A carbene has a general formula of R₂C:. Carbenes are one of the principle groups of reactive organic intermediaries and they are highly reactive due to the two unpaired electrons. Photolysis of a diazoalkane or a simple elimination reaction will produce a carbene. Dihalocarbenes such as Cl₂C: have the greatest synthetic value as they frequently act as intermediates in organic syntheses.

Carbenes can act as electron donors in chemical reactions. They are intermediates in such reactions as the change of a carboxylic acid to its next highest homologue or to a derivative of a homologous acid such as an ester or an amide. (A homologous series is a group of organic compounds that have the same functional group and a regular structural pattern so that a member of the series differs from the next member by a fixed number of atoms.) The reaction of a base with a trihalogen derivative of methane is an example of a commonly encountered reaction which proceeds via a carbene intermediate.

Carbenes are so reactive that they are unlikely to be frequently encountered. In any reaction of which they are a part, they rapidly move to the next stage of the synthesis. It is only in reactions with transition metals that carbenes are stable, due to the distribution of electrons within the structure formed.