How does carbene dimerization produce an olefin?

The dimerization of carbenes is an intermolecular reaction in which two molecules of a carbene combine to form an olefin (alkene).1

Because carbenes are highly reactive molecules containing a divalent carbon atom—meaning the carbon utilizes only two of its four potential valence bonds—they are typically transient intermediates. When two carbene molecules undergo dimerization, they join together to produce a product with a molecular formula exactly twice that of the original carbene. A representative example of this process is the formation of tetramethoxyethylene from two molecules of dimethoxycarbene.21