Carbon-bromine bond, dissociation

Reaction (IV) is possible and in fact quite likely. The breaking of a carbon-bromine bond involves the absorption of 58,000 calories and this minimum is close enough to the 55,000 calories to be within the limit of uncertainty of the constants involved. This reaction can account for the experimental results and is in line with spectroscopic evidence that ethyl halides dissociate photo-chemically into the halogen atom and the free radical. 

As in direct metalation, the reaction occurs at the metal surface. An electron is transferred from the surface to the <7 antibonding orbital of the carbon-bromine bond to produce the anion radical in the rate-determining step " (equation 1). The anion radical can then dissociate at the surface to the 1-methyl-2,2-diphenylcyclopropyl radical (equation 2). At this point some racemization may occur and the radical can undergo a number of indistinguishable reactions. The radical may pick up another electron to yield the anion (equation 3) or since mercury is such an efficient radical trap, the radical may become adsorbed on the mercury surface (equation 4) from which it can either take another electron to yield the anion (equation 5) or combine with another adsorbed radical to produce a dicyclopropylmercury (equation 6). 

Delocalized highly polarizable anionic transition states occur in certain elimination reactions, too. The breakdown of erythro-dibromophenylpropanoic acid 32 provides an example which can serve to test the influence of the tetrahedral ammonium hosts upon two alternative competing reaction pathways. The investigations of Groven-stein and Cristol revealed that 32 may either undergo rate limiting dissociation of the benzylic carbon-bromine bond which ultimately leads to the formation of... 

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 ... 

Mungull et al. [704] reported that 1,2-dibromotetrafluoroethane was an efficient initiator for the photo-polymerization of TFE. However, no polymer formation was detected when a Pyrex filter was used instead of the quartz window. Since 1,2-dibromotetrafluoroethane absorbs strongly at X < 270 nm photo-dissociation of the carbon-bromine bond was made responsible for the initiation. Other halocarbons were tested as initiators in this reaction CCI4, iodomethane, bromoethane, iodoethane, bromotrifluoro-methane, and pentafluoroiodoethane. All of these materials did act as initiators but yielded only oily products. 

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-... 

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. 

Generally, in bromine addition to carbon-carbon double bonds, bromine bridging, solvent dependent dissociation of the ionic intermediates, steric interactions between the counteranion and the first bonded halogen during the nucleophilic step, the possibility of carbon-carbon rotation in the carbenium ion intermediate, preassociation phenomena and nucleophilic assistance determine the stereochemical behavior of the reaction . Several of these factors have been invoked to explain the stereochemistry of bromine addition to dienes, although others have been completely ignored or neglected. Bromine addition to cyclopentadiene, 1,3-cyclohexadiene, 2,4-hexadienes and 1,3-pentadienes has been examined repeatedly by Heasley and coworkers and the product distribution has been... 

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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