BDEs of C-halogen bonds

The reactivity shown in Scheme 3 results from the low bond dissociation energy (BDE) of the P-H bond [11] k=l.2 10 M s for the H-transfer from R02P(0)H to a primary C-centered radical) and the fast halogen-atom transfer from a C-halogen bond to a phosphonyl radical [9,12] (fc=4 10 M s for f-Bu-Br and k=83 10 M s for Cl3C-Br). Piettre et al. [13] pointed out that these chain reactions were even more efficient when dialkylthiophosphites and the corresponding dialkylphosphinothioyl radicals were involved. 

Until now, we have focused on reactions of alkanes. Now let s consider the radical halogenation of alkenes. For example, consider what outcome you might expect when cyclohexene undergoes radical bromination. Begin by comparing all C—H bonds to identify the bond that is most easily broken. Specifically, compare the BDE for each type of C—H bond in cyclohexene. 

There is a dramatic effect on the C—F BDE. The increase of almost 100 kJ moT1 in the BDE is not reflected in the C—H BDE, which is essentially constant. The effect of increasing C—X BDE with increasing X substitution is not seen for any other halogen. If, for example, we examine the C—Cl bond dissociation energies with increasing chlorination in the chlorinated methanes—the CHC1 series ... 

We can summarize the above discussion as follows increasing the number of fluorines on a carbon atom increases the C—F and C—Cl bond dissociation energy, but it does not have a systematic effect on the C—H BDE. Increasing the number of other halogen atoms on the carbon atom does not systematically increase BDEs and seems to show in some cases a slight systematic decrease in BDE. 

The likelihood of bond homolysis is directly related to the BDE for that bond. The BDE for the H-H bond is 104 kcal/mol, whereas the BDE for the Br Br bond is 46 kcal/mol, so the likelihood of H-H homolysis is much smaller than the likelihood of Br-Br homolysis. Sigma bonds that are particularly prone to homolysis include N-O and 0-0 bonds, bonds between C and very heavy atoms like Pb and I, halogen-halogen bonds, and very strained bonds. 

The BDEs in C-, Si-, Sn-, Pb-, N-, P-, As-, Sb-, Bi-, O, S-, Se-, Te-, and halogen-clusters or complexes are presented in Chapter 25 through Chapter 28, respectively. The arrangements of BDE data in these neutral, cations, anions, clusters, and complexes are shown in the following Periodic Table. Here the numbers represent chapters of the BDE data for these bonds. 

The group of Groves recently reported a series of Mn-catalyzed, highly selective halogenation [173-176] and azidation [177] of aliphatic C(sp )-H bonds (Scheme 2.32). In general, the regioselectivity depends on the BDE of aliphatic... 

The validity of equation (12) has been checked for several families of alkyl halides for which D and E /x- are known (Ref. 32, see particularly figure 6 therein). It was thus found that for v = 0.1 V s the constant is equal to 0.3 eV at 20°C (expressing D in eV and the potentials in V). Equation (12) was then applied to the approximate determination of unknown BDEs in several series of compounds undergoing dissociative electron transfer, namely, TV-halosultams,32 sulfonium cations,33 vicinal dihalides,34 1,3-dihaloadamantes, 1,4-dihalo-bicyclo[2.2.2]octanes, and l,3-dihalobicyclo[l.l.l]pentanes.35 In the latter case, the mutual influence of the two halogens could be rationalized thanks to the conversion of the peak potential data to bond dissociation energies. 

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