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Which of the following carbocations is the most stable?

1. Cyclopropylmethyl cation
2. Cyclobutyl cation
3. Cyclopentyl cation
4. None of these

Correct answer: Cyclopropylmethyl cation

Solution

The cyclopropylmethyl (cyclopropylcarbinyl) cation is unusually stable because the high p-character of the strained cyclopropane bent bonds overlaps efficiently with the empty p orbital of the adjacent cationic carbon, strongly delocalising the positive charge. This stabilisation exceeds that of the simple secondary cyclobutyl and cyclopentyl cations.

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