JEE Main 2022 · 24 Jun · Shift 2 · Chemistry Q73

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ChemistrySingle correct+4 / -1Question 73 of 90

Arrange the following carbocations in decreasing order of stability.

Three five-membered ring cations: A, the cyclopentyl cation; B, a tetrahydrofuran ring with the positive charge on the carbon next to oxygen (C-2); C, a tetrahydrofuran ring with the positive charge on C-3.

A > C > B

A > B > C

C > B > A

C > A > B

Answer and worked solution

Official answer: B

The cations. A is cyclopentyl (secondary, no heteroatom). B has the charge on the carbon next to ring oxygen. C has it one carbon further from oxygen.

B against C. In C the oxygen only withdraws electrons inductively, which destabilises the cation, so B is above C. In B the oxygen lone pair also delocalises the charge (an oxocarbenium ion).

Keyed order. NTA's key gives A > B > C. Note: by resonance B would usually be ranked above A (B > A > C), an order none of the options offers. The keyed option ranks A first and B above C.

(B)

Topics:Reactive intermediates Electronic effects (inductive, resonance, hyperconjugation)

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JEE Main 2022 · 24 Jun · Shift 2, Chemistry question 73