Cyclooctatetraene Identifiers CAS number ChemSpider RTECS number CY1400000 Jmol-3D images Image 1 Properties Molecular formula C8H8 Molar mass 104.15 g/mol Appearance Clear yellow Density 0.9250 g/cm3, liquid Melting point
-5 – -3 °C (268 – 270 K)
142 – 143 °C (415 – 416 K)
Solubility in water immiscible Hazards EU classification Flammable (F)
Carc. Cat. 1
Muta. Cat. 2
R-phrases , , , ,
S-phrases , NFPA 704 Flash point -11 °C Autoignition
561 °C Related compounds Related hydrocarbons Cyclooctane
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because of its stoichiometric relationship to benzene, COT has been the subject of much research and some controversy.
Unlike benzene, C6H6, however, cyclooctatetraene, C8H8, is not aromatic, although its dianion, C8H82- (cyclooctatetraenide), is. Its reactivity is characteristic of an ordinary polyene, i.e. it undergoes addition reactions. Benzene, by contrast, characteristically undergoes substitution reactions, much as alkanes do, not additions.
Willstätter noted that the compound did not exhibit the expected aromaticity. Between 1939 and 1943, chemists throughout the US unsuccessfully attempted to synthesize COT. They rationalized their lack of success with the conclusion that Willstätter had not actually synthesized the compound but instead its isomer, styrene. Willstätter responded to these reviews in his autobiography, where he noted that the American chemists were 'untroubled' by the reduction of his cyclooctatetraene to cyclooctane (a reaction impossible for styrene). In 1947, Walter Reppe at Ludwigshafen at last repeated Willstätter's synthesis.
Structure and bonding
Early studies demonstrated that COT did not display the chemistry of an aromatic compound. Then, early electron diffraction experiments concluded that the C-C bond distances were identical. However, X-ray diffraction data from H. S. Kaufman demonstrated cyclooctatetraene to adopt several conformations and to contain two distinct C-C bond distances. This result indicated that COT is an annulene with fixed alternating single and double C-C bonds.
In its normal state, cyclooctatetraene is non-planar and adopts a tub conformation with angles C=C-C = 126.1° and C=C-H = 117.6° (of the Cx atom in C-HCx=C bound).
Richard Willstätter's original synthesis (4 consecutive elimination reactions on a cyclooctane framework) gives relatively low yields. Reppe's synthesis of cyclooctatetraene, which involves treating acetylene at high pressure with a warm mixture of nickel cyanide and calcium carbide, was much better, with chemical yields near 90%:
COT can also be prepared by photolysis of barrelene, one its structural isomers, the reaction proceeding via another isolable isomer, semibullvalene. COT derivatives can also be synthesised by way of semibullvalene intermediates. In the sequence illustrated below, octaethylcyclooctatetraene (C8Et8) is formed by thermal isomerisation of octaethylsemibullvalene, itself formed by copper(I) bromide mediated cyclodimerisation of 1,2,3,4-tetraethyl-1,4-dilithio-1,3-butadiene.
Because COT is unstable and easily forms explosive organic peroxides, a small amount of hydroquinone is usually added to commercially available material. Testing for peroxides is advised when using a previously opened bottle; white crystals around the neck of the bottle may be composed of the peroxide, which may explode when mechanically disturbed.
Cyclooctatetraene has been isolated from certain fungi.
The π bonds in COT react as usual for olefins, rather than as aromatic ring systems. Mono- and polyepoxides can be generated by reaction of COT with peroxy acids or with dimethyldioxirane. Various other addition reactions are also known. Furthermore, polyacetylene can been synthesized via the ring-opening polymerization of cyclooctatetraene. COT itself —and also analogs with side-chains— have been used as metal ligands and in sandwich compounds.
Cyclooctatetraenide as a ligand
Cyclooctatetraene forms organometallic complexes with some metals, including yttrium and lanthanides. One-dimensional Eu-COT sandwiches have been described as nanowires. The sandwich compounds U(COT)2, or uranocene and Fe(COT)2, are known.
Because COT changes conformation between tub-shaped and planar with addition or subtraction of electrons, it could, in principle, be used to construct artificial muscles. Such devices have been contemplated to be makeable by grafting COT derivatives to a backbone of a suitable conducting polymer, which would supply or remove the reducing equivalents.
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- ^ Two Undergraduate Experiments in Organic Polymers: The Preparation of Polyacetylene and Telechelic Polyacetylene via Ring-Opening Metathesis Polymerization Eric J. Moorhead and Anna G. Wenzel Journal of Chemical Education • Vol. 86 No. 8 August 2009 973
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- ^ JST Nanostructed Materials Project Highlights- Prof. Nakajima's Presentation
- ^ Crystalline Graphite from an Organometallic Solution-Phase Reaction Erich C. Walter, Tobias Beetz, Matthew Y. Sfeir, Louis E. Brus, and Michael L. Steigerwald J. Am. Chem. Soc.; 2006; 128(49) pp 15590 - 15591; (Communication) doi: 10.1021/ja0666203
- ^ UCR Fiat Lux: Muscle building - UCR researchers hope to create artificial muscles
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