:Cyclodecapentaene

{{Multiple images

| perrow = 2

| total_width = 440

| image1 = Planar cis-10annulene.svg

| alt1 = SMILES: C1=CC=CC=CC=CC=C1; all cis-, all in a plane, making a regular decagon

| caption1 = ({{va|Im1|text=1}}): a hypothetical planar con­forma­tion for all-cis-[10]annulene.

| image2 = Boat cis-10annulene.svg

| alt2 = SMILES: C1=CC=CC=CC=CC=C1; all cis-, but the outer two double-bonds are tilted up, in the same direction, making a sort of bowl shape

| caption2 = ({{va|Im2|text=2}}): the lowest-energy con­forma­tion for all-cis-[10]annulene

| image3 = Planar alternant 10annulene.svg

| alt3 = SMILES: C1=CC=CC=CC=CC=C1; isomer trans,cis,trans,cis,cis, making a pinched oval, with two hydrogens leaping out of the plane in the same direction at the pinch

| caption3 = ({{va|Im3|text=3}}): one hypothetical planar con­form­ation for trans,­cis,­trans,­cis,­cis-[10]annulene

| image4 = Figure-8 alternant 10annulene.svg

| alt4 = SMILES: C1=CC=CC=CC=CC=C1; isomer trans,cis,cis,cis,cis, making a "figure-8"

| caption4 = ({{va|Im4|text=4}}): one conformation for trans,­cis,­cis,­cis,­cis-[10]annulene

| layout = 2 / 2

| align = left

}}

Although not aromatic itself, [10]annulene can transition between different con­forma­tional isomers through aromatic or quasi­aromatic excited states, such that its con­forma­tional iso­mer­ism is fixed only at extreme cryogenic temperatures.{{Cite journal |last1=Castro |first1=Claire |last2=Karney |first2=William L. |last3=McShane |first3=Colleen M. |last4=Pemberton |first4=Ryan P. |date=2006-04-01 |title=[10]Annulene: Bond Shifting and Conformational Mechanisms for Auto­meriz­ation |journal=The Journal of Organic Chemistry |language=en |volume=71 |issue=8 |pages=3001–3006 |doi=10.1021/jo0521450 |pmid=16599594 |issn=0022-3263}} Under­stand­ing the com­posi­tion and react­ivity of these mix­tures com­put­ation­ally has proven difficult,{{Cite journal |last1=Slayden |first1=Suzanne W. |last2=Liebman |first2=Joel F. |date=2001-05-01 |title=The Energetics of Aromatic Hydrocarbons: An Experimental Thermo­chemical Perspective |journal=Chemical Reviews |language=en |volume=101 |issue=5 |pages=1545–1546 |doi=10.1021/cr990324+ |issn=0009-2665}} because a large number of conformations all minimize the energy locally.{{Cite journal |last1=Xie Yaoming |last2=Schaefer |first2=Henry F. |last3=Liang Guyan |last4=Bowen |first4=J. Phillip |date=Feb 1994 |title=[10]Annulene: The Wealth of Energetically Low-Lying Structural Isomers of the Same (CH)10 Connectivity |journal=Journal of the American Chemical Society |language=en |volume=116 |issue=4 |pages=1442–1449 |doi=10.1021/ja00083a032 |issn=0002-7863 |ref={{harvid|Xie|Schaefer|Liang|Bowen|1994}}}}

The all-cis isomer (1), a fully convex decagon, would have bond angles of 144°, which creates large amounts of angle strain relative to the ideal 120° in sp² atomic hybridization. Instead, the all-cis isomer adopts a planar boat-like conformation (2) to relieve the angle strain,{{Harvnb|Xie|Schaefer|Liang|Bowen|1994}}, though note that {{Harvnb|Kemp-Jones|Masamune|1973|pp=126-7}} instead proposes a "twist" conformation, with 6 atoms coplanar and the remaining 4 in a raised handle. although it, too, is less stable than the next planar isomer, trans,cis,trans,cis,cis-[10]annulene (3).{{cite needed|date=July 2023}} Yet even isomer (3) is unstable, suffering from steric repulsion between the two internal hydrogen atoms,{{Cite journal |last1=Sulzbach |first1=Horst M. |last2=Schleyer |first2=Paul v. R.|author3=Jiao Haijun|author4=Xie Yaoming |last5=Schaefer |first5=Henry F. |date=Feb 1995 |title=A [10]Annulene Isomer May Be Aromatic, After All! |journal=Journal of the American Chemical Society |language=en |volume=117 |issue=4 |pages=1369–1373 |doi=10.1021/ja00109a021 |issn=0002-7863}} and tends to distort into the perimeter of two fused circles, one larger and the other smaller, as in azulene. The nonplanar trans,­cis,­cis,­cis,­cis isomer is the most stable of all possible isomers,{{Cite needed|date=July 2023}} although it is unclear whether it too has a boat-like configuration as in conformer (4), or the "heart" configuration produced if one internal hydrogen in conformer (3) were flipped inside-out.

Cyclodecapentaene can undergo an electrocyclic rearrangement to{{Cite journal |author1=Masamune S.|last2=Seidner |first2=R. T. |date=1969 |title=[10]Annulenes |journal=Journal of the Chemical Society D: Chemical Communications |language=en |issue=10 |pages=542 |doi=10.1039/c29690000542 |issn=0577-6171}} or from dihydronaphthalene. Photolysis of the latter generates [10]annulene, but it quickly reverts to the reactant, even at cryogenic temperatures.{{Rp|page=122}}

File:Bridged_10annulene.svg

File:Twice-bridged_10annulene.svg

Aromaticity can be induced in compounds having a [10]annulene-type core if planarity is forcibly imposed by other substituents. Two methods to do so are known.

One method is to formally replace two hydrogen atoms by a methylene bridge ({{chem2|\sCH2\s}}); this gives the planar bicyclic 1,6-methano­[10]annulene (5). Indeed, 1,6-methano­[10]annulene has no bond length alternation in its X-ray structure and signs of a telltale diamagnetic ring current in its NMR spectrum. Likewise, a tricyclic methine bridge gives an aromatic structure (6) similar to the stable oxonium ion oxatriquinacene.{{cite journal |last1=Gilchrist |first1=Thomas L. |last2=Rees |first2=Charles W. |last3=Tuddenham |first3=David |last4=Williams |first4=David J. |year=1980 |title=7b-Methyl-7bH-cyclopent[cd]indene-1,2-dicarboxylic acid, a new 10-electron aromatic system; X-ray crystal structure |journal=Journal of the Chemical Society, Chemical Communications |issue=15 |pages=691–692 |doi=10.1039/C39800000691}}

File:Cyano 10annulene derivative.svg

When de­proton­ated to form the anion this type of compound is even more stabilized. The central carbanion enhances the molecule's planarity and the number of resonance structures that can be drawn is extended to 7, including two resonance forms with a complete benzene ring. Computational chemistry suggests a tricyclic[10]annulene derivative with an annulated benzene ring and a full set of cyano substituents (7) would be one of the most acidic compounds known, with a computed pK_a in DMSO of −30.4 (compared to for instance −20 for magic acid).{{cite journal |title= Extremal acidity of Rees polycyanated hydrocarbons in the gas phase and DMSO—a density functional study |first1= Robert |last1= Vianello |first2= Zvonimir B. |last2= Maksi |journal= Chemical Communications |year= 2005|issue= 27 |pages= 3412–3414 |doi= 10.1039/B502006A |pmid= 15997281 }}

The other method is to further remove hydrogens and develop triple bonds or cyclopropanes along the ring. Thus com­puta­tional studies suggest that cyclo­deca­tetraene­yne is (although formally a 12-π system) planar and aromatic,{{Cite journal |last1=Navarro Vázquez |first1=Armando |last2=Schreiner |first2=Peter R. |date=2005-06-01 |title=1,2-Didehydro­[10]annulenes: Structures, Aromaticity, and Cycl­iz­ations |journal=Journal of the American Chemical Society |language=en |volume=127 |issue=22 |pages=8150–8159 |doi=10.1021/ja0507968 |issn=0002-7863}} as is bicyclo[8.1.0]undeca-1,3,7,9-tetraen-5-yne.{{Cite journal |last1=Parmar |first1=Karnjit |last2=Blaquiere |first2=Christa S. |last3=Lukan |first3=Brianna E. |last4=Gengler |first4=Sydnie N. |last5=Gravel |first5=Michel |date=Sep 2022 |title=Synthesis of a highly aromatic and planar dehydro­[10]annulene derivative |journal=Nature Synthesis |language=en |volume=1 |issue=9 |pages=696–700 |doi=10.1038/s44160-022-00135-z |bibcode=2022NatSy...1..696P |issn=2731-0582}} Predicting the aroma­ticity of these compounds is not always obvious: the polycyclic hydrocarbon tetra­dihydro­naphtho­[10]annulene, in which a valence isomer of [10]annulene is fused to two naphthalenes, does not exhibit aromaticity inside the central 10-π ring.{{Cite journal |author1=Umeda Rui |author2=Hibi Daijiro |author3=Miki Koji |author4=Tobe Yoshito |date=2009-09-17 |title=Tetradehydro­dinaphtho­[10]annulene: A Hitherto Unknown Dehydro­annulene and a Viable Precursor to Stable Zethrene Derivatives |journal=Organic Letters |language=en |volume=11 |issue=18 |pages=4104–4106 |doi=10.1021/ol9015942 |issn=1523-7060}}

• Azulene is also a 10 π-electron system in which aromaticity is maintained by direct trans­annular bonding to form a fused 7–5 bicyclic molecule.
• Cyclodecatetraene is a stable, non-aromatic 8 π-electron system with no ring strain.{{Rp|page=131}}

{{clear}}

{{reflist}}

{{Annulenes}}

Category:Annulenes

Category:Fully conjugated nonaromatic rings

Category:Ten-membered rings