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

(C) II $<$ I $<$ III $<$ IV

(A) IV $<$ III $<$ II $<$ I

(B) III $<$ I $<$ II $<$ IV

(D) I $<$ II $<$ III $<$ IV

Haloalkanes and Haloarenes — JEE Main · 2026 · 22 Jan 2026 · Shift 1 Question with Solution

The correct order of the rate of reaction of the following reactants with nucleophile by

S_{N} 1

mechanism is :
(Given : Structure I and II are rigid)

(A)IV $<$ III $<$ II $<$ I
(B)III $<$ I $<$ II $<$ IV
(C)II $<$ I $<$ III $<$ IV✓ Correct
(D)I $<$ II $<$ III $<$ IV

Answer: (C)

Solution

S_{N} 1

reaction passes through a carbocation, so its rate rises with the stability of the cation formed when bromide leaves. Structures I and II are rigid bridgehead bromides, the bicyclo[2.2.2]octane cage and the smaller bicyclo[2.2.1] cage: a bridgehead carbon cannot flatten to the planar geometry a carbocation needs, so both ionise with great difficulty, and the more compact bicyclo[2.2.1] framework (II) resists flattening even more than the bicyclo[2.2.2] one (I), making II the slowest and I the next slowest. The tertiary centre of tert-butyl bromide (III) gives a planar, hyperconjugatively stabilised cation and reacts readily, while triphenylmethyl bromide (IV) gives a cation delocalised over three rings and is the fastest. The order of reactivity is

II < I < III < IV

JEE Main · 2026 · 22 Jan 2026 · Shift 1 · Single correct option

JEE Main · 2026 · 22 Jan 2026 · Shift 1 · Haloalkanes and Haloarenes

Solution

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