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Chemistry · Aldehydes and Ketones

JEE Main 2025 — 4 April, Morning Shift — Question 4

Given below are two statements, one is labelled as Statement I and the other is labelled as Statement II.

Statement I: The dipole moment of CH34−CH3=CH2−CH1=O\overset{4}{\text{CH}_3} - \overset{3}{\text{CH}} = \overset{2}{\text{CH}} - \overset{1}{\text{CH}} = \text{O} is greater than CH34−CH23−CH22−CH1=O\overset{4}{\text{CH}_3} - \overset{3}{\text{CH}_2} - \overset{2}{\text{CH}_2} - \overset{1}{\text{CH}} = \text{O}.

Statement II: C1_1 - C2_2 bond length of CH34−CH3=CH2−CH1=O\overset{4}{\text{CH}_3} - \overset{3}{\text{CH}} = \overset{2}{\text{CH}} - \overset{1}{\text{CH}} = \text{O} is greater than C1_1 - C2_2 bond length of CH34−CH23−CH22−CH1=O\overset{4}{\text{CH}_3} - \overset{3}{\text{CH}_2} - \overset{2}{\text{CH}_2} - \overset{1}{\text{CH}} = \text{O}.

In the light of the above statements, choose the correct answer from the options given below :

  1. Option A:

    Both Statement-I and Statement-II are false

  2. Option B:

    Statement-I is true but Statement-II is false

    Correct
  3. Option C:

    Both Statement-I and Statement-II are true

  4. Option D:

    Statement-I is false but Statement-II is true

Answer: B

Step-by-step solution

In CH3−CH=CH−CH=O\mathrm{CH_3{-}CH{=}CH{-}CH{=}O} the C=C\mathrm{C=C} bond is conjugated with the carbonyl group. Conjugation increases electron withdrawal toward the carbonyl group and enhances its polarity, hence the dipole moment of the conjugated aldehyde is greater than that of the non-conjugated aldehyde CH3−CH2−CH2−CH=O\mathrm{CH_3{-}CH_2{-}CH_2{-}CH{=}O}.

Due to resonance, the C1−C2\mathrm{C_1{-}C_2} bond in the conjugated aldehyde possesses partial double bond character and is therefore shorter than the corresponding C1−C2\mathrm{C_1{-}C_2} single bond in the non-conjugated aldehyde.

Thus, StatementI is true while StatementII is false.

Answer key and solution verified before publishing.

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Exam
JEE Main 2025
Paper
4 April, Morning Shift
Subject
Chemistry
Chapter
Aldehydes and Ketones
Topic
Physical properties, Uses and Tests for Carbonyl Compound

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