JEE Main · 2022 · 28 Jun 2022 · Shift 1 · General Organic Chemistry

Which of the following structures are aromatic in nature?

(A)A,B,C and D
(B)Only A and B
✓ Correct
(C)Only A and C
(D)Only B, C and D

Answer: (B)

Solution

A compound is aromatic if it satisfies Hückel's rule: it must be cyclic, planar, fully conjugated (all atoms in the ring have a p-orbital), and contain

(4 n + 2) π

electrons (where

n

is a non-negative integer).

From the structures:

A: Cyclopentadienyl anion ( $C_{5} H_{5}^{-}$ ) has 6 π electrons ( $4 (1) + 2 = 6$ , $n = 1$ ). It is cyclic, planar, and fully conjugated → aromatic.
B: Tropylium cation ( $C_{7} H_{7}^{+}$ ) has 6 π electrons ( $4 (1) + 2 = 6$ , $n = 1$ ). It is cyclic, planar, and fully conjugated → aromatic.
C: Cyclooctatetraene ( $C_{8} H_{8}$ ) has 8 π electrons ( $4 (2) = 8$ , does not satisfy $4 n + 2$ ). It is non-planar (tub-shaped) → non-aromatic.
D: Cyclopentadienyl cation ( $C_{5} H_{5}^{+}$ ) has 4 π electrons ( $4 (1) = 4$ , does not satisfy $4 n + 2$ ) → anti-aromatic (or non-aromatic depending on context).

Only A and B are aromatic.

JEE Main · 2022 · 28 Jun 2022 · Shift 1 · Single correct option

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JEE Main · 2022 · 28 Jun 2022 · Shift 1 · General Organic Chemistry

Solution

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