Carbon-bromine bond, dissociation energy

The last example represents a fairly rare elimination of hydrogen fluoride in preference to hydrogen chloride, a reaction that deserves a more detailed discussion A comparison of bond dissociation energies of carbon-halogen bonds shows that the carbon-fluorine bond is much stronger than the carbon-chlorine, carbon-bromine, and carbon-iodme bonds 108-116, 83 5, 70, and 56 kcal/mol, respec-... 

The dissociation energy of a carbon bromine bond is typically about 210 kJ/moL (a) What is the maximum wavelength of photons that can cause C—Br bond dissociation (b) Which kind of electromagnetic radiation— ultraviolet, visible, or infrared—does the wavelength you calculated in part (a) correspond to ... 

Although free-radical halogenation is a poor synthetic method in most cases, free-radical bromination of alkenes can be carried out in a highly selective manner. An allylic position is a carbon atom next to a carbon-carbon double bond. Allylic intermediates (cations, radicals, and anions) are stabilized by resonance with the double bond, allowing the charge or radical to be delocalized. The following bond dissociation enthalpies show that less energy is required to form a resonance-stabilized primary allylic radical than a typical secondary radical. 

The initiation step provides a radical source by thermal or photochemical dissociation of initiators, which then provides bromine radicals by reaction with Br2. Initiators are sometimes present in the alkene as allyl hydroperoxides which may be present due to inadvertent, prior autooxidation. Bromine or HBr may be present in trace amounts in NBS. Reaction of the bromine radical 20 with the substrate 1 proves selective for allylic or benzylic hydrogens due to the near thermoneutral nature of the reaction which breaks the C-H bond and forms the H-Br bond. Reaction of the formed carbon-centered radical 21 with Br2 provides the desired bromide 3 and Br 20. Hydrogen bromide 17 reacts with NBS to form succinimide 4 and resupplies the required low concentration of Br2. Alternatively, reaction of substrate radical 21 with NBS 2 provides product 3 and succinimidyl radical 22 (S ). Due to energy and kinetics considerations, abstraction of the allylic hydrogen by the S should be slower than abstraction of bromine from NBS by an allyl radical. In using solvents in which NBS, succinimide 4 or it s radical 22 are not very soluble, S is not the key chain-carrier. Byproducts and side-reactions can occur with S. ... 

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