Carbenes

unstable organic compounds containing an electrically neutral divalent carbon atom R’R“C: (the dots denote two electrons); they are intermediate particles in many organic reactions. The simplest carbene, methylene, :CH₂, is formed in thermal or photochemical dissociation of diazomethane or ke-tene:

CH₂N₂→:CH₂ + No

Other carbenes may be produced analogously. Dihalocarbenes form during thermal dissociation of alkali salts of trihaloacetic acids:

CCl₃COONa → :CC1₂ + NaCl + CO₂

Various methods are used for stabilizing carbenes, depending on the conditions of generation and the nature of the compounds interacting with the carbenes. For example, the following can occur: (1) dimerization of carbenes, :CH₂ + :CH ₂→CH₂═CH₂; (2) intrusion of carbenes through the carbon-hydrogen bond (for example, into hydrocarbons), R−H + :CH₂ → R− CH₃; or (3) combination of carbenes by a multiple bond (for example, ethylene), with the formation of a three-member ring,

The last reaction is widely used in the synthesis of various three-member cyclic compounds.