In chemistry a carbene is a short-lived and highly reactive organic molecule with a divalent carbon atom with only 6 valence electrons and the general formula: R₁R₂C:. The carbon atom is sp² hybridised with a empty p-orbital extending above and below a plane containing R₁ and R₂ and the free electron pair.

The parent carbene is H₂C: also called methylene. An often encountered carbene is Cl₂C: or dichlorocarbene which can be generated in situ from chloroform and a strong base.

Generally there are two types of carbenes; singlet or triplet carbenes. Singlet carbenes have their electrons paired; triplet carbenes have their electrons unpaired (the nomenclature difference comes from disparate signatures in EPR spectra). Singlet carbenes generally participate in electrocyclic reactions as either electrophiles or nucleophiles. Triplet carbenes should be considered to be diradicals, and participate in stepwise radical additions. Since triplet carbenes are excited state species, they will over time decompose into singlet carbenes if no reaction proceeds.

The reaction of methylene generated from photolysis of diazomethane with cis-2-butene and trans-2-butene is stereospecific which proves that in this reaction methylene is a singlet.^[1]

Carbenes can be stabilized as organometallic species, these fall into two categories:

• Fischer carbenes in which carbenes are tethered to a metal and an electron-withdrawing group (usually a carbonyl),
• schrock carbenes ; in which carbenes are tethered to a metal and an electron-donating group. The reactions that such carbenes participate in are very different from those in which organic carbenes participate.

A carbene is a reactive intermediate but certain organic carbenes discovered in the 1990's are also quite stable. Singlet carbenes derived from imidazolium or thiazolium salts, for example, can be crystallized and isolated as free carbenes and stored without chemical decomposition, if kept in an oxygen- and moisture-free atmosphere ^[2]

Ad stands for the adamantane substituent

references

• ^[1]STRUCTURE OF CARBENE, CH2 Philip S. Skell, Robert C. Woodworth; J. Am. Chem. Soc.; 1956; 78(17); 4496-4497.
• ^[2] A stable crystalline carbene Anthony J. Arduengo, , III Richard L. Harlow, Michael Kline; J. Am. Chem. Soc.; 1991; 113(1); 361-363.