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MCQMediumJEE 2025Electronic Effects (Inductive, Resonance, Hyperconjugation)

JEE Chemistry 2025 Question with Solution

Given below are two statements. In the light of the above statements, choose the correct answer from the options given below:

Image showing Statement I and Statement II comparing crotonaldehyde and butyraldehyde dipole moment and C1-C2 bond length with carbon numbering 4, 3, 2, 1 marked on both structures.
  • A

    Statement I is false but Statement II is true

  • B

    Both Statement I and Statement II are false

  • C

    Statement I is true but Statement II is false

  • D

    Both Statement I and Statement II are true

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: Two statements compare CH3−CH=CH−CHO\mathrm{CH_3-CH{=}CH-CHO} and CH3−CH2−CH2−CHO\mathrm{CH_3-CH_2-CH_2-CHO}.

Find: Which statement is true and which is false.

For the conjugated aldehyde CH3−CH=CH−CHO\mathrm{CH_3-CH{=}CH-CHO}, conjugation between C=C\mathrm{C{=}C} and C=O\mathrm{C{=}O} produces resonance and increases charge separation. The important resonance idea is

O=C−C=C⇌O−−C+−C=C\mathrm{O{=}C-C{=}C \rightleftharpoons O^- - C^+ - C{=}C}

Greater charge separation generally gives a greater dipole moment. Therefore,

μ ⁣(CH3−CH=CH−CHO)>μ ⁣(CH3−CH2−CH2−CHO)\mu\!\left(\mathrm{CH_3-CH{=}CH-CHO}\right) > \mu\!\left(\mathrm{CH_3-CH_2-CH_2-CHO}\right)

So Statement I is true.

Now compare the C1−C2\mathrm{C_1-C_2} bond length. In CH3−CH=CH−CHO\mathrm{CH_3-CH{=}CH-CHO}, the C1−C2\mathrm{C_1-C_2} bond has partial double-bond character due to conjugation. A bond with partial double-bond character is shorter than a pure single bond. Hence,

ℓ ⁣(C1−C2 in CH3−CH=CH−CHO)<ℓ ⁣(C1−C2 in CH3−CH2−CH2−CHO)\ell\!\left(\mathrm{C_1-C_2\ in\ CH_3-CH{=}CH-CHO}\right) < \ell\!\left(\mathrm{C_1-C_2\ in\ CH_3-CH_2-CH_2-CHO}\right)

So Statement II is false.

Therefore, the correct option is C: Statement I is true but Statement II is false.

Resonance-Based Explanation

Given:

  • Molecule CH3−CH=CH−CHO\mathrm{CH_3-CH{=}CH-CHO} is a conjugated enal.
  • Molecule CH3−CH2−CH2−CHO\mathrm{CH_3-CH_2-CH_2-CHO} is a saturated aldehyde.

Find: Truth value of the two statements.

Analysis of Statement I: Dipole moment depends on charge magnitude and separation distance. In the conjugated system CH3−CH=CH−CHO\mathrm{CH_3-CH{=}CH-CHO}, resonance affects electronic distribution and creates greater charge separation than in the saturated compound. Therefore, its dipole moment is greater.

Analysis of Statement II: In CH3−CH=CH−CHO\mathrm{CH_3-CH{=}CH-CHO}, the C1−C2\mathrm{C_1-C_2} bond has partial double-bond character. Partial double-bond character decreases bond length. Compared with the pure single bond in CH3−CH2−CH2−CHO\mathrm{CH_3-CH_2-CH_2-CHO}, the bond is actually shorter, not greater. Hence Statement II is false.

Therefore, the correct conclusion is: Statement I is true; Statement II is false. The correct option is C.

Common mistakes

  • Assuming that conjugation always only delocalizes electrons without changing dipole moment is incorrect. Here conjugation creates resonance forms with larger charge separation. Compare the polarity of the resonance contributors before deciding the dipole moment.

  • Treating the C1−C2\mathrm{C_1-C_2} bond in the conjugated aldehyde as a normal single bond is wrong. Due to partial double-bond character, its bond length decreases. Use resonance to judge bond order before comparing bond lengths.

  • Confusing bond length with bond polarity leads to error. A more polar bond is not necessarily longer. In this question, increased conjugation raises dipole moment but shortens the adjacent C1−C2\mathrm{C_1-C_2} bond.

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