JEE Main levelSingle correct optionDifficulty 2 of 3g3025
The cyclobutylmethyl cation, c-CX4HX7−CHX2X+ (a 1∘ cation), ring-expands readily to:
Athe cyclohexyl cation
Bthe cyclopentyl cation (a 2∘ cation) with relief of ring strain
Cthe cyclopropylmethyl cation
Dthe 1-methylcyclobutyl cation by ring contraction only
Answer: B
A ring C-C bond migrates to the cationic carbon, so the four-membered ring becomes five-membered. The charge moves from a primary carbon to a secondary ring carbon and the angle strain is reduced.