Cyclohexene bond dissociation energy

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. 

Any of the four equivalent aUyhc hydrogen atoms of cyclohexene can react under these conditions. The vinyl C—H bonds do not react because the C—H bond energy of sp -hybridized carbon atoms is larger than the bond energy of aUyhc C—bonds. The four C—H bonds located at non-allylic sites are also less reactive than the aUyhc C—H bonds. We expect this result based on the difference in the bond dissociation energies cited above for propane versus propene. 

The various radicals produced can all abstract an allylic hydrogen from the organic substrate, cyclohexene in this case. With BDEs of 372kJ/mol, allylic C-H bonds are considerably weaker than regular alkyl-H bonds, whose dissociation energies vary from about 404 kJ/mol for a tertiary C-H bond to as high as 439 kJ/mol for methane. Allylic hydrogen abstraction is shown below for the benzoyloxy radical, a major radical species in the system ... 

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