Question 69 of 83
Q.Which one of the following is aromatic ?
(a) benzene ring (six-membered, three alternating double bonds)
(b) cyclooctatetraene (eight-membered ring, four alternating double bonds)
(c) cyclopentadiene (five-membered ring, two double bonds)
(d) both
(a) and (b)
Puducherry TnboardTamil Nadu HSC First Year (DGE) Board 2023MCQ· 1mImportance★★★★★
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Start your 14-day free trial to unlock the full solution →Benzene, with 6 delocalised pi electrons in a planar hexagonal ring, satisfies Huckel's (4n+2) rule for n=1 and is aromatic; the other two ring systems shown are not.
Huckel's rule states that a cyclic, planar, fully conjugated system is aromatic only if it contains (4n+2) pi electrons, where n = 0, 1, 2, ...
- Benzene: a planar hexagon with three alternating double bonds, giving 6 pi electrons total. 6 = 4(1)+2, so n=1 satisfies Huckel's rule -- benzene is aromatic.
- Cyclooctatetraene: an eight-membered ring with four alternating double bonds, giving 8 pi electrons. 8 does not fit 4n+2 for any integer n, and the real molecule is also non-planar (tub-shaped) to avoid the instability of forcing 8 pi electrons into a planar ring -- so it is non-aromatic. …
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