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Shouldn't cyclopropenyl anion be aromatic as it will keep its lone pair in an unhybridized orbital? My teacher says that the lone pair will be a part of a hybrid orbital and the compound will be anti-aromatic.

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  • $\begingroup$ Welcome to chemistry.se! In $\ce{C3H3-}$ (with the hypothetical $D_\mathrm{3h}$ symmetry) you would have 4 electrons in the pi-system (all p orbitals). This is only a minimum on the potential energy surface in a triplet state, which is higher in energy than the distorted singlet state. The anion is not aromatic, there are no stable (ground state) anti-aromatic compounds. $\endgroup$ – Martin - マーチン May 18 '17 at 8:03
  • $\begingroup$ No stable antiaromatic compounds? Some call that a challenge ... cns.utexas.edu/news/new-breed-anti-aromatics. $\endgroup$ – Oscar Lanzi May 18 '17 at 10:08
  • $\begingroup$ @Martin-マーチン are there really no stable anti-aromatic compounds? 5,5-difluoro-1,3-cyclopentadienes are antiaromatic and you can buy that one. $\endgroup$ – DSVA May 18 '17 at 13:39
  • $\begingroup$ @DSVA I don't see how the compound you cite is anti aromatic, there is no circular conjugated pi system. If a compound is destabilised with respect to another, distorted arrangement of the 'same' bonding pattern, it cannot be the ground state. The definition of anti-aromaticity dictates that it is less stable than a distorted geometry, because it cites this the reason for the distortion. $\endgroup$ – Martin - マーチン May 18 '17 at 14:25
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    $\begingroup$ @Martin-マーチン that compound basically has a conjugated 4pi-electron system since the CF2 group can participate "hyperconjugatively" with the ring system (original publication pubs.acs.org/doi/abs/10.1021/ol401154r , follow up by Houk: onlinelibrary.wiley.com/doi/10.1002/jcc.24191/full ). That compound is planar and shows antiaromaticity in every metric we got (aromatic stabilization energies, NICS, ACID, Diels-Alder reactivity,...) $\endgroup$ – DSVA May 18 '17 at 14:51