A transient species of
the form RR′C:, with two valence electrons
that do not form bonds. The simplest example
is methylene, H2C:. Carbenes are
short-lived intermediates in some organic
reactions. They attack double bonds to
form three-membered rings (i.e. cyclopropane
derivatives). They also attack single
bonds in insertion reactions, for
example:
R–O–H + R2C: → R-O-C(R2)H
R–H + R2C: → R–C(R2)-H
Carbenes are important ‘reagents’ in organic
synthesis and various methods exist
for generating them in the reaction
medium.
An organic radical containing divalent carbon. Some divalent-carbon derivatives where the carbon is multiple bonded to oxygen or nitrogen are stable compounds (carbon monoxide); most are highly reactive units known only as reaction intermediates. Carbenes, carbonium ions, carbanions, and free radicals are the four most important classes of organic reaction intermediates containing carbon in an unstable valence state. A typical synthesis involving a carbene is that of cyclopropanes by the addition of carbenes to olefins.
