What Is Cyclobutadiene?
Cyclobutadiene (C4H4) is the simplest of the cyclic hydrocarbons containing a conjugated double bond.
In 1965, R. Pettit showed that cyclobutadiene could be liberated by reacting the iron carbonyl chain of cyclobutadiene with a tetravalent cerium salt.
The number of π-electrons is stable when it consists of six (4π+2), as in benzene.
Cyclobutadiene forms a square by sigma bonding and is unstable because it consists of 4 pi-electrons, making it anti-aromatic.
Uses of Cyclobutadiene
Cyclobutadiene is a molecule that cannot exist because of its anti-aromatic property, consisting of four carbon atoms.
Cyclobutadiene was elucidated to be unstable based on Hückel’s rule, but in the 1970s, the rectangular singlet cyclobutadiene was found to be the most stable structure.
In 2009, Prof. Sekiguchi et al. at the University of Tsukuba, in collaboration with Prof. Apeloig at the Israel Institute of Technology, succeeded for the first time in the world in optically observing cyclobutadiene in the triplet state.