Bromine bond strengths

In Fig. 1 the relative metal-chlorine and metal-bromine bond strengths are plotted against the atomic number of the metal. The filled-in circles and squares represent the values corrected for the crystal field stabilisation energy. 

The relationships between rate of cleavage, bond strength and radical-anion redox potential can be combined in one concept. In this, cleavage rate is dependent on a reaction driving force, defined as the difference between the redox potential of the substrate radical-anion and the redox potential of the product anion in equ-librium with the coiresponding radical (E° for bromine ion, bromine radical as an example). 

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 ... 

In spite of the difference in bond strengths, hydrogen abstraction is favoured over bromine abstraction by a factor of about 2 1. All the processes discussed above represent chain propagating steps, but often the chains are short and... 

Ah initio calculations were performed to obtain the bond strength and the relative rate coefficients, using the Caussian-3 methodology previously extended to brominated compounds by Curtiss et al. [40,41]. The results of the calculations... 

Energetics Bond strength tables allow the calculation of the AW for the process. Highly exothermic abstractions tend to have poor selectivity. For example, C-H abstractions by a bromine radical (endothermic) are much more selective than C-H abstractions by a chlorine radical (exothermic). 

Another example of data of this type pertains to the reaction of f-butoxy radicals with hydrocarbons. The f-butoxy radical is more reactive and less selective than bromine atoms and the are quite low. There is a correlation corresponding to Equation (3.32), with a = 0.3 for hydrocarbons with C-H bond strengths greater than 90kcal/mol. For weaker bonds, the levels off at about 2kcal/mol. We say more about the relationship between E for hydrogen abstraction and the enthalpy of the reaction in Topic 11.2. 

The role of the halogen atom in the precursors was also explored. Although bromine was mostly used, iodine and chlorine were also employed. While the former gave very efficient cydisation (but with a far more complex mixture of products), the latter gave no reaction at all, reflecting the order of C-X bond strength, Cl>Br>I. 

Radicals are very unstable and reactive, and these bromine radicals may simply recombine or they may react with other compounds. You already know that bromide anions are good nucleophiles in Sfj2 reactions, but bromine radicals do two quite different reactions abstraction and addition. The Br radical may abstract a hydrogen atom from the alkene or it may add to the tc bond. Notice that each reaction produces a new carbon-centred radical and, in the first case, a molecule of HBr. Whereas the Br—Br bond is weak, the H—Br bond is much stronger (366 kj mol" ) and, unlike ionic reactions, radical reactions are dominated by bond strength. 

---