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JEE Chemistry 2026 Question with Solution

The correct order of the rate of reaction of the following reactants with nucleophile by SN1\mathrm{S_N1} mechanism is:

Four brominated substrates labeled I, II, III and IV are shown for comparing their SN1 reactivity, with I and II being rigid bicyclic structures, III a tertiary alkyl bromide, and IV a triphenylmethyl bromide.

(Given: Structures II and IIII are rigid)

  • A

    III<I<II<IVIII < I < II < IV

  • B

    I<II<III<IVI < II < III < IV

  • C

    II<I<III<IVII < I < III < IV

  • D

    IV<III<II<IIV < III < II < I

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: Four reactants I,II,III,IVI, II, III, IV are compared for rate of reaction with a nucleophile by SN1\mathrm{S_N1} mechanism.

Find: The correct increasing order of rate.

In an SN1\mathrm{S_N1} reaction, the rate depends on the stability of the carbocation formed after the leaving group departs. Resonance stabilization is more effective than hyperconjugation for stabilizing the carbocation.

From the given explanation:

  • Structure II: Due to rigid bicyclic framework, carbocation formation is highly strained and least stable.
  • Structure I: Slightly less strained than IIII, but still rigid, so the carbocation is unstable.
  • Structure III: Forms a tertiary carbocation stabilized by hyperconjugation.
  • Structure IV: Forms a triphenylmethyl carbocation highly stabilized by resonance with three phenyl rings.

Therefore, the increasing order of carbocation stability, and hence the increasing order of SN1\mathrm{S_N1} rate, is

II<I<III<IV\text{II}<\text{I}<\text{III}<\text{IV}

Thus, the correct option is C.

Common mistakes

  • Assuming that a more substituted carbon always reacts faster in SN1\mathrm{S_N1}. That is incomplete here because rigidity and strain in bicyclic systems can strongly destabilize the carbocation. First examine whether the carbocation can actually form comfortably.

  • Ignoring resonance stabilization in IVIV. A triphenylmethyl carbocation is far more stable than an ordinary tertiary carbocation because the positive charge is delocalized over three phenyl rings. Do not compare IIIIII and IVIV only by degree of substitution.

  • Treating structures II and IIII as equivalent just because both are rigid. The solution states that IIII is more strained and gives the least stable carbocation, while II is slightly less strained. Compare relative strain, not only the word rigid.

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