Cyclobutadiene
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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 and a rectangular structure (with a square form hypothetically possible). Though it has alternating single and double bonds, it fails Huckel's rule.
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 antiaromatic rather than aromatic. As a result, it is highly reactive and has a very short lifetime. In fact cyclobutadiene is so unstable and reactive that two cyclobutadiene molecules will instantaneously dimerize by a Diels-Alder reaction under nearly any conditions, even at -78°C.
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 Ce4+.