Allyl bromide, bond dissociation energy

Arenesulfonyl iodide and bromide are rather unstable compounds because of low bond dissociation energies of their S02-I and S02-Br. Therefore, treatment of p-tosyl bromide (47) with alkene or allene (48) produces 1,2-adduct (49) through the addition of the formed p-tosyl radical onto the allene as shown in eq. 4.19 [52]. Here, the p-tosyl radical attacks the central sp carbon of the allene group to generate the stable allylic radical, and then it reacts with p-tosyl bromide to give 1,2-adduct (49) and a p-tosyl radical again, i.e., chain pathway. So, this is also an atom(group)-transfer reaction. 

The main products of the reaction are hydrogen bromide, propene and benzene, with smaller amounts of 1-bromopropene and 2-bromopropane. The mechanism requires the overall activation energy to be equal to that for the first step. While these two energies are very close for allyl bromide (estimates are 45.5 and 47.5 kcal.mole for reaction 1, and 45.4 kcal.mole" for the overall activation energy ), the C-Br bond dissociation energy of 65 kcal.mole for ethyl bromide is considerably greater than the overall activation energy of 53 kcal. 

A more detailed analysis of the radical mechanisms has been presented . Generally, all three processes show first-order kinetics but Ej reactions do not exhibit an induction period and are unaffected by radical inhibitors such as nitric oxide, propene, cyclohexene or toluene. For the non-chain mechanism, the activation energy should be equivalent to the homolytic bond dissociation energy of the C-X bond and within experimental error this requirement is satisfied for the thermolysis of allyl bromide For the chain mechanism, a lower activation energy is postulated, hence its more frequent occurrence, as the observed rate coefficient is now a function of the rate coefficients for the individual steps. Most alkyl halides react by a mixture of chain and E, mechanisms, but the former can be suppressed by increasing the addition of an inhibitor until a constant rate is observed. Under these conditions a mass of reliable reproducible data has been compiled for Ej processes. Necessary conditions for this unimolecular mechanism are (a) first-order kinetics at high pressures, (b) Lindemann fall-off at low pressures, (c) the absence of induction periods and the lack of effect of inhibitors and d) the absence of stimulation of the reaction in the presence of atoms or radicals. 

In contrast, the less strained four-7r-electron cyclopentadienyl cation is very unstable. Its p r+ has been estimated as --40, using an electrochemical cycle. The heterolytic bond dissociation energy to form the cation from cyclopentadiene is 258 kcal/mol, which is substantially more than for formation of an allylic cation from cyclopentene but only slightly more than the 252 kcal/mol for formation of an unstabilized secondary carbocation. " Solvolysis of cyclopentadienyl halides assisted by silver ion is extremely slow, even though the halide is doubly allylic. When the bromide and antimony pentafluoride react at -78°C, the EPR spectrum observed indicates that the cyclopentadienyl cation is a triplet. Similar studies indicate that pentachlorocyclopentadienyl cation is also a triplet, but the ground state of the pentaphenyl derivative is a singlet. 

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. ... 

---