Cyclooctatetraene

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Cyclooctatetraene
Cyclooctatetraen.svg Cyclooctatetraene-profile-3D-balls.png
Cyclooctatetraene-3d.png
Identifiers
CAS number 629-20-9 YesY
ChemSpider 553448 YesY
EC-number 211-080-3
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 to −3 °C (23 to 27 °F; 268 to 270 K)
Boiling point 142 to 143 °C (288 to 289 °F; 415 to 416 K)
Solubility in water immiscible
Hazards
EU classification Flammable (F)
Carc. Cat. 1
Muta. Cat. 2
Toxic (T)
R-phrases R45, R46, R11, R36/38,
R48/23/24/25, R65
S-phrases S53, S45
NFPA 704
Flammability code 4: Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily. Flash point below 23 °C (73 °F). E.g., propane Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point −11 °C (12 °F; 262 K)
Autoignition temperature 561 °C (1,042 °F; 834 K)
Related compounds
Related hydrocarbons Cyclooctane
Tetraphenylene
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]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, cyclooctatetraene, C8H8, is not aromatic, although its dianion, C
8
H2−
8
(cyclooctatetraenide) is. Its reactivity is characteristic of an ordinary polyene, i.e. it undergoes addition reactions. Benzene, by contrast, characteristically undergoes substitution reactions, not additions.

History[edit]

1,3,5,7-Cyclooctatetraene was initially synthesized by Richard Willstätter at Munich in 1905:[1][2]

Willstätter's synthesis of cyclooctatetraene (1905)

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). During World War 2, Walter Reppe at BASF Ludwigshafen developed a simple, one-step synthesis of cyclooctatetraene from acetylene, providing material identical to that prepared by Willstätter.[3] Any remaining doubts on the accuracy of Willstätter's original synthesis were resolved when Arthur C. Cope and co-workers at MIT reported, in 1947, a complete repetition of the Willstätter synthesis, step by step, using the originally reported techniques. They obtained the same cyclooctatetraene,[4] and they subsequently reported modern spectral characterization of many of the intermediate products, again confirming the accuracy of Willstätter's original work.[5]

Structure and bonding[edit]

Cyclooctatetraene in its native "tub-shaped" conformation.

Early studies demonstrated that COT did not display the chemistry of an aromatic compound.[6] Then, early electron diffraction experiments concluded that the C-C bond distances were identical.[7] However, X-ray diffraction data from H. S. Kaufman demonstrated cyclooctatetraene to adopt several conformations and to contain two distinct C-C bond distances.[8] 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).[9] The point group of cyclooctatetraene is D2d.[10]

Synthesis[edit]

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%:[3]

Reppe's synthesis of cyclooctatetraene

COT can also be prepared by photolysis of barrelene, one its structural isomers, the reaction proceeding via another isolable isomer, semibullvalene.[11] 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.[12]

Synthesis of octaethylsemibullvalene from 1,2,3,4-tetraethyl-1,4-diiodo-1,3-butadiene and its thermal isomerisation to octaethylcyclooctatetraene

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.

Natural occurrence[edit]

Cyclooctatetraene has been isolated from certain fungi.[13]

Reactions[edit]

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.[14] COT itself —and also analogs with side-chains— have been used as metal ligands and in sandwich compounds.

Cyclooctatetraenide as a ligand[edit]

COT readily reacts with potassium metal to form the salt K2COT, which contains the dianion C8H82−.[15] The dianion is both planar and octagonal in shape and aromatic with a Huckel electron count of 10.

Cyclooctatetraene forms organometallic complexes with some metals, including yttrium and lanthanides. One-dimensional Eu-COT sandwiches have been described as nanowires.[16] The sandwich compounds U(COT)2 (uranocene), and Fe(COT)2, are known.

Uranocene, a sandwich compound containing two COT2− rings.

The compound Fe(COT)2, when refluxed in toluene with dimethyl sulfoxide and dimethoxyethane for 5 days, is found to form magnetite and crystalline carbon also containing carbon nanotubes.[17]

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.[18]

See also[edit]

References[edit]

  1. ^ Mason, S. "The Science and Humanism of Linus Pauling (1901-1994)", Chemical Society Reviews 26, 1 (February 1997).
  2. ^ Richard Willstätter, Ernst Waser (1911). "Über Cyclo-octatetraen". Berichte der deutschen chemischen Gesellschaft 44 (3): 3423–3445. doi:10.1002/cber.191104403216. 
  3. ^ a b Walter Reppe, Otto Schlichting, Karl Klager, Tim Toepel (1948). "Cyclisierende Polymerisation von Acetylen I Über Cyclooctatetraen". Justus Liebigs Annalen der Chemie 560 (1): 1–92. doi:10.1002/jlac.19485600102. 
  4. ^ Arthur C. Cope, C. G. Overberger (1947). "The synthesis of cycloöctatetraene from pseudopelletierine". Journal of the American Chemical Society 69 (4): 976. doi:10.1021/ja01196a513. 
  5. ^ Arthur C. Cope, C. G. Overberger (1947). "Cyclic Polyolefins. I. Synthesis of Cycloöctatetraene from Pseudopelletierine". Journal of the American Chemical Society 70 (4): 1433–1437. doi:10.1021/ja01184a041. 
  6. ^ Johnson, A.W., Sci. Progress; 506; 1947; 35.
  7. ^ Bastiensen, O.; Hassel, O.; Langseth, A. (1947). "The ‘Octa-Benzene’, Cyclo-octatetraene (C8H8)". Nature 160 (4056): 128. Bibcode:1947Natur.160..128B. doi:10.1038/160128a0. 
  8. ^ Kaufman, H. S.; Fankuchen, I.; Mark, H. (1948). "Structure of Cyclo-octatetraene". Nature 161 (4083): 165. Bibcode:1948Natur.161..165K. doi:10.1038/161165a0. 
  9. ^ Thomas, P. M.; Weber, A. (1978). "High resolution Raman spectroscopy of gases with laser sources. XIII - the pure rotational spectra of 1,3,5,7-cyclooctatetraene and 1,5-cyclooctadiene". Journal of Raman Spectroscopy 7 (6): 353–357. Bibcode:1978JRSp....7..353T. doi:10.1002/jrs.1250070614. 
  10. ^ Claus, K. H.; Krüger, C. (15 September 1988). "Structure of cyclooctatetraene at 129 K". Acta Crystallographica Section C Crystal Structure Communications 44 (9): 1632–1634. doi:10.1107/S0108270188005840. 
  11. ^ Zimmerman, H. E.; Grunewald, G. L. (1966). "The Chemistry of Barrelene. III. A Unique Photoisomerization to Semibullvalene". J. Am. Chem. Soc. 88 (1): 183–184. doi:10.1021/ja00953a045. 
  12. ^ Wang, C.; Yuan, J.; Li, G.; Wang, Z.; Zhang, S.; Xi, Z. (2006). "Metal-Mediated Efficient Synthesis, Structural Characterization, and Skeletal Rearrangement of Octasubstituted Semibullvalenes". J. Am. Chem. Soc. 128 (14): 4564–4565. doi:10.1021/ja0579208. PMID 16594680. 
  13. ^ Stinson, M.; Ezra, D.; Hess, W. M.; Sears, J.; Strobel, G. “An endophytic Gliocladium sp. of Eucryphia cordifolia producing selective volatile antimicrobial compounds” Plant Science 2003, volume 165, pp. 913-922. doi:10.1016/S0168-9452(03)00299-1
  14. ^ 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
  15. ^ The cyclooctatetraenyl dianion Thomas J. Katz J. Am. Chem. Soc.; 1960; 82(14); 3784-3785. doi:10.1021/ja01499a077
  16. ^ JST Nanostructed Materials Project Highlights- Prof. Nakajima's Presentation
  17. ^ 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
  18. ^ UCR Fiat Lux: Muscle building - UCR researchers hope to create artificial muscles