| Revision as of 23:35, 20 April 2011 editWhy Not A Duck (talk | contribs)Extended confirmed users, Rollbackers11,866 edits Undid revision 425000266 by 14.139.160.2 (talk) -- would its aromaticity actually depend on temperature? seems dubious. source or explanation?← Previous edit |
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{{Chembox |
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| verifiedrevid = 409197833 |
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| ImageFile = (18)Annulene.svg |
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| verifiedrevid = 425085548 |
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| ImageSize = 170px |
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| ImageFileL1 = (18)Annulene-3D-balls.png |
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| ImageName = Skeletal formula |
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| ImageFile1 = (18)Annulene-3D-balls.png |
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| ImageFileR1 = (18)Annulene.svg |
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| ImageSize1 = 210px |
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| ImageSize = 170px |
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| ImageFile2 = 18annulene-crystal.png |
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| ImageName1 = Ball-and-stick model |
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| ImageCaption2 = "Herringbone" crystal structure of annulene |
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| IUPACName = |
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| ImageAlt = Structural formula of cyclooctadecanonaene |
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| OtherNames = |
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| PIN = (1''Z'',3''E'',5''E'',7''Z'',9''E'',11''E'',13''Z'',15''E'',17''E'')-Cyclooctadeca-1,3,5,7,9,11,13,15,17-nonaene |
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| Section1 = {{Chembox Identifiers |
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| OtherNames = |
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| Section1 = {{Chembox Identifiers |
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| CASNo_Ref = {{cascite|changed|??}} |
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| CASNo = 2040-73-5 |
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| CASNo = 2040-73-5 |
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| PubChem = |
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| PubChem = 137294 |
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| ChemSpiderID_Ref = {{chemspidercite|changed|chemspider}} |
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| SMILES = |
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| ChemSpiderID = 120988 |
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| InChI=1S/C18H18/c1-2-4-6-8-10-12-14-16-18-17-15-13-11-9-7-5-3-1/h1-18H/b2-1-,3-1+,4-2+,5-3+,6-4+,7-5-,8-6-,9-7+,10-8+,11-9+,12-10+,13-11-,14-12-,15-13+,16-14+,17-15+,18-16+,18-17- |
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| SMILES = C1=CC=CC=CC=CC=CC=CC=CC=CC=C1 |
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| InChIKey = STQWAGYDANTDNA-DWSNDWDZSA-N |
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| InChI = 1/C18H18/c1-2-4-6-8-10-12-14-16-18-17-15-13-11-9-7-5-3-1/h1-18H/b2-1-,3-1+,4-2+,5-3+,6-4+,7-5-,8-6-,9-7+,10-8+,11-9+,12-10+,13-11-,14-12-,15-13+,16-14+,17-15+,18-16+,18-17- |
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| InChIKey = STQWAGYDANTDNA-DWSNDWDZBB |
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| StdInChI_Ref = {{stdinchicite|changed|chemspider}} |
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| StdInChI = 1S/C18H18/c1-2-4-6-8-10-12-14-16-18-17-15-13-11-9-7-5-3-1/h1-18H/b2-1-,3-1+,4-2+,5-3+,6-4+,7-5-,8-6-,9-7+,10-8+,11-9+,12-10+,13-11-,14-12-,15-13+,16-14+,17-15+,18-16+,18-17- |
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| StdInChIKey_Ref = {{stdinchicite|changed|chemspider}} |
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| StdInChIKey = STQWAGYDANTDNA-DWSNDWDZSA-N |
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| Section2 = {{Chembox Properties |
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| Section2 = {{Chembox Properties |
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| C = 18 | H = 18 |
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| C=18 | H=18 |
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| MolarMass = |
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| MolarMass = 234.3 g·mol<sup>−1</sup> |
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| Appearance = |
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| Appearance = red-brown crystals |
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| Density = |
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| Density = 1.134 g/cm<sup>3</sup> (calc.)<ref name=str/> |
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| Solubility = }} |
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| Section3 = {{Chembox Hazards |
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| Section3 = {{Chembox Structure |
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| Structure_ref =<ref name=str>{{cite journal|doi=10.1107/S0108768195004927 |title=[18]-Annulene, C<sub>18</sub>H<sub>18</sub>, structure, disorder and Hückel's 4 ''n'' + 2 rule |date=1995 |last1=Gorter |first1=S. |last2=Rutten-Keulemans |first2=E. |last3=Krever |first3=M. |last4=Romers |first4=C. |last5=Cruickshank |first5=D. W. J. |journal=Acta Crystallographica Section B Structural Science |volume=51 |issue=6 |pages=1036–1045 |bibcode=1995AcCrB..51.1036G }}</ref> |
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| CrystalStruct = |
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| SpaceGroup = ], P2<sub>1</sub>/a |
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| PointGroup = |
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| LattConst_a = 1.4984 (5) nm |
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| LattConst_b = 0.4802(2) nm |
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| LattConst_c = 1.0260(3) nm |
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| LattConst_alpha = |
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| LattConst_beta = 111.52(1) |
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| LattConst_gamma = |
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| LattConst_ref = |
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| LattConst_Comment = |
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| UnitCellFormulas = 2 |
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| Coordination = |
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}} |
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| Section4 = {{Chembox Hazards |
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| FlashPt = |
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| Autoignition = }} |
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'''Cyclooctadecanonaene''' or '''annulene''' is an ] with chemical formula C<sub>18</sub>H<sub>18</sub>. This ] obeys ] and is therefore an ] compound. The compound was first synthesised by ].<ref>In the literature and some internet references Sondheimer is misspelled as Sandheimer.</ref> The original synthesis started by the ] of the di-] 1,5-hexadiyne with ] in ] to give the trimer, followed by ] and ] with ] ] in ] and was concluded with ] ] with the ].<ref>{{cite journal | title = Unsaturated Macrocyclic Compounds. XXIII. The Synthesis of the Fully Conjugated Macrocyclic Polyenes Cyclooctadecanonaene (Annulene), Cyclotetracosadodecaene (Annulene), and Cyclotriacontapentadecaene (Annulene). | author = Sondheimer, F., Wolovsky, R. and Amiel, Y. | journal = ] | year = 1962 | volume = 68 | issue = 2 | pages = 274–284 | doi = 10.1021/ja00861a030}}</ref> <ref>{{OrgSynth | title = <nowiki>Annulene</nowiki> | author = K. Stöckel and F. Sondheimer | collvol = 6 | collvolpages = 68 | year = 1988 | prep = cv6p0068}}</ref> |
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'''Cyclooctadecanonaene''' or '''annulene''' is an ] with chemical formula {{chem|C|18|H|18}}. It belongs to the class of highly ] compounds known as ] and is ]. The usual isomer that annulene refers to is the most stable one, containing six interior hydrogens and twelve exterior ones, with the nine formal double bonds in the ''cis'',''trans'',''trans'',''cis'',''trans'',''trans'',''cis'',''trans'',''trans'' configuration. It is reported to be a red-brown crystalline solid. |
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==Aromaticity== |
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Notably, annulene is the first annulene after ] (annulene) to be fully ]: its π-system contains 4n + 2 electrons (n = 4), and it is large enough to comfortably accommodate six hydrogen atoms in its interior, allowing it to adopt a planar shape, thus satisfying ]. The discovery of aromatic stabilization for annulene is historically significant for confirming earlier theoretical predictions based on ], since simple versions of valence bond theory did not readily explain 4''n'' + 2 rule. |
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The <sup>1</sup>H ] of this compound exhibits hallmarks of a system with an ], with the 12H signal of exterior hydrogens at 9.25 ppm, while 6H signal of interior hydrogens resonates at a remarkable −2.9 ppm in THF-''d<sub>8</sub>'' at −60 °C. On the other hand, a single signal at 5.45 ppm (weighted average of two individual signals) is observed at 120 °C. This is consistent with rapid exchange of exterior and interior hydrogens at that temperature. The ] in annulene are in between those of single and double carbon–carbon bond, with two bond lengths observed ]: 138.9 pm (concave edges) and 140.7 pm (convex edges) and are indicative of significant ]. The favorability of delocalization is, in turn, interpreted as evidence for aromaticity. For comparison, these values are close to bond length of benzene (140 pm).<ref name="Jux2016">{{Cite journal|last1=Jux|first1=Norbert|last2=R. Schleyer|first2=Paul v|last3=Majetich|first3=George|last4=Meyer|first4=Karsten|last5=Hampel|first5=Frank|last6=W. Heinemann|first6=Frank|last7=V. Nizovtsev|first7=Alexey|last8=Lungerich|first8=Dominik|date=2016|title=Annulene put into a new perspective|journal=Chemical Communications|language=en|volume=52|issue=25|pages=4710–4713|doi=10.1039/C6CC01309K|pmid=26953607|doi-access=free}}</ref> |
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annulene are around 139 to 141 pm, indicating delocalization and a bond order between 1 and 2. In general, carbon-carbon single bonds and double bonds are 154 pm and 134 pm, respectively, while benzene, with a bond order of 1.5, the bond-length is 140 pm.]] |
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Based on enthalpy of hydrogenation, overall ] energy has been estimated to be 37 kcal/mol.<ref>{{Cite journal|date=1974-11-06|title=The Stabilization Energy of Annulene. A thermochemical determination|journal=Helvetica Chimica Acta|volume=57|issue=7|pages=2276–2288|doi=10.1002/hlca.19740570745|issn=0018-019X|last1=Oth|first1=Jean F. M.|last2=Bünzli|first2=Jean-Claude|last3=De Julien De Zélicourt|first3=Yves}}</ref> This is about the same as that of benzene; however, this energy is spread out over 18 atoms instead of 6, so annulene experiences a weaker stabilization than benzene. In terms of reactivity, it is somewhat more air- and light-stable than annulene]] and annulene]], which are, respectively, weakly aromatic and nonaromatic due to transannular interactions. Nevertheless, it rapidly undergoes ]s, much like other ], and attempts to effect ] on annulene failed.<ref>{{cite journal|author1=Sondheimer, F.|author2=Wolovsky, R. |author3=Amiel, Y.|year=1962|title=Unsaturated Macrocyclic Compounds. XXIII. The Synthesis of the Fully Conjugated Macrocyclic Polyenes Cyclooctadecanonaene (Annulene), Cyclotetracosadodecaene (Annulene), and Cyclotriacontapentadecaene (Annulene).|journal=]|volume=68|issue=2|pages=274–284|doi=10.1021/ja00861a030}}</ref> |
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Despite the usual interpretation of annulene as an 18-electron aromatic system, a 2014 theoretical study suggested that annulene may be thought of as having only three completely delocalized ] associated with its aromaticity, while other six π bonds represent conjugated ] ("3c-2e") π bonds on periphery of molecule.<ref>{{cite journal|last=Ivanov|first=A.|author2=Boldyrev. A|title=Deciphering aromaticity in porphyrinoids via adaptive natural density partitioning|journal=Org. Biomol. Chem.|volume=12|issue=32|pages=6145–6150|year=2014|doi=10.1039/C4OB01018C|pmid=25002069}}</ref> |
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==Synthesis== |
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The compound was first synthesised by ].<ref>In the literature and some internet references, Sondheimer is sometimes misspelled as Sandheimer.</ref> The original synthesis started by ] of di-] 1,5-hexadiyne with ] in ] to give the trimer, followed by ] and ] with ] in ] and was concluded with ] ] with ].<ref>{{OrgSynth | title = <nowiki>Annulene</nowiki> | author1 = Stöckel, K. |author2=Sondheimer, F. | collvol = 6 | collvolpages = 68 | year = 1988 | prep = cv6p0068}}</ref> |
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==See also== |
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==References== |
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==References== |
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