Methyl iodide, bond dissociation energy

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 same argument will apply to a localized a bond in a saturated molecule. In this case dissociation gives rise to a localized radical or a pair of such radicals. The photochemical dissociation of an alkyl iodide RI into an alkyl radical R and an iodine atom is a simple example. A further extension is seen if one or both of the new radical centers is adjacent to an even AH. Such a radical will be an odd AH in which the odd electron occupies a NBMO, i.e., an orbital of the same energy as an AO. In this case, dissociation is the reverse of an analog of union (Fig. 6.27). The situation differs from normal union only in the mode of overlap of the two interacting orbitals. Union of an odd AH with methyl gives rise to a product C=CH2 with a localized CC n bond combination of the same radical with methyl gives rise to a product CH—CH3 with a localized CC <7 bond. 

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