The correct stability order for the following species is (I) (see structure) (II) (see structure) (III) (see structure) (IV) (see structure)IIIIIIIV
(A)(II) > (IV) > (I) > (III)
(B)(I) > (II) > (III) > (IV)
(C)(II) > (I) > (IV) > (III)
(D)(I) > (III) > (II) > (IV)✓ Correct
Answer: (D)
Solution
The stability order is determined by resonance stabilization and hyperconjugation. Species (I) is a benzylic carbocation stabilized by resonance with the benzene ring and has six α-hydrogen atoms available for hyperconjugation. Species (III) is also resonance-stabilized but has only three α-hydrogen atoms. Species (II) is a secondary carbocation with five α-hydrogen atoms. Species (IV) is a primary carbocation with only two α-hydrogen atoms. Therefore, the order is (I) > (III) > (II) > (IV).
JEE Advanced · 2008 · 13 Apr 2008 · Paper 2 · Single correct option
