Cyclobutadiene
| Cyclobutadiene | |
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1,3-Cyclobutadiene |
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Other names
Cyclobutadiene |
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| Identifiers | |
| CAS number | 1120-53-2  |
| PubChem | 136879 |
| ChemSpider | 120626 |
| ChEBI | CHEBI:33657 |
| Jmol-3D images | Image 1 Image 2 |
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| Properties | |
| Molecular formula | C4H4 |
| Molar mass | 52.07 g/mol |
 (verify) (what is:  / ?)Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
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 C4H4. It is believed to be in equilibrium between a pair of homologous rectangular/nonplanar ground states and a square, excited triplet state based upon theoretical calculations[1], and through spectroscopic[2] and crystallographic[3][4] investigation of substituted cyclobutadienes, in an argon matrix[citation needed] and inside carceplexes[2]. Though it has alternating single and double bonds, it is predicted triplet, unstable and antiaromatic by Hückel's rule [5], because its ring has 4 π-electrons, and 4 is not twice an odd number. Some cyclobutadiene-metal compounds are stable, thought to be caused by the metal atom providing 2 more electrons to the system. Nonplanar, rectangular distortion to a singlet ground state is caused by the Jahn-Teller Effect[6]. Tetra-t-butyl substituted cyclobutadiene is sufficiently stable to gather x-ray crystallographic images of the structure which has a distorted, nonplanar configuration and double bond lengths longer than normal (1.464 vs expected 1.34)[4]
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[citation needed] 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 electrons 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 dimerizes by a Diels-Alder reaction at 35 K[citation needed]. The monomeric form has been studied at higher temperatures by trapping with matrix isolation in a noble gas[citation needed].
[edit] Synthesis
After numerous attempts, it was first synthesized in 1965 by Rowland Pettit of the University of Texas, although he could not isolate it. Cyclobutadiene can be generated through degradation from a cyclobutadiene metal compound for example C4H4Fe(CO)3 with ammonium cerium(IV) nitrate. This cyclobutadieneiron tricarbonyl complex was prepared from Fe4(CO)9 and cis-dichlorocyclobutene in a double dehydrohalogenation.[7][8]
Cyclobutadiene when liberated from the iron complex reacts with electron-deficient alkynes to a Dewar benzene:[9]
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) is a cyclooctatetraene:[10]
![Acetylene-Acetylene [2 + 2] Cycloadditions Chung-Chieh Lee 2006](/wiki/File:CyclobutadienSynthDessyWhite.png)
[edit] See also
[edit] References
- ^ A theoretical study of the structure of cyclobutadiene H. Kollmar, V. Staemmler; J. Am. Chem. Soc., 1977, 99 (11), pp 3583–3587 [1] doi:10.1021/ja00453a009
- ^ a b The Taming of Cyclobutadiene Donald J. Cram, Martin E. Tanner, Robert Thomas; Angewandte Chemie International Edition in English Volume 30, Issue 8, pages 1024–1027, August 1991 [2]
- ^ Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix Yves-Marie Legrand, Arie van der Lee and Mihail Barboiu; Science 16 July 2010: Vol. 329 no. 5989 pp. 299-302 [3] doi:10.1126/science.1188002
- ^ a b Structure of Tetra-tert-butylcyclobutadiene Hermann Irngartinger, Norbert Riegler1. Klaus-Dieter Malsch, Klaus-Albert Schneider, Günther Maier; Angewandte Chemie International Edition in English Volume 19, Issue 3, pages 211–212, March 1980 [4] doi:10.1002/anie.198002111
- ^ Huckel Theory II J. M. LoBue, Copyright 2002 by the Division of Chemical Education, Inc., American Chemical Society. Link
- ^ A simple quantum mechanical model that illustrates the Jahn-Teller effect Peter Senn; J. Chem. Educ., 1992, 69 (10), p 819 [5] doi:10.1021/ed069p819
- ^ Cyclobutadiene- and Benzocyclobutadiene-Iron Tricarbonyl Complexes G. F. Emerson, L. Watts, R. Pettit; J. Am. Chem. Soc.; 1965; 87(1); 131-133. First Page
- ^ Iron, tricarbonyl (η4-1,3-cyclobutadiene)- R. Pettit and J. Henery Organic Syntheses, Coll. Vol. 6, p.310 (1988); Vol. 50, p.21 (1970) Link
- ^ Cyclobutadiene L. Watts, J. D. Fitzpatrick, R. Pettit J. Am. Chem. Soc.; 1965; 87(14); 3253-3254. Abstract
- ^ 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
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