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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3g3049

Which of the following cations are aromatic (planar, cyclic, fully conjugated, with $4 n + 2$ pi electrons)?

Athe cyclobutadiene dication, $C X_{4} H X_{4} X^{2 +}$ (planar ring, 2 pi electrons)
Bcycloprop-2-en-1-ylium (cyclopropenyl cation), $C X_{3} H X_{3} X^{+}$
Ccyclopenta-2,4-dien-1-ylium (cyclopentadienyl cation), $C X_{5} H X_{5} X^{+}$
Dcyclohepta-2,4,6-trien-1-ylium (tropylium), $C X_{7} H X_{7} X^{+}$

Answer: A, B, D

Cyclopropenyl has 2 pi electrons (

n = 0

) and tropylium has 6 (

n = 1

), so both are aromatic. The planar

C X_{4} H X_{4} X^{2 +}

has 2 pi electrons, so it is aromatic too. The cyclopentadienyl cation has 4 pi electrons (

4 n

), so it is antiaromatic.

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