A molecule ("P") on treatment with acid undergoes rearrangement and gives ("Q") ("Q") on ozonolysis followed by reflux under alkaline condition gives ("R"). The structure of ("R") is given below:
The image displays the chemical structure of product R obtained from rearrangement and subsequent reaction steps.
The structure of (P)$(\mathrm{P})$ is
A.
The image displays the chemical structure of product R obtained from rearrangement and subsequent reaction steps.
B.
The image displays the chemical structure of product R obtained from rearrangement and subsequent reaction steps.
C.
The image displays the chemical structure of product R obtained from rearrangement and subsequent reaction steps.
D.
The image displays the chemical structure of product R obtained from rearrangement and subsequent reaction steps.
Solution & Explanation
Core Logic
The reaction sequence indicates that molecule "P" undergoes an acid-catalyzed rearrangement to produce alkene/alcohol intermediate "Q". Subsequent ozonolysis breaks down the double bond system, and alkaline reflux sets up an intramolecular aldol condensation sequence to form the cyclic ketone system "R". Following the detailed ring contraction/expansion step templates outlined below:
The image displays the chemical structure of product R obtained from rearrangement and subsequent reaction steps.
The image displays the chemical structure of product R obtained from rearrangement and subsequent reaction steps.
The image displays the chemical structure of product R obtained from rearrangement and subsequent reaction steps.
The image displays the chemical structure of product R obtained from rearrangement and subsequent reaction steps.
The image displays the chemical structure of product R obtained from rearrangement and subsequent reaction steps.
The image displays the chemical structure of product R obtained from rearrangement and subsequent reaction steps.
The image displays the chemical structure of product R obtained from rearrangement and subsequent reaction steps.
The image displays the chemical structure of product R obtained from rearrangement and subsequent reaction steps.
Pattern Recognition
Sees: Acidic rearrangement arrow$\rightarrow$ ozonolysis arrow$\rightarrow$ intramolecular aldol condensation.
Shortcut: Work backwards from the dicarbonyl fragments formed after opening the final product ring system.
Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
More Aldehydes, Ketones and Carboxylic Acids Previous-Year Questions — Page 4
Qjee_main_2025_02_april_morningReactions of Phenolic Benzaldehydes
Given below are two statements :
Statement (I): Vanillin
Reactions of Phenolic Benzaldehydes will react with NaOH and also with Tollen's reagent.
Statement (II) : Vanillin
Reactions of Phenolic Benzaldehydes will undergo self aldol condensation very easily.
In the light of the above statements, choose the most appropriate answer from the options given below:
A.(1) Statement I is incorrect but Statement II is correct$(1)\ \text{Statement I is incorrect but Statement II is correct}$
B.(2) Statement I is correct but Statement II is incorrect$(2)\ \text{Statement I is correct but Statement II is incorrect}$
C.(3) Both Statement I and Statement II are incorrect$(3)\ \text{Both Statement I and Statement II are incorrect}$
D.(4) Both Statement I and Statement II are correct$(4)\ \text{Both Statement I and Statement II are correct}$
Solution
Related Formula
Phenolic protons react with standard strong bases:
Aldol condensation structural requirement: Requires presence of acidic α$\alpha$-hydrogen atoms connected to carbonyl centers.
Core Logic
Let's analyze functional groups within the Vanillin molecular framework:
Vanillin contains a phenolic hydroxyl group, an aromatic ether, and a formyl functional group (benzaldehyde derivative).
Statement I: The presence of the phenolic -OH$-\mathrm{OH}$ group allows acid-base reaction with NaOH$\mathrm{NaOH}$ directly Vanillin structural functional group verification for Q36. The aldehyde center readily reduces Tollen's reagent to produce a silver mirror. (Statement I is accurate).
Statement II: Vanillin lacks any alpha-hydrogens adjacent to its carbonyl carbon, preventing it from undergoing self-aldol condensation. (Statement II is false).
Pattern Recognition
Benzaldehyde and its substituted derivatives (like vanillin or benzaldehyde itself) never undergo self-aldol condensation because they lack α$\alpha$-carbons with abstractable protons. Instead, they typically perform Cannizzaro transformations when exposed to highly concentrated alkaline media.
Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
The major product (P) in the following reaction is :
Benzil-Benzilic Acid Rearrangement
A.
B.
C.
D.
Solution
Related Formula
Intramolecular Cannizzaro-type reaction or Benzil-Benzilic acid rearrangement involves nucleophilic attack of hydroxide at a carbonyl group, followed by hydride transfer to the adjacent carbonyl carbon.
Core Logic
Let's analyze the starting compound, phenylglyoxal:
Ph-CO-CHO$$\mathrm{Ph-CO-CHO}$$
The aldehyde carbon (-CHO$-CHO$) is much more electrophilic than the ketone carbon (-CO-$-CO-$) due to less steric hindrance and absence of phenyl group electron donation.
Hydroxide ion (OH^-$\mathrm{OH}^-$) selectively attacks the aldehyde carbonyl carbon, forming a tetrahedral intermediate.
Benzil-Benzilic Acid Rearrangement
Step 1: Hydride Transfer Mechanism
The tetrahedral intermediate collapses, prompting an intramolecular hydride (H^-$H^-$) transfer to the adjacent ketone carbonyl carbon:
In asymmetrical 1,2-dicarbonyl systems with an aldehyde and a ketone, nucleophilic addition occurs preferentially at the more reactive aldehyde carbon. The hydrogen is then transferred as a hydride to the ketone carbon, yielding an α$\alpha$-hydroxy carboxylate salt.
Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
Qjee_main_2025_08_april_eveningAldol Condensation
When the dicarbonyl compound shown below undergoes an base-catalyzed intramolecular aldol condensation reaction, the major structural product formed is:
{{Q_IMG1}}
The image features a symmetrical open-chain diketone containing branching elements, poised for intramolecular cyclization.
A.
B.
C.
D.
Solution
Core Logic
Intramolecular aldol condensations are governed heavily by thermodynamic stability, favoring the formation of 5-membered or 6-membered rings over strained 3-, 4-, or large 7-membered options.
Enolate Generation: Base abstracts an α$\alpha$-proton to form a nucleophilic carbanion enolate.
Attack Vector Selection: Deprotonation at the outer methyl site enables a ring-closing attack on the distant carbonyl carbon, perfectly designing a highly stable cyclopentene ring skeleton.
Dehydration: Heating drives the loss of a water molecule (-H₂O$-\text{H}_2\text{O}$), introducing an α,β$\alpha,\beta$-unsaturated carbonyl arrangement that provides stabilization via conjugated resonance. The image features a symmetrical open-chain diketone containing branching elements, poised for intramolecular cyclization.
Pattern Recognition
Count the intervening carbon chain carefully. Intramolecular aldol pathways will always selectively build 5- or 6-membered rings due to favorable ring strain kinetics. Eliminating choices based on incorrect ring sizes isolates Option (1) instantly.
Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
Q30jee_main_2025_08_april_eveningReactions of Cycloalkenes and Alkynes
Identify the major product 'P' in the given reaction sequence starting from 1,2-dibromocyclooctane:
1,2-dibromocyclooctane (i) KOH (alc.) (ii) NaNH₂ (iii) Hg²⁺/H^+ (iv) Zn-Hg/HCl 'P (Major product)'$$\text{1,2-dibromocyclooctane} \xrightarrow{\text{(i) KOH (alc.)}} \xrightarrow{\text{(ii) NaNH}_2} \xrightarrow{\text{(iii) Hg}^{2+}/H^+} \xrightarrow{\text{(iv) Zn-Hg/HCl}} \text{'P (Major product)'}$$
Let us systematically follow the transformation steps:
First Elimination: 1,2-dibromocyclooctane reacts with alcoholic KOH$\text{KOH}$ to remove one molecule of HBr$\text{HBr}$, resulting in a bromocyclooctene intermediate.
Second Elimination: Treatment with the stronger base NaNH₂$\text{NaNH}_2$ removes the second molecule of HBr$\text{HBr}$, forming an alkyne inside the 8-membered ring: cyclooctyne.
Kucherov Reaction: Hydration of cyclooctyne using Hg²⁺/H^+$\text{Hg}^{2+}/H^+$ creates an enol intermediate that undergoes tautomerization to form a stable ketone: cyclooctanone.
Clemmensen Reduction: Subjecting cyclooctanone to zinc amalgam and hydrochloric acid (Zn-Hg/HCl$\text{Zn-Hg/HCl}$) completely reduces the carbonyl group (>C=O$>C=O$) to a methylene group (-CH₂-$-\text{CH}_2-$), finishing with cyclooctane. Complete mechanistic sequence mapping for cyclooctane product formation
Pattern Recognition
A vicinal dihalide treated with sequential strong bases creates an alkyne path. Alkyne hydration creates a ketone body. Finally, Clemmensen reduction takes the ketone down to a simple hydrocarbon skeleton. Recognizing this terminal reduction loop establishes cyclooctane as the undisputed answer.
Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
Class 11 Chemistry: Hydrocarbons
In the Claisen-Schmidt reaction to prepare dibenzalacetone from 5.3 g$5.3\text{ g}$ of benzaldehyde, a total of 3.51 g$3.51\text{ g}$ of product was obtained.
The percentage yield in this reaction was __%.
Numerical Answer.Answer: 60 to 60
Solution
Core Logic
The balanced chemical equation for the synthesis of dibenzalacetone is:
Molar mass of benzaldehyde (PhCHO) = 106 g/mol$$\text{Molar mass of benzaldehyde } (PhCHO) = 106\text{ g/mol}$$Moles of benzaldehyde used = (5.3)/(106) = 0.05 mol = (1)/(20) mol$$\text{Moles of benzaldehyde used} = \frac{5.3}{106} = 0.05\text{ mol} = \frac{1}{20}\text{ mol}$$
Claisen-Schmidt Condensation diagram for Q48 - JEE Main 2025 Evening
According to the reaction stoichiometry, 2 moles of PhCHO$2\text{ moles of } PhCHO$ yield 1 mole of dibenzalacetone$1\text{ mole of dibenzalacetone}$.
Theoretical moles of product = (0.05)/(2) = 0.025 mol$$\text{Theoretical moles of product} = \frac{0.05}{2} = 0.025\text{ mol}$$
Step 1: Yield Evaluation
$Molar mass of dibenzalacetone (C17H14O) = 234 g/mol$$$\text{Molar mass of dibenzalacetone } (C{17}H{14}O) = 234\text{ g/mol}$$Theoretical mass = 0.025 × 234 = 5.85 g$$\text{Theoretical mass} = 0.025 \times 234 = 5.85\text{ g}$$Percentage yield = Actual massTheoretical mass × 100 = (3.51)/(5.85) × 100 = 60%$$\text{Percentage yield} = \frac{\text{Actual mass}}{\text{Theoretical mass}} \times 100 = \frac{3.51}{5.85} \times 100 = 60%$$
Pattern Recognition
Always remember the stoichiometric ratio: It takes 2 moles of benzaldehyde to condense with 1 mole of acetone to form the symmetrical dibenzalacetone product.
Chapter Mix
Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids
Practice past-year questions one chapter at a time. Pick an exam → subject → chapter and get every PYQ for that topic — pulled together from all past papers — with the chapter's key formulas alongside.
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