Free carbanion

Carbocations, Carbanions, Free Radicals, Carbenes, and Nitrenes... 

CARBOCATIONS, CARBANIONS, FREE RADICALS, CARBENES, AND NITRENES... 

Most of the reactions of triplet carbenes discussed in this chapter will deal with reactions in solution, but some reactions in the gas phase will also be included. Triplet carbenes may be expected to show a radical-like behaviour, since their reactions usually involve only one of their two electrons. In this, triplet carbenes differ from singlet carbenes, which resemble both carbenium ions (electron sextet) and carbanions (free electron pair). Radical like behaviour may, also be expected in the first excited singlet state Sr e.g. the state in CH2) since here, too, two unpaired electrons are present in the reactive intermediate. These Sj-carbenes are magnetically inert, i.e., should not show ESR activity. Since in a number of studies ESR spectra could be taken of the triplet carbene, the reactions most probably involved the Ti-carbene state. However, this question should be studied in more detail. 

The major carbon centered reaction intermediates in multistep reactions are carboca-tions (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [ ]+, [ ]-, [ ], and [ CI I21-... 

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

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