Indian Forest Service (Mains), Chemistry optional · 2016 · Paper II

Q1(d). Order of carbocation stability

Question

Arrange the following carbocations in decreasing order of their stability. Give reason in support of your answer.
[structures given in the paper]

C > A > B > D : cyclopropyl σ donation > tertiary hyperconjugation > primary benzylic resonance > non-planar bridgehead.

C, dicyclopropylmethyl. In the bisected conformation the bent (Walsh) C–C σ bonds of each cyclopropane overlap the empty p orbital. Two cyclopropyls make it the most stable here. For comparison, the tricyclopropylmethyl cation…
A, t-butyl. Tertiary: nine C–H hyperconjugating bonds and the +I effect of three CH₃.
B, benzyl. Charge delocalised into the ring (resonance), but the cation is primary; pKR⁺ below -20 and gas-phase hydride affinity 239 against 237 kcal mol⁻¹ for t-butyl, so slightly less stable than A. Many texts place B before A.
D, 1-adamantyl. Tertiary, but at a bridgehead: the cage holds it pyramidal, it cannot flatten fully, and solvation from behind is blocked. A stable bridgehead ion, but less stable than an open-chain tertiary cation.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Coversempty p orbitalnine C–H hyperconjugatingdelocalised into the ringcannot flatten