Cyclooctatetraene

Cyclooctatetraene, also known as annulene, is an unsaturated derivative of cyclooctane with the formula C8H8[1][2]. It is a colorless to light yellow flammable liquid at room temperature[2]. This polyunsaturated hydrocarbon has been the subject of much research and some controversy due to its stoichiometric relationship to benzene[2].

Structure and Bonding

Cyclooctatetraene does not display the chemistry of an aromatic compound, unlike benzene[2]. Early electron diffraction experiments concluded that the C-C bond distances were identical, but X-ray diffraction data demonstrated that cyclooctatetraene adopts several conformations and contains two distinct C–C bond distances, indicating that it is an annulene with fixed alternating single and double C-C bonds[2]. 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°, and its point group is D2d[2].

Synthesis

Cyclooctatetraene was initially synthesized by Richard Willstätter in Munich in 1905 using pseudopelletierine as the starting material and the Hofmann elimination as the key transformation[2]. It can also be prepared by photolysis of barrelene, one of its structural isomers, the reaction proceeding via another isolable isomer, semibullvalene[2]. Cyclooctatetraene derivatives can also be synthesized by way of semibullvalene intermediates[2].

Hazard Information

Cyclooctatetraene is a flammable liquid and a dangerous fire hazard[3]. It can affect individuals when breathed in, and contact can irritate the skin and eyes[3]. Breathing cyclooctatetraene can irritate the nose and throat[3]. Good work practices, such as changing into clean clothing promptly after contamination, providing eye wash fountains and emergency shower facilities, and washing or showering to remove the chemical on skin contact, can help reduce hazardous exposures[3].

Commercial Availability

Cyclooctatetraene is available commercially from suppliers like Sigma-Aldrich, where it is listed as a 98% liquid with 0.1% hydroquinone as an inhibitor[4]. It has a refractive index (n20/D) of 1.537, a boiling point of 142-143 °C, and a melting point of −5-−3 °C[4]. The product is also listed with various hazard warnings, including Aquatic Chronic 3, Asp. Tox. 1, Eye Irrit. 2, Flam. Liq. 2, Skin Irrit. 2, and STOT SE 3[4].

Citations:

[1] https://webbook.nist.gov/cgi/cbook.cgi?ID=C629209&Mask=608&Units=SI

[2] https://en.wikipedia.org/wiki/Cyclooctatetraene

[3] https://nj.gov/health/eoh/rtkweb/documents/fs/0581.pdf

[4] https://www.sigmaaldrich.com/US/en/product/aldrich/138924

[5] https://pubchem.ncbi.nlm.nih.gov/compound/1_3_5_7-cyclooctatetraene