There are two types of carbenes: singlets or triplets, depending upon their electronic structure.<ref>Template:Cite book</ref> The different classes undergo different reactions.
Mercuric and organomercury halides (except fluorides) can stably store a wide variety carbenes as the α-halomercury adduct until a mild thermolysis.Template:Cn For example, the "Seyferth reagent" releases CCl2 upon heating:
C6H5HgCCl3 → CCl2 + C6H5HgCl
It remains uncertain which (if any) of such metallated reagents form truly free carbenes, instead of a reactive metal-carbene complex. Nevertheless, reactions with such metallocarbenes generally give the same organic products as with other carbene sources.Template:Sfn
Small-molecule extrusion
Separately, carbenes can be produced from an extrusion reaction with a large free energy change. Diazirines and epoxides photolyze with a tremendous release in ring strain to carbenes, the former to inert nitrogen gas. Epoxides typically give reactive carbonyl wastes, and asymmetric epoxides can potentially form two different carbenes. Typically, the C-O bond with lesser fractional bond order (fewer double-bond resonance structures) breaks. For example, when one substituent is alkyl and another aryl, the aryl-substituted carbon is usually released as a carbene fragment.
The two classes of carbenes are singlet and triplet carbenes. Triplet carbenes are diradicals with two unpaired electrons, typically form from reactions that break two σ bonds (α elimination and some extrusion reactions), and do not rehybridize the carbene atom. Singlet carbenes have a single lone pair, typically form from diazo decompositions, and adopt an sp2 orbital structure.Template:Sfn Bond angles (as determined by EPR) are 125–140° for triplet methylene and 102° for singlet methylene.
At a very high level of generality, carbenes behave like aggressive Lewis acids. They can attack lone pairs, but their primary synthetic utility arises from attacks on π bonds, which give cyclopropanes; and on σ bonds, which cause carbene insertion. Other reactions include rearrangements and dimerizations. A particular carbene's reactivity depends on the substituents, including any metals present.
Singlet and triplet carbenes exhibit divergent reactivity.<ref>Template:March6th</ref>Template:Page needed<ref>Contrariwise, Template:Harvnb states: "The reactivities of carbenes and carbenoids are the same no matter how they are generated." Grossman's analysis is not supported by modern physical organic chemistry texts, and likely refers to rapid equilibration between carbene states following most carbene generation methods.</ref>
Triplet carbenes are diradicals, and participate in stepwise radical additions. Triplet carbene addition necessarily involves (at least one) intermediate with two unpaired electrons.
Insertions are another common type of carbene reaction,Template:Sfn a form of oxidative addition. Insertions may or may not occur in single step (see above). The end result is that the carbene interposes itself into an existing bond, preferably X–H (X not carbon), else C–H or (failing that) a C–C bond. Alkyl carbenes insert much more selectively than methylene, which does not differentiate between primary, secondary, and tertiary C-H bonds.
The 1,2-rearrangement produced from intramolecular insertion into a bond adjacent to the carbene center is a nuisance in some reaction schemes, as it consumes the carbene to yield the same effect as a traditional elimination reaction.Template:Sfn Generally, rigid structures favor intramolecular insertions. In flexible structures, five-membered ring formation is preferred to six-membered ring formation. When such insertions are possible, no intermolecular insertions are seen. Both inter- and intra-molecular insertions admit asymmetric induction from a chiral metal catalyst.
Electrophilic attack
Carbenes can form adducts with nucleophiles, and are a common precursor to various 1,3-dipoles.Template:Sfn
Carbenes and carbenoid precursors can dimerize to alkenes. This is often, but not always, an unwanted side reaction; metal carbene dimerization has been used in the synthesis of polyalkynylethenes and is the major industrial route to Teflon (see Template:Slink). Persistent carbenes equilibrate with their respective dimers, the Wanzlick equilibrium.
Ligands in organometallic chemistry
In organometallic species, metal complexes with the formulae LnMCRR' are often described as carbene complexes.<ref>For a concise tutorial on the applications of carbene ligands also beyond diaminocarbenes, see Template:Cite journal</ref> Such species do not however react like free carbenes and are rarely generated from carbene precursors, except for the persistent carbenes.Template:Cn<ref>Contrariwise, Template:Harvnb</ref> The transition metal carbene complexes can be classified according to their reactivity, with the first two classes being the most clearly defined:
Fischer carbenes, in which the carbene is bonded to a metal that bears an electron-withdrawing group (usually a carbonyl). In such cases the carbenoid carbon is mildly electrophilic.
Schrock carbenes, in which the carbene is bonded to a metal that bears an electron-donating group. In such cases the carbenoid carbon is nucleophilic and resembles a Wittig reagent (which are not considered carbene derivatives).
Carbene radicals, in which the carbene is bonded to an open-shell metal with the carbene carbon possessing a radical character. Carbene radicals have features of both Fischer and Schrock carbenes, but are typically long-lived reaction intermediates.
A large-scale application of carbenes is the industrial production of tetrafluoroethylene, the precursor to Teflon. Tetrafluoroethylene is generated via the intermediacy of difluorocarbene:<ref name="William">Template:Cite book</ref>
