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MCQEasyJEE 2026Electronic Effects (Inductive, Resonance, Hyperconjugation)

JEE Chemistry 2026 Question with Solution

Arrange the following carbanions in the decreasing order of stability:

I. pp-Br−C6H4−CH2−\mathrm{Br{-}C_6H_4{-}CH_2^-}

II. C6H5−CH2−\mathrm{C_6H_5{-}CH_2^-}

III. pp-CH3O−C6H4−CH2−\mathrm{CH_3O{-}C_6H_4{-}CH_2^-}

IV. pp-CHO−C6H4−CH2−\mathrm{CHO{-}C_6H_4{-}CH_2^-}

V. pp-CH3−C6H4−CH2−\mathrm{CH_3{-}C_6H_4{-}CH_2^-}

Choose the correct answer from the options given below:

  • A

    IV >> I >> II >> V >> III

  • B

    I >> IV >> II >> V >> III

  • C

    I >> II >> IV >> V >> III

  • D

    IV >> II >> I >> III >> V

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: The stability order of substituted benzyl carbanions has to be arranged.

Find: The decreasing order of stability and the correct option.

Step 1: Stability principle for carbanions. Carbanions are stabilized by electron withdrawing groups and destabilized by electron donating groups.

Step 2: Analysing substituent effects.

  • (IV) −CHO-CHO: Strong −M-M and −I-I effect, maximum stabilization.
  • (I) −Br-Br: −I-I effect dominates, stabilizing the carbanion.
  • (II) H: No substituent effect, moderate stability.
  • (V) −CH3-CH_3: +I+I effect, destabilizes carbanion.
  • (III) −OCH3-OCH_3: Strong +M+M effect, maximum destabilization.

Step 3: Ordering stability.

IV>I>II>V>III\text{IV}>\text{I}>\text{II}>\text{V}>\text{III}

Step 4: Final conclusion. Therefore, the correct decreasing order of stability is IV >> I >> II >> V >> III. The correct option is A.

Common mistakes

  • Treating −OCH3-OCH_3 as stabilizing only because oxygen is electronegative is incorrect. Its strong +M+M effect donates electron density into the ring and destabilizes the carbanion. Check the dominant mesomeric effect, not just electronegativity.

  • Assuming −Br-Br stabilizes more than −CHO-CHO is incorrect. Halogen shows mainly a −I-I effect here, but −CHO-CHO provides both strong −I-I and −M-M withdrawal, so it stabilizes the carbanion more.

  • Ignoring the unsubstituted benzyl carbanion (II) as the reference is a common error. First place the neutral reference, then compare whether each substituent withdraws or donates electron density relative to hydrogen.

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