CARBENE

any member of a class of highly reactive molecules containing divalent carbon atomsthat is, carbon atoms that utilize only two of the four bonds they are capable of forming with other atoms. Occurring usually as transient intermediates during chemical reactions, they are important chiefly for what they reveal about chemical reactions and molecular structure. In addition, some chemical compounds, particularly those in which the molecules contain carbon atoms arranged in small rings, can best be prepared by the use of carbenes. According to the electronic theory of bonding, bonds between atoms are formed by a sharing of electrons. In terms of this theory, then, a carbene is a compound in which only two of the four valence, or bonding, electrons of a carbon atom are actually engaged in bonding with other atoms. By contrast, in multiple bonded compounds, such as hydrogen cyanide, all four of the valence electrons of the atoms are involved in bonds with other atoms. Because there is no excess or deficiency of electrons in the molecules of carbenes, they are electrically neutral (nonionic). Additional reading W. Kirmse, Carbene Chemistry (1964); G.L. Closs, Topics in Stereochemistry, vol. 3 (1968).