Cyclobutadiene
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Image:Cyclobutadiene.png 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 C{{#if:{{{1|}}}|{{{1}}}|}}4H{{#if:{{{1|}}}|{{{1}}}}}4 and a rectangular structure (with a square form hypothetically possible). Though it has alternating single and double bonds, it fails Huckel'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,
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 C{{#if:{{{1|}}}|{{{1}}}|}}4H{{#if:{{{1|}}}|{{{1}}}}}4Fe{{#if:{{{1|}}}|{{{1}}}|}}(C{{#if:{{{1|}}}|{{{1}}}|}}O{{#if:{{{1|}}}|{{{1}}}|}})3 with ammonium cerium(IV) nitrate. This cyclobutadiene iron tricarbonyl complex was prepared from Fe{{#if:{{{1|}}}|{{{1}}}|}}4(C{{#if:{{{1|}}}|{{{1}}}|}}O{{#if:{{{1|}}}|{{{1}}}|}})9 and cis-dichlorocyclobutene 1.
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References
- 1 Cyclobutadiene- and Benzocyclobutadiene-Iron Tricarbonyl Complexes G. F. Emerson, L. Watts, R. Pettit; J. Am. Chem. Soc.; 1965; 87(1); 131-133. First Pagear:بيوتادايين حلقي