Which of the following carbocations is the least stable?

Acyclohexyl cation
B2-propyl (isopropyl) cation
Ccyclopentyl cation
Dcyclopropyl cation

Answer: D

All four are secondary, but a cationic carbon wants bond angles of 120 degrees. In a three-membered ring the angle is 60 degrees, so planarising the carbon raises angle strain greatly. The cyclopropyl cation is the least stable.

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