Cyclobutadiene
843920
223268761
2008-07-03T09:50:04Z
Nergaal
4761569
{{Chembox new
| Name = '''Cyclobutadiene'''
| ImageFileL1 = Cyclobutadiene structure.svg
| ImageSizeL1 = 100px
| ImageNameL1 = Cyclobutadiene
| ImageFileR1 = Cyclobutadiene3D.png
| ImageSizeR1 = 120px
| IUPACName = 1,3-Cyclobutadiene
| OtherNames = Cyclobutadiene<br />[4]Annulene
| Section1 = {{Chembox Identifiers
| CASNo = 1120-53-2
| SMILES = C1=CC=C1
}}
| Section2 = {{Chembox Properties
| Formula = C<sub>4</sub>H<sub>4</sub>
| MolarMass = 52.07 g/mol
| Density =
| MeltingPt =
| BoilingPt =
}}
}}
'''Cyclobutadiene''' is the smallest [''n'']-[[annulene]] ([4]-annulene), an extremely unstable [[hydrocarbon]] having a lifetime shorter than five seconds in the free state. It has [[chemical formula]] {{carbon}}<sub>4</sub>{{hydrogen}}<sub>4</sub> and a [[rectangle|rectangular]] structure verified by infrared studies. This is in contrast to the [[Square (geometry)|square]] geometry predicted by simple Hückel theory. Though it has alternating single and double [[covalent bond|bonds]], it fails [[Hückel's rule]], because its ring has 4 π-electrons, and 4 is not [[singly even|twice an odd number]]. Some cyclobutadiene-metal compounds are stable because the metal atom provides 2 more electrons to the system.
The [[pi electron]] energy of cyclobutadiene is higher than that of its open-chain counterpart, 1,3-[[butadiene]], and it is therefore said to be anti-aromatic rather than ''[[aromatic]]''. Recent studies on cyclobutadiene show that it has a rectangular structure as opposed to a square structure and two different 1,2-dideuterio-1,3-cyclobutadiene stereoisomers. This indicates that the [[pi electron]]s are localized and therefore not considered to be [[antiaromatic]]. However it is far from stable, it is highly reactive and has a very short lifetime. Cyclobutadiene [[dimer]]izes by a [[Diels-Alder reaction]] at 35 K. The monomeric form has been studied at higher temperatures by trapping with [[matrix isolation]] in a [[noble gas]].
==Synthesis==
After numerous attempts, it was first synthesized in [[1965]] by Rowland Pettit of the [[University of Texas at Austin|University of Texas]], although he could not isolate it. Cyclobutadiene can be generated through degradation from a cyclobutadiene [[metal]] compound for example {{carbon}}<sub>4</sub>{{hydrogen}}<sub>4</sub>{{iron}}({{carbon}}{{oxygen}})<sub>3</sub> with [[ammonium cerium(IV) nitrate]]. This [[cyclobutadieneiron tricarbonyl]] complex was prepared from {{iron}}<sub>4</sub>({{carbon}}{{oxygen}})<sub>9</sub> and cis-dichlorocyclobutene in a double [[dehydrohalogenation]].<ref> ''Cyclobutadiene- and Benzocyclobutadiene-Iron Tricarbonyl Complexes'' G. F. Emerson, L. Watts, R. Pettit; [[J. Am. Chem. Soc.]]; '''1965'''; 87(1); 131-133. [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1965/87/i01/f-pdf/f_ja01079a032.pdf First Page]</ref><ref> ''Iron, tricarbonyl (η4-1,3-cyclobutadiene)-'' R. Pettit and J. Henery [[Organic Syntheses]], Coll. Vol. 6, p.310 ('''1988'''); Vol. 50, p.21 ('''1970''') [http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv6p0310 Link]</ref>
Cyclobutadiene when liberated from the iron complex reacts with electron-deficient [[alkyne]]s to a [[Dewar benzene]]:<ref>''Cyclobutadiene'' L. Watts, J. D. Fitzpatrick, R. Pettit [[J. Am. Chem. Soc.]]; '''1965'''; 87(14); 3253-3254. [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1965/87/i14/f-pdf/f_ja01092a049.pdf Abstract]</ref>
:[[Image:Cyclobutadiene-DewarbenzeneConversion.png|600px|Cyclobutadiene to Dewarbenzene conversion]]
The Dewar benzene converts to [[dimethyl phthalate]] on heating at 90°C.
One cyclobutadiene derivative is also accessible through a [2+2][[cycloaddition]] of a di-[[alkyne]]. In this particular reaction the [[trapping reagent]] is ''2,3,4,5-tetraphenylcyclopenta-2,4-dienone'' and one of the final products (after expulsion of [[carbon monoxide]]) a [[cyclooctatetraene]]:<ref>''Revisit of the Dessy-White Intramolecular Acetylene-Acetylene [2 + 2] Cycloadditions'' Chung-Chieh Lee, Man-kit Leung, Gene-Hsiang Lee, Yi-Hung Liu, and Shie-Ming Peng [[J. Org. Chem.]]; '''2006'''; 71(22) pp 8417 - 8423; (Article) {{DOI|10.1021/jo061334v}}</ref>
:[[Image:CyclobutadienSynthDessyWhite.png|400px|Acetylene-Acetylene [2 + 2] Cycloadditions Chung-Chieh Lee 2006]]
==See also==
* [[Cyclobutene]]
* [[Butadiene]]
==References==
<references/>
{{cycloalkenes}}
[[Category:Annulenes]]
[[ar:بيوتادايين حلقي]]
[[de:Cyclobutadien]]
[[el:Κυκλοβουταδιένιο]]
[[it:Ciclobutadiene]]
[[pt:Ciclobutadieno]]
[[ru:Циклобутадиен]]
[[zh:环丁二烯]]