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Question 56

The correct order of the rate of reaction of the following reactants with nucleophile by $$S_{N}1$$ mechanism is :
(Given: Structures I and II are rigid)

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Solution

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  • (IV) Triphenylmethyl bromide: The resulting carbocation is highly stabilized by resonance with three phenyl rings. This is the most stable cation in the set, leading to the fastest rate.
  • (III) tert-Butyl bromide: The carbocation is stabilized by the inductive effect and hyperconjugation from three methyl groups. While stable, it lacks the extensive resonance of (IV).
  • (I) & (II) Bridgehead Bromides: These react the slowest due to Bredt's Rule.
    • An $$S_{N}1$$ reaction requires the carbocation to adopt a planar $$sp^{2}$$ geometry.
    • In rigid bicyclic systems, the bridgehead carbon cannot become planar because of the geometric constraints of the rings. This creates immense "ring strain."
    • Structure (II) is smaller and more rigid than Structure (I), making it even harder to achieve planarity. Therefore, (II) is the least reactive.

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