Antiaromaticity 923714 178302027 2007-12-16T15:44:10Z 68.127.229.114 [[Image:Antiaromats2.png|300px|right|Antiaromatic compounds]] '''Antiaromatic''' or '''pseudoaromatic''' [[molecule]]s are [[cyclic]] systems containing alternating single and [[double bond]]s, where the [[pi electron]] energy of antiaromatic compounds is higher than that of its open-chain counterpart. Therefore antiaromatic compounds are unstable and highly reactive; often antiaromatic compounds distort themselves out of planarity to resolve this instability. Antiaromatic compounds usually fail the [[Hückel's rule]] of [[aromatic]]ity. Examples of antiaromatic systems are [[cyclobutadiene]] ('''A'''), the cyclopentadienyl cation ('''B''') and the cyclopropenyl anion ('''C'''). [[Cyclooctatetraene]] is a 4n system but neither aromatic or antiaromatic because the molecule escapes a planar geometry. By adding or removing an electron pair via a [[redox]] reaction, a π system can become aromatic and therefore more stable than the original non- or anti-aromatic compound, for instance the [[cyclooctatetraenide dianion]]. The [[IUPAC]] criteria for '''antiaromaticity''' are as follows:<ref>[[Compendium of Chemical Terminology]], [http://goldbook.iupac.org/A00382.html antiaromatic compounds], accessed 1 Feb 2007.</ref> # The molecule must have 4''n'' π electrons where ''n'' is any integer. # The molecule must be cyclic. # The molecule must have a conjugated pi electron system. # The molecule must be planar. However, most chemists agree on the definition based on empirical (or simulated) energetic observations.{{Fact|date=February 2007}} It is observed that the energy difference between aromatic and antiaromatic compounds diminishes with increasing size <ref>''A Thiadiazole-Fused N,N-Dihydroquinoxaline: Antiaromatic but Isolable'' Shaobin Miao, [[Paul von Ragué Schleyer|Paul v. R. Schleyer]], Judy I. Wu, Kenneth I. Hardcastle, and Uwe H. F. Bunz [[Org. Lett.]]; '''2007'''; 9(6) pp 1073 - 1076; (Letter) {{DOI|10.1021/ol070013i}}</ref>. For instance the 12-pi system [[diphenylene]] is an antiaromatic compound but stable and even commercially available. The low energy penalty for antiaromaticity is also demonstrated in certain [[pyrazine]]-dihydropyrazine pairs: :[[Image:AromaticAntiaromaticConversion.png|400px|''A Thiadiazole-Fused N,N-Dihydroquinoxaline]] The compound on the left is a 14 pi-electron aromatic compound ([[NICS (chemistry)|NICS]] value -26.1 ppm) which can be [[organic reduction|reduced]] in a strongly [[exothermic reaction]] to the 16 pi-electron antiaromatic compound (NICS +27.7 ppm) on the right <ref>Reducing agent: [[Sodium hypophosphite]], tips stands for [[triisopropylsilyl]]</ref>. The dihydropyrazine slowly converts back to the pyrazine under the action of [[oxygen]]. It shows that other electronic factors can overpower aromaticity. Antiaromaticity is also observed in a [[chemical equilibrium]] between these two [[porphyrin]] derivatives <ref>''22-Hydroxybenziporphyrin: Switching of Antiaromaticity by Phenol-Keto Tautomerization Marcin Stpie'', Lechosaw Latos-Grayski, and Ludmia Szterenberg [[J. Org. Chem.]]; '''2007'''; 72(7) pp 2259 - 2270; (Article) {{DOI|10.1021/jo0623437}}</ref>: :[[Image:Antiaromaticityinporphyrinesystem.png|400px|Switching of Antiaromaticity by Phenol-Keto Tautomerization]] A regular porphyrin is an 18 electron aromatic compound (not counting two non-contributing double bonds) but on substituting a [[pyrrole]] ring by a [[phenylene|meta-phenylene ring]] aromaticity is lost due to lack of [[conjugation]]. In this system the phenylene group is also a [[phenol]] and structure '''A''' is found to interconvert with 20 electron antiaromat '''B''' via [[tautomerism|phenol-keto tautomerism]]. Antiaromaticity is evident from [[NMR spectroscopy]] with the inner NH protons shifting [[chemical shift|downfield]] by 10 ppm to 21 ppm. The NICS values compare +0.7 for '''A''' (non-aromatic) and +5 (antiaromatic) for '''B''' and other [[in silico]] experimentation predicts that B is actually more stable than A. ==References== {{reflist}} [[Category:Physical organic chemistry]] [[ar:ضد أروماتي]] [[es:Antiaromaticidad]] [[it:Composti antiaromatici]]