Is cyclohepta-1,3,5-triene aromatic?
I think it should not be, because even though the π-system possesses six π-electrons, the conjugation is not complete around the ring.
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$\begingroup$ Related: chemistry.stackexchange.com/questions/28342/are-these-aromatic/… $\endgroup$ – Klaus-Dieter Warzecha Jul 16 '16 at 6:07
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You are correct. The conjugation does not extend all the way around the ring, so the molecule is not aromatic. In addition, the single methylene group ($\ce{CH2}$) sticks out of the plane of the rest of the molecule.
(On the other hand, removing a hydride ion $\ce{H-}$ from the $\ce{CH2}$ group leaves the tropylium ion $\ce{C7H7+}$, which is planar and aromatic, with resonance structures putting the positive charge on any of the seven carbons.)
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3$\begingroup$ If the world was black and white, you'd be correct. Unfortunately, aromaticity is among those concepts which have a large grey area: en.wikipedia.org/wiki/Homoaromaticity $\endgroup$ – Martin - マーチン♦ Jul 14 '17 at 5:23
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$\begingroup$ But where is the gray area for single hydrocarbon cycles? $\endgroup$ – Oscar Lanzi Jan 9 '18 at 23:26
