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Among the following, which carbocation is the least likely to form as a reaction intermediate?

1. (C6H5)3C+ (triphenylmethyl cation)
2. a cyclohexenyl (allylic, resonance-stabilized) carbocation
3. a bridged bicyclic (bridgehead) carbocation
4. CH3-CH2+ (ethyl cation, primary)

Correct answer: a bridged bicyclic (bridgehead) carbocation

Solution

Triphenylmethyl and allylic (cyclohexenyl) cations are strongly resonance-stabilized; the ethyl cation, although a primary carbocation, can still form. A bridgehead carbocation in a small bridged bicyclic system is forbidden by Bredt's rule because the bridgehead carbon cannot achieve the planar sp2 geometry a carbocation requires, making it the least likely to form.

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