Radical and carbene chemistry

Organic halides play a fundamental role in organic chemistry. These compounds are important precursors for carbocations, carbanions, radicals, and carbenes and thus serve as an important platform for organic functional group transformations. Many classical reactions involve the reactions of organic halides. Examples of these reactions include the nucleophilic substitution reactions, elimination reactions, Grignard-type reactions, various transition-metal catalyzed coupling reactions, carbene-related cyclopropanations reactions, and radical cyclization reactions. All these reactions can be carried out in aqueous media. 

In organo-fluorine chemistry many reactions are known where more or less stable ionic or radical intermediates are formed, for example, carbocations, carbanions. and partially fluorinated and perfluorinated radicals and carbenes. While some of these species are short lived, others are surprisingly stable and isolable. In the latter case electronic and steric arrangements often kinetically stabilize the intermediate. 

Vasella has reviewed the chemistry of glycosyl carbanions, carbocations, radicals and carbenes and related work covering his own important contributions and those of others. Specific areas to have been reviewed are carba-sugars (with ring oxygen atoms replaced by carbon), the enediyne anticancer antibiotics notably calicheamycin and esperamycin (Nicolaou s synthetic work on the disaccharide component especially). ... 

KiRMSE, w. Carbene Chemistry (Academic Press, 2nd Edition, 1971). KLUMPP, G. w. Reactivity in Organic Chemistry (Wiley, 1982). KOCHI, J. K. (Ed.). Free Radicals (Wiley-Interscience, Vols. I and II,... 

The origin of the difference lies in the fact that triplet carbenes are biradicals (or diradicals) and exhibit chemistry similar to that exhibited by radicals, while singlet carbenes incorporate both nucleophilic and electrophilic sites, e.g., for singlet and triplet methylene. 

A radical, often called a. free radical, is a highly reactive and short lived species with an unpaired electron. Free radicals are electron-deficient species, but usually uncharged. So their chemistry is very different from the chemistry of even-electron and electron-deficient species, e.g. carbo-cations and carbenes. A radical behaves like an electrophile, as it requires only a single electron to complete its octet. 

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