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Question 45

Choose the halogen which is most reactive towards SN1 reaction in the given compounds (A, B, C & D)
A.

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B.

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C.

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D.

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Solution

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The rate of an SN1 reaction depends entirely on the stability of the carbocation intermediate formed during the rate-determining step after the leaving group departs.

  • Carbocation Stability Order: 3° (Tertiary) > 2° (Secondary) > 1° (Primary)
  • Leaving Group Ability: Better leaving groups (like I− > Br− > Cl−) drop off faster, but carbocation structural stability is the primary deciding factor if degrees differ.
Analyzing Compound A :

Compound A contains two bromine atoms labeled as Br(a) and Br(b):

  • Br(a): Attached to a highly hindered quaternary carbon center. If it leaves, it forms a highly stable 3° (tertiary) carbocation.
  • Br(b): Located at the end of the chain. If it leaves, it forms an unstable 1° (primary) carbocation.

Therefore, for Compound A, Br(a) is significantly more reactive towards an SN1 pathway.

General Matching Rules for A, B, C, D:

By applying the same structural principles across all four standard system variants:

  • A: Reacts via Br(a) to give the tertiary carbocation intermediate.
  • B: Reacts via I(a) due to better carbocation stability/leaving group parameters.
  • C: Reacts via Br(a) to generate the more stable carbon center.
  • D: Reacts via Br(a) for the same tertiary/secondary structural benefit.
Correct Option: A − Br(a) ; B − I(a) ; C − Br(a) ; D − Br(a)

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