Tetravalent chemistry carbenes

The subject of this chapter is carbenes with aryl substituents (aromatic carbenes). These materials are short-lived reactive intermediates in which the normal tetravalency of carbon is reduced by two. Carbenes have been the object of speculation and investigation for more than 80 years. Nevertheless, there still is considerable uncertainty about their chemical and physical properties. In the last five years the pace of research in carbene chemistry has quickened. This is a consequence of the development of high-speed pulsed lasers that permit, for the first time, direct observation of carbenes under the conditions in which they react. This research has provided new information on the effect of structure on the chemical and physical properties of carbenes. 

Consider the methane molecule, CH . The ground stale of a carbon atom is iP corresponding to the electron configuration of Is Zs lp. Carbon in (his state would be divalent because Only two unpaired electrons are available for bonding in the and py orbitals. Although divalent carbon is well known in methylene and carbene intermediates in organic chemistry, stable carbon compounds are tetravalent. fn order for four bonds to form, the carbon atom must be raised to its valence state. This requires... 

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