Cyclobutadiene

Structural formula
General
Surname	Cyclobutadiene
other names	1,3-cyclobutadiene [4] Annulene
Molecular formula	C 4 H 4
External identifiers / databases
CAS number 1120-53-2 PubChem 136879 Wikidata Q415302
properties
Molar mass	52.07 g mol −1
safety instructions
GHS hazard labeling no classification available
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Cyclobutadiene ( CBD ) is an extremely unstable diene with the molecular formula C ₄ H ₄ . In free form it breaks down to 1,3-butadiene within 5 seconds . Cyclobutadiene is the smallest annulene and, as such, can only be trapped in solid matrices of inert gases at low temperatures without dimerizing. It is a cyclic system with conjugated double bonds . With an even number of electron pairs, it is an anti-aromatic according to Hückel's rule and thus a di- radical with a low mesomerism stabilization energy . Therefore it is highly reactive and dimerizes under almost all conditions by the Diels-Alder reaction , the reaction takes place even at −78 ° C. The endo isomer is preferably formed , while the exo isomer is only formed as a by-product.

After numerous unsuccessful attempts, Rowland Pettit was able to manufacture it for the first time at the University of Texas in 1965, but he was unable to isolate it. Cyclobutadiene can be produced by reducing its metal compounds.

Cyclobutadiene is a stable ligand in many metal complexes. This is due to the fact that two electrons are pushed by the metal into ligand orbitals, which creates a hückel-aromatic 6π-electron system. CBD can mostly be split off from the metal center by oxidation (e.g. by cerium (IV) nitrate ).

Once released from the metal complexes, cyclobutadiene can be reacted with electron-poor alkynes. First, Dewar benzene is produced , which then slowly rearranges to form a benzene derivative . Furthermore, cyclobutadiene can be obtained from the decomposition of 2-pyrone . After the thermally controlled ring closure, the diene is obtained by splitting off CO ₂ .

Individual evidence

1. ↑ This substance has either not yet been classified with regard to its hazardousness or a reliable and citable source has not yet been found.
2. ↑ G. Maier: The cyclobutadiene problem . In: Angewandte Chemie . tape 86 , no. 14 , 1974, p. 491-505 , doi : 10.1002 / anie.19740861402 .
3. ↑ Joachim Buddrus: Fundamentals of Organic Chemistry , Walter de Gruyter Verlag, Berlin, 4th edition, 2011, p. 431, ISBN 978-3-11-024894-4 .