Effect of fluorine substituents on the structure, stability, and

9 Effect of fluorine substituents on the structure stability, and reactivity of fluoroalkyl radicals 

Taking into account the marked pyramidalization of carbon radicals by a-fluorine substituents, it is interesting to see whether benzylic or allylic conjugation would make the fluoroalkyl radicals planar or not. The observed a(19F) hfs constants (42.6 and 39.7 G) for 1,1,3,3-tetrafluoroallyl radical [18] are smaller than that for CH3CF2 radical (94.0 G) 

Generation and stereospecific reaction of fluorocyclopropyl radicals provided experimental evidence for the nonplanarity of the fluoroalkyl radical [23]. Hunsdiecker reaction 

Relative BDEs have been used to estimate the relative radical stabilities. Therefore, the smaller BDE is a measure for the higher stability of the radicals that are formed by the homolytic dissociation of the bond. For example, BDEs of a series of substituted alkyl C—H bonds decrease in the order of the degree of substitution, CH3—H (104.8), CH3CH2—H (101.1), 

The ionization potentials (IP), electron affinities (EA), and absolute electronegativities of fluoroalkyl radicals are useful in order to elucidate the nucleophilic and electrophilic reactivities of the fluoroalkyl radicals. Table 1.36 summarizes available IP and EA data, indicating that (1) all of the a-fluoromethyl radicals have lower IPs than methyl radicals in spite of the strong inductive effect of the fluorine atom, and (2) trifluoromethyl and pentafluoroethyl radicals are, of course, more electrophilic than methyl and ethyl radicals because of their higher values of EA [30]. The former result may arise from the electron-donating conjugation of lone-pair electrons on the fluorine atom, and the latter is due to the strong inductive effect of the fluorine atom. 

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