Carbanions, Free Radicals, Carbenes, and Nitrenes

Carbocations, Carbanions, Free Radicals, Carbenes, and Nitrenes 

There are four types of organic species in which a carbon atom has a valence of only 2 or 3/ They are usually very short lived, and most exist only as intermediates that are quickly converted to more stable molecules. However, some are more stable than others and fairly stable examples have been prepared of three of the four types. The four types of species are carhocations (A), free radicals (B), carbanions (C), and carbenes (D). Of the four, only carbanions have a complete octet around the carbon. There are many other organic ions and radicals with charges and unpaired electrons on atoms other than carbon, but we will discuss only nitrenes (E), the nitrogen analogs of carbenes. 

Each of the five types is discussed in a separate section, which in each case includes brief summaries of the ways in which the species form and react. These summaries are short and schematic. The generation and fate of the five types are more fully treated in appropriate places in Part 2 of this book. 

we must say a word about the naming of A. For many years these species were called carbonium ions , though it was suggested as long ago as 1902 that this was inappropriate because -onium usually refers to a covalency higher than that of the neutral atom. Nevertheless, the name carbonium ion was well established and created few problems until some years ago, when George Olah and his co-workers found evidence for another type of intermediate in which there is a positive charge at a carbon atom, but in which the formal covalency of the carbon atom is 5 rather than 3. The simplest example is the methanonium ion CHj (see p. 770). Olah proposed that the name carbonium ion be henceforth reserved for penta-coordinated positive 

Carbocations are intermediates in several kinds of reactions. The more stable ones have been prepared in solution and in some cases even as solid salts, and X-ray crystallographic structures have been obtained in some cases. An isolable dioxa-stabilized pentadienylium ion was isolated and its structure was determined by h, C NMR, mass spectrometry (MS), and IR. A P-fluoro substituted 4-methoxy-phenethyl cation has been observed directly by laser flash photolysis. In solution, the carbocation may be free (this is more likely in polar solvents, in which it is solvated) or it may exist as an ion pair, which means that it is closely associated with a negative ion, called a counterion or gegenion. Ion pairs are more likely in nonpolar solvents. 

CARBOCATIONS, CARBANIONS, FREE RADICALS, CARBENES, AND NITRENES 

First we must say a word about the naming of A. For many years these species were called carbonium ions, though it was suggested3 as long ago as 1902 that this was inappropriate 

Subsequently it was found that the same cations could also be generated from alcohols in super acid-S02 at - 60°C 1 and from alkenes by the addition of a proton from super acid or HF-SbF5 in S02 or SO20IF at low temperatures.12 Even alkanes give carbocations in super acid by loss of H. For example,13 isobutane gives the r-butyl cation 

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CARBOCATIONS, carbanions, free radicals, carbenes, and nitrenes... 

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