carbenium ions in protic solvents. Base-induced elimination HX from haloforms (CHX3) under phase-transfer conditions. Photolysis of diazirines and epoxides can also be employed. Diazirines are cyclic forms of diazoalkanes. The strain of the small ring makes photoexcitation easy. Photolysis of epoxides gives carbonyl compounds as side products. With asymmetric epoxides, two different carbonyl compounds can potentially form. The nature of substituents usually favors formation of one over the other. One of the C-O bonds will have a greater double bond character and thus will be stronger and less likely to break. Resonance structures can be drawn to determine which part will contribute more to the formation of carbonyl. When one substituent is alkyl and another aryl, the aryl-substituted carbon is usually released as a carbene fragment. Carbenes are intermediates in the Wolff rearrangement Applications of carbenes A large scale application of carbenes is the industrial production of tetrafluoroethylene, the precursor to Teflon. Tetrafluoroethylene is generated via the intermediacy of difluorocarbene: CHClF2 → CF2 + HCl 2 CF2 → F2C=CF2 The insertion of carbenes into C–H bonds has been exploited widely, e.g. the functionalization of polymeric materials and electro-curing of adhesives. The applications rely on synthetic 3-aryl-3-trifluoromethyldiazirines, a carbene precursor that can be activated by heat, light, or voltage. History Carbenes had first been postulated by Eduard Buchner in 1903 in cyclopropanation studies of ethyl diazoacetate with toluene. In 1912 Hermann Staudinger also converted alkenes to cyclopropanes with diazomethane and CH2 as an intermediate. Doering in 1954 demonstrated with dichlorocarbene synthetic utility. See also Transition metal carbene complexes Atomic carbon a single carbon atom with the chemical formula :C:, in effect a twofold carbene. Also has been used to make "true carbenes" in situ. Foiled carbenes derive their stability from proximity of a double bond (i.e. their ability to form conjugated systems). Carbene analogs and carbenoids Carbenium ions, protonated carbenes Ring opening metathesis polymerization References External links Reactive intermediates Functional groups