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MCQEasyJEE 2024Electronic Effects (Inductive, Resonance, Hyperconjugation)

JEE Chemistry 2024 Question with Solution

The correct stability order of the following resonance structures of CH3CH=CHCHO\text{CH}_3\text{CH}=\text{CHCHO} is:

  • A

    II > III > I

  • B

    III > II > I

  • C

    I > II > III

  • D

    II > I > III

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: The resonance structures of CH3−CH=CH−CHO\text{CH}_3 - \text{CH} = \text{CH} - \text{CHO} are to be arranged in order of stability.

Find: The correct stability order among I, II, and III.

The stability of resonance structures is judged using these factors:

  1. Octet rule compliance
  2. Minimum formal charge separation
  3. Placement of negative charge on the more electronegative atom

From the given solution:

  • Structure I has a negative charge on carbon and a positive charge on oxygen, which is unfavorable.
  • Structure II has the positive charge on carbon and the negative charge on oxygen, so the charge distribution is more favorable.
  • Structure III has no charges and all atoms satisfy the octet rule, so it is the most stable.

Therefore, the stability order is III > II > I. Hence, the correct option is B.

Image-Based Reasoning

Three resonance structures of CH3CH=CHCHO labeled III, II, and I with charge placements and notes comparing their relative stability.

The image-based explanation states:

  • III is a non-polar resonance structure with more covalent bond character, so it is most stable.
  • II places the negative charge on oxygen, which is more electronegative, so it is more stable than I.
  • I places the negative charge on carbon and the positive charge on oxygen, which is less favorable.

Thus, the order remains III > II > I.

Common mistakes

  • Placing a negative charge on carbon as equally stable as on oxygen is incorrect because oxygen is more electronegative and stabilizes negative charge better. Prefer the resonance form with negative charge on oxygen.

  • Ignoring the neutral resonance form is a mistake because a structure with no charge separation is usually more stable than charged forms when octets are satisfied. Check for the neutral contributor first.

  • Judging stability only by the number of resonance forms is wrong here because the quality of each contributor matters more than count. Compare octet completion and charge distribution before ranking.

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