Chlorination versus bromination

The greater selectivity of bromination can be illustrated by comparing the reaction coordinate diagrams for the formation of primary and tertiary free radicals by hydrogen atom abstraction from 2-methylpropane with bromine versus chlorine radicals (Figure 4.20). 

CS tensor parameters were sensitive to the heteroligand coordinated to Zr, although the EFG tensor stiU dominated spectral appearance. The CS span increased along the ligand series of Cl, Br, and Me, while the differences in basicity between a regular Cp and a methylated Cp ligand were also found to affect CS values. Brominated species exhibited more positive values and larger Q values versus chlorinated zirconocenes, while Cq values were similar. Near-axial CS tensors were reported for many of the compounds. 

Chapter 4, the absorption cross sections of the major sources of bromine to the stratosphere, CH3Br, CBrClF2 (Halon-1211), and CBr3F (Halon-1301), are substantial well out into the 250-nm region. The C-Br bond is even weaker than the C-Cl bond, as is generally the case for bromine compared to chlorine bonds (e.g., Lee et al., 1999a). For example, the C-Br bond strength is 70 kcal mol-1 versus 85 kcal mol-1 for C-Cl and 110 kcal mol-1 for C-F the C-Br bond breaks first ... 

Urry (13) studied hydrogen abstraction from toluene a-d versus toluene by chlorine, bromine and CH COCV radicals and found that the attack on toluene a-d. is slower than that on toluene. The isotope effect was found to be in the range of 2.2 to 9.9. 

To gain more information as to the effect of counter anions on the relative rates of elimination and termination, t-BuBr initiator has been used instead of t-BuQ at selected temperatures in MeCl, MeBr and CH2CI2 solvents. The use of f-BuBr initiator is expected to yield a counter anion with a bromine instead of a chlorine atom. In particular, the object of these experiments was to determine C 2/Ci3 and Cr2/Ci2H + Ci4 ratios in a series of runs and thus to compare the effect of the nature of the halogen in the counter anion on the relative rates of elimination versus termination. Results are shown in Table 7. 

The answer is found by inspection of Figure 3-7 and Table 3-5. The chlorine atom is much more reactive than the bromine atom, as indicated by the Aff values by the hydrogen abstraction step -1-2 kcal mol versus -1-18 kcal mol. Since the transition states of atom abstraction reflect the relative stabilities of the products, it is evident that it is much easier to get to (CH3 -I- HCl) thantofCHj + HBr). 

Under these conditions, the ring is not brominated, and the reaction is quite selective. This is made possible by the fact that the benzylic C-H bond is relatively weak—368 versus 436 kj moH for methane. The same process is possible for chlorine but is a little less selective—the chlorine radical is more reactive than the bromine radical, and polychlorination occurs to some extent. The fluorine radical is too reactive for practical use—this is related to the strength of the H—F bond that is formed on hydrogen abstraction (see Section 12.2). The HI bond, by contrast, is weak, and the process of radical formation is endothermic. Many sets of reaction conditions for this process are known— AT-chlorosuccinimide or AT-bromosuccinimide in the presence of a radical initiator is also widely used (Figure 13.32). We should recall that the Sfjl or 5 2 substitution of benzyl bromide is very facile, so we can use this process to put a range of substituents into the benzylic position (e.g., as in Figure 13.33). 

---