karbonene

Carbenes, sometimes referred to in Norwegian as karbonene, are reactive chemical species containing a neutral carbon atom that bears only six valence electrons. The carbon is typically divalent, bonded to two substituents and bearing two nonbonding electrons. Traditional descriptions distinguish singlet carbenes, in which the two nonbonding electrons are paired in one orbital, from triplet carbenes, with two unpaired electrons in separate orbitals. The simplest example is methylene, :CH2.

Reactivity depends on spin state. Singlet carbenes are generally electrophilic and tend to form covalent bonds by inserting into C–H or C=C bonds and by cyclopropanation of alkenes. Triplet carbenes behave more like diradicals and often react by radical-type pathways or combine with other radicals. This duality underpins many synthetic applications.

Carbenes can be generated by several methods. From diazo compounds undergoing thermolysis or photolysis with extrusion of nitrogen gas, or by dehalogenation of dihalomethanes such as chloroform to give dichlorocarbene. Highly stabilized carbenes, notably N-heterocyclic carbenes (NHCs), are isolable and widely used as ligands in organometallic catalysis and as organocatalysts. Other stable carbenes include cyclic and mesoionic carbenes.

Applications of carbenes span organic synthesis, notably cyclopropanation and X–H insertion reactions, and organometallic chemistry, where NHC ligands support many cross-coupling and polymerization catalysts. The study of carbenes also informs fundamental reaction mechanisms and the development of metal-free catalysis and novel reagents. Safety and handling reflect their high reactivity and, for many reactive singlet and triplet species, short lifetimes.