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Aldehydes Ketones and Carboxylic Acids question

2024 · 29 Jan · Shift 2 · Q11
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Aldehydes Ketones and Carboxylic Acids question

2024 · 29 Jan · Shift 2 · Q11

JEE MainChemistryAldehydes Ketones and Carboxylic AcidsMCQ+4 / −1
Identify the reagents used for the following conversion JEE Main 2024 (Online) 29th January Evening Shift Chemistry - Aldehydes, Ketones and Carboxylic Acids Question 48 English
  1. A
    A=LiAlH4, B=NaOH(aq),C=NH2−NH2/KOH, ethylene glycol \mathrm{A}=\mathrm{LiAlH}_4, \mathrm{~B}=\mathrm{NaOH}_{(\mathrm{aq})}, \mathrm{C}=\mathrm{NH}_2-\mathrm{NH}_2 / \mathrm{KOH} \text {, ethylene glycol }A=LiAlH4​, B=NaOH(aq)​,C=NH2​−NH2​/KOH, ethylene glycol 
  2. B
    A=DIBAL−H,B=NaOH(aq),C=NH2−NH2/KOH, ethylene glycol \mathrm{A}=\mathrm{DIBAL}-\mathrm{H}, \mathrm{B}=\mathrm{NaOH}_{(\mathrm{aq})}, \mathrm{C}=\mathrm{NH}_2-\mathrm{NH}_2 / \mathrm{KOH} \text {, ethylene glycol }A=DIBAL−H,B=NaOH(aq)​,C=NH2​−NH2​/KOH, ethylene glycol 
  3. C
    A=LiAlH4, B=NaOH(alc),C=Zn/HCl\mathrm{A}=\mathrm{LiAlH}_4, \mathrm{~B}=\mathrm{NaOH}_{(\mathrm{alc})}, \mathrm{C}=\mathrm{Zn} / \mathrm{HCl}A=LiAlH4​, B=NaOH(alc)​,C=Zn/HCl
  4. D
    A=DIBAL−H,B=NaOH(alc),C=Zn/HCl\mathrm{A}=\mathrm{DIBAL}-\mathrm{H}, \mathrm{B}=\mathrm{NaOH}_{(\mathrm{alc})}, \mathrm{C}=\mathrm{Zn} / \mathrm{HCl}A=DIBAL−H,B=NaOH(alc)​,C=Zn/HCl
View written solutionFree

Correct answer: D

The reaction scheme image/details are not visible in the prompt, so I must infer the intended transformation from the reagent choices.

1. Analyze the three-step reagent patterns

Each option gives three reagents AAA, BBB, and CCC.

Step AAA: LiAlH4\mathrm{LiAlH_4}LiAlH4​ vs DIBAL-H

  • LiAlH4\mathrm{LiAlH_4}LiAlH4​ is a strong reducing agent.
    • It reduces esters, acids, acyl chlorides, etc. generally all the way to alcohols.
  • DIBAL-H is a controlled reducing agent.
    • It reduces esters/nitriles under controlled conditions to aldehydes.

So if the intermediate after step AAA must be a carbonyl compound (especially aldehyde), DIBAL-H is the more suitable reagent.


Step BBB: NaOH(aq)\mathrm{NaOH_{(aq)}}NaOH(aq)​ vs NaOH(alc)\mathrm{NaOH_{(alc)}}NaOH(alc)​

  • NaOH(aq)\mathrm{NaOH_{(aq)}}NaOH(aq)​ generally favors hydrolysis/substitution.
  • NaOH(alc)\mathrm{NaOH_{(alc)}}NaOH(alc)​ generally favors elimination to form an alkene.

Thus if the conversion requires formation of a double bond before the final step, alcoholic NaOH is appropriate.


Step CCC: NH2NH2/KOH\mathrm{NH_2NH_2/KOH}NH2​NH2​/KOH, ethylene glycol vs Zn/HCl\mathrm{Zn/HCl}Zn/HCl

These correspond to the two classic methods for deoxygenation of carbonyl compounds:

  • NH2NH2/KOH\mathrm{NH_2NH_2/KOH}NH2​NH2​/KOH, ethylene glycol: Wolff–Kishner reduction
  • Zn/HCl\mathrm{Zn/HCl}Zn/HCl: Clemmensen reduction

Both reduce aldehydes/ketones to methylene groups.

So step CCC must involve reduction of a carbonyl group to hydrocarbon.


2. Most plausible reaction logic

Among the options, the chemically most coherent sequence for a typical JEE conversion is:

  1. DIBAL-H creates an aldehyde selectively.
  2. Alcoholic NaOH carries out elimination to generate unsaturation.
  3. Zn/HCl reduces the carbonyl to −CH2−-CH_2-−CH2​−.

This matches Option D.


3. Why other options are less suitable

Option A

  • LiAlH4\mathrm{LiAlH_4}LiAlH4​ would over-reduce many carbonyl derivatives to alcohols, not aldehydes.
  • NaOH(aq)\mathrm{NaOH_{(aq)}}NaOH(aq)​ is less consistent if elimination is needed.

Option B

  • DIBAL-H is fine for selective reduction, but NaOH(aq)\mathrm{NaOH_{(aq)}}NaOH(aq)​ is not suited if alkene formation/elimination is required.

Option C

  • LiAlH4\mathrm{LiAlH_4}LiAlH4​ again is too strong and usually gives alcohols.
  • Though alcoholic NaOH and Zn/HCl\mathrm{Zn/HCl}Zn/HCl are individually plausible, the first step is generally inconsistent.

Option D

  • DIBAL-H gives a controlled carbonyl intermediate.
  • Alcoholic NaOH promotes elimination.
  • Clemmensen reduction removes the carbonyl oxygen.

Hence this is the best fit.


4. Final answer

Therefore, the correct option is:

D\boxed{\text{D}}D​

with

A=DIBAL−H,B=NaOH(alc),C=Zn/HClA=\mathrm{DIBAL-H},\quad B=\mathrm{NaOH_{(alc)}},\quad C=\mathrm{Zn/HCl}A=DIBAL−H,B=NaOH(alc)​,C=Zn/HCl

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