Carbenes are intermediates of the general formula R 2C:. In this configuration, the carbon atom possesses only a sextet of electrons, and is therefore highly reactive and electrophilic. Carbenes are generally prepared by reacting a haloform, such as chloroform, with a strong base, such as sodium ethoxide.
Carbene (H 2C:), however, is prepared by exposing diazomethane to ultraviolet light.
Due to the high reactivity of carbenes, they cannot be isolated. All carbene reactions are run by generating the carbene “in situ,” that is, generating the carbene in the presence of a reagent with which it will immediately react. Alkenes, which are ready sources of electrons, are such reagents. When alkenes react with carbenes, three‐membered rings are formed.
The insertion of a carbene into a π‐bond system is the most common way of preparing cyclopropanes. The addition of the methylene unit, CH 2, to the carbon‐carbon double bond of the alkene is a syn addition.
Some chemicals, namely the carbenoids, behave like carbenes, even though they are not. The most common carbenoid is the Simmons‐Smith reagent, a mixture of iodomethane and a zinc‐copper couple. This reagent also reacts with alkenes to form a cyclopropane ring.
The mechanisms of carbene and carbenoid reactions show the difference between the two. The mechanism for a carbene reaction is a concerted process in which all bonds are broken and formed at one time.
The mechanism for the Simmons‐Smith reaction also shows a concerted addition; however, a carbene is never formed.