IIT-JEE 2007 · Paper 2 · Chemistry Q28

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ChemistrySingle correct+3 / -1Question 28 of 66

Among the following, the least stable resonance structure is

Resonance form of 1-nitrobuta-1,3-diene: CH₂=CH-CH⁻-CH⁺-N⁺(=O)O⁻ (carbanion at C3, carbocation at C4 next to the positive nitrogen).

Resonance form: CH₂=CH-CH⁺-CH⁻-N⁺(=O)O⁻ (carbocation at C3, carbanion at C4 next to the nitro group).

Resonance form: ⁺CH₂-CH⁻-CH=CH-N⁺(=O)O⁻ (charge separated at C1 and C2).

Resonance form: ⁺CH₂-CH=CH-CH=N⁺(O⁻)O⁻ (positive charge at C1, two negative oxygens).

Answer and worked solution

Official answer: A

Judge the charges. A charge-separated structure is least stable when like charges sit next to each other.

In (A) the carbocation is on the carbon bonded to the positively charged nitrogen of the nitro group: two adjacent positive centres, and the negative charge is on the carbon that is not stabilised. In (B) the negative charge is next to N⁺ (favourable). (C) and (D) keep the opposite charges apart from the nitrogen.

(A)

Topics:Resonance Electronic effects (inductive, resonance, hyperconjugation)

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IIT-JEE 2007 · Paper 2, Chemistry question 28