Bicyclobutane

Bicyclobutane is an organic compound with the formula C₄H₆. It is a bicyclic molecule consisting of two cis-fused cyclopropane rings, and is one of the most strained compounds isolatable on a large scale. It is a colorless easily condensed gas.

The first reported bicyclobutane was the carboxyethyl derivative, C₄H₅CO₂Et, which was prepared by dehydrohalogenation the corresponding bromocyclobutanecarboxylate ester. The parent hydrocarbon was prepared by conversion of bromocyclobutanecarboxylate ester to 1-bromo-3-chlorocyclobutane, followed intramolecular Wurtz coupling using molten sodium:

It is a nonplanar molecule, with a dihedral angle of 123°.