Carbon-iodine bond, dissociation energy

With the assumptions that (2) represents the slow step and that kj = Ogg calculated Arrhenius parameters for the concurrent bimolecular (1) and unimolec-ular (2) processes. The Arrhenius activation energy for step (2) was calculated to be 43 kcal.mole for each of the alkyl iodides, and, assuming Ogg s mechanism, this value should equal the carbon-iodine bond dissociation energy in each of these molecules. Modern studies show that the D(R—I) values are in the range 50-55 kcal.mole" and for this and other reasons Ogg s mechanism is now considered unsatisfactory. 

From this mechanism, the carbon-iodine bond dissociation energies were estimated by Sullivan as 55 and 52 kcal.mole for methyl iodide and ethyl iodide respectively, these values being in good agreement with those obtained by other tech-niques . Semenov has proposed a similar mechanism but without steps (c) and (e). 

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-iodine bonds 108-116, 83 5, 70, and 56 kcal/mol, respec-... 

In the case of HI the weak carbon-iodine bond (bond dissociation energy = 52 keal/mol [217 kJ/mol]) causes the addition step to be slow ... 

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