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Cyclobutadiene is the smallest [n]-annulene (-annulene), an extremely unstable hydrocarbon having a lifetime shorter than five seconds in the free state. It has chemical formula C4H4 and a rectangular structure verified by infrared studies. This is in contrast to the square geometry predicted by simple Hückel theory. Though it has alternating single and double bonds, it fails Hückel's rule, because its ring has 4 π-electrons, and 4 is not twice an odd number. Some cyclobutadiene-metal compounds are stable because the metal atom provides 2 more electrons to the system.
Additional recommended knowledge
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 non-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 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.
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 compounds 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.
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:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Cyclobutadiene". A list of authors is available in Wikipedia.|