
- $\mathrm{CH}_3-\mathrm{C}(\mathrm{OH})(\mathrm{CH}_3)-\mathrm{CH}_2-\mathrm{NH}_2$
- $\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}_2-\mathrm{OH}$
- $\mathrm{CH}_3-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}(\mathrm{CH}_3)-\mathrm{NH}_2$
### Core Logic
Breaking down the multi-step reaction path sequence:
1. Hydration of propyne using Kucherov's trigger condition ($Hg^{2+}, H_2SO_4$) adds water across the triple bond via Markovnikov's rule. The intermediate enol undergoes tautomerization to yield **acetone** ($CH_3-CO-CH_3$).
2. Reacting acetone with $HCN$ drives nucleophilic addition at the carbonyl carbon, forming a **cyanohydrin** intermediate: $CH_3-C(OH)(CH_3)-CN$.
3. Introducing a reducing agent ($H_2/Ni$) selectively converts the nitrile group ($-CN$) into a primary amine side chain ($-CH_2-NH_2$).
The final synthesized structure is: $\mathrm{CH}_3-\mathrm{C}(\mathrm{OH})(\mathrm{CH}_3)-\mathrm{CH}_2-\mathrm{NH}_2$. {{SOL_IMG1}}
### Pattern Recognition
Alkyne hydration produces a ketone carbonyl system. Cyanohydrin synthesis introduces a carbon coordinate, which subsequently reduces to a primary aliphatic amine functional group.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
Q48 (2025)
Xg of benzoic acid on reaction with aq. $\mathrm{NaHCO_3}$ release $\mathrm{CO}_{2}$ that occupied 11.2 L volume at STP. X is ______ g.
### Related Formula
$$\text{Moles of gas at STP} = \frac{V\text{ in Liters}}{22.4 \text{ L/mol}}$$
### Core Logic
The balanced acid-base reaction equation is:
$$C_{6}H_{5}COOH + NaHCO_{3} \rightarrow C_{6}H_{5}COO^{-}Na^{+} + H_{2}O + CO_{2}$$
This stoichiometry shows a $1:1$ molar ratio between benzoic acid and the released carbon dioxide gas.
Calculate the total moles of evolved $CO_2$ gas at standard conditions:
$$\text{moles of } CO_2 = \frac{11.2\text{ L}}{22.4\text{ L mol}^{-1}} = 0.5\text{ moles}$$
Thus, the reaction consumed exactly $0.5\text{ moles}$ of benzoic acid ($C_6H_5COOH$, molecular weight = $122\text{ g/mol}$):
$$\text{mass consumed (X)} = 0.5 \times 122 = 61\text{ grams}$$
### Pattern Recognition
Carboxylic acids react with sodium bicarbonate in a straightforward $1:1$ molar ratio, releasing exactly 1 mole of carbon dioxide gas per mole of acid group.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
Q37 (2025)
The total number of compounds from below when treated with hot $KMnO_{4}$ giving benzoic acid is:
{{IMG1}}
- 3
- 4
- 6
- 5
### Related Formula
Side-chain oxidation criteria:
$$\text{Ar-CH(R)}_2 \xrightarrow{\text{hot } KMnO_4} \text{Ar-COOH}$$
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 $KMnO_4$, provided the benzylic carbon contains at least one hydrogen atom.
Evaluating the structures from the diagram:
1. Toluene (contains $3$ benzylic H) $\rightarrow$ **Yields benzoic acid**
2. Ethylbenzene (contains $2$ benzylic H) $\rightarrow$ **Yields benzoic acid**
3. Isopropylbenzene / Cumene (contains $1$ benzylic H) $\rightarrow$ **Yields benzoic acid**
4. tert-Butylbenzene (contains $0$ benzylic H) $\rightarrow$ **Resists oxidation**
5. Isobutylbenzene (contains $2$ benzylic H) $\rightarrow$ **Yields benzoic acid**
6. 2-Phenylpropan-2-ol (contains $0$ benzylic H, tertiary alcohol center) $\rightarrow$ **Resists oxidation**
7. n-Propylbenzene (contains $2$ benzylic H) $\rightarrow$ **Yields benzoic acid**
8. 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.
{{SOL_IMG1}}
### 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 $KMnO_4$ side-chain oxidation.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
Q1316 (2025)
The product (P) formed in the following reaction is:
{{IMG1}}
### Related Formula
$$\mathrm{R-CO-R'} \xrightarrow[\mathrm{HCl}]{\mathrm{Zn-Hg}} \mathrm{R-CH_2-R'}$$
### Core Logic
The provided reaction relies on classic Clemmensen Reduction reagent configurations ($\mathrm{Zn-Hg} / \mathrm{HCl}$). This reagent explicitly selective targets ketone/aldehyde carbonyl centers ($>\mathrm{C}=\mathrm{O}$) and converts them cleanly into methylene units ($-\mathrm{CH}_2-$). Any free aliphatic alcohol functionalities ($-\mathrm{OH}$) located on the ring are preserved under these reductive conditions. Thus, the ketone group on the ring is cleanly converted to $-\mathrm{CH}_2-$, yielding option (3):
{{SOL_IMG1}}
### Pattern Recognition
Clemmensen routes deoxygenate carbonyl clusters selectively without compromising adjacent standalone cyclic alcohol structural components.
### Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
