Carbon-centered radicals definitions

Definitive kinetic data for the reactivity of cyclopropyl and vinyl radicals were provided by work of Ingold and co-workers [26a,b]. Their investigation of the reactivity of some carbon-centered radicals with tri-n-butylstannane at 30 C [27] showed that the absolute rate constant for those reactions decreased along the series C Hj (59 ) (CH3)2X=CH (35)>c-C3H 5 (8.5)>CH 3 (1.2)>RCH2CH3 (0.27)>(CH3)2CH (0.15) (u rate constant 10 " k ys , sec at 30 C). 

According to the classical definition. Barton esters are mixed anhydrides of carboxylic acids with thio-hydroxamic acid such as I (Scheme Ij. This class of compound was originally developed to allow the transformation of carboxylic acids to a convenient source of radicals for synthetic application. Even now, they are one of the most important entries to C-radicals. Over time, the scope of the reaction was broadened, allowing the generation of heteroatom-centered radicals, particularly oxyl-, aminyl-, and iminyl radicals of synthetic interest. For these transformations, carbonates and carbamates (II), acetates (IV), and ethers (V) were developed (Scheme 1). Finally, oxalates (III) were used for deoxygenation of secondary and tertiary alcohols. The radical fragmentation reaction of these compounds can be carried out either by irradiation or by thermal activation. Both methods are discussed here briefly. 

As seen in the previous section, one characteristic of the triplet state is its paramagnetism. This alone would of course not suffice as a definition of the triplet since there are many odd-electron species that also exhibit paramagnetism but do not exist as triplets. Thus we might state that a triplet is a paramagnetic even-electron species. This still does not constitute a limiting definition since compounds containing even numbers of electrons may exhibit two, three, or even five distinct electronic levels. For example, when in a biradical the radical centers are separated by several carbon atoms as below, no interaction between the electron spins occurs and the radicals appear as two doublet states ... 

---