Free-radical reactions atom localization energy

A second theoretical index, and one for which there appears to be more justification in its application to free-radical reactions, is the atom localization energy. This index is a measure of the energy required to localize one electron of the 7r-electron system in the aromatic molecule at the point of attack of the radical. The formation of the intermediate adduct in a free-radical aromatic substitution may be regarded as the sum of two processes one, the localization of an electron at the point of attack and the other, the pairing of this... 

The dimerization of butadiene, aryl olefins and ethyl vinyl ether is best rationalized by postulating a radical cation 89 (Eq. (174) ) as first intermediate. As the fi -carbon of 89 has the highest free valence, the highest positive charge density and the lowest atom localization energy radical or electrophilic reactions of 89... 

The free-radical arylation of pyridine N-oxides has not been studied systematically, alkylation not at all. When pyridine A-oxide was treated with benzene- and p-chlorobenzenediazonium salts only the 2-arylpyridine jV-oxides were isolated.393 No mention was made of the formation of the 3- and 4-aryl derivatives expected to be produced as well. The phenylation of pyridine N-oxide (diazoaminobenzene at 131° or 181° was found to be the most convenient source of phenyl radicals) was reinvestigated,394 and the reactivities of the nuclear positions found to be in the order 2 > 4 > 3, which is also that predicted6 on the basis of atom localization energy calculations. 2-Phenyl-pyridine N-oxide formed 71-81% of the total phenylation products, whereas the 3-isomer comprised only 5.6-9.6% of that total. The phenylpyridines were found among the by-products of the reaction. 

The potential I speak of is usually called the potential of average force. Insofar as it is to be identified to a thermodynamic potential it is a local Helmholtz free energy as a function of the coordinate positions of all the atoms (or radicals) that must change relative positions in the reaction it may be defined by... 

There is considerable support for the hypothesis that a molecule decomposes unimolecularly when a sufficient amount of energy becomes localized in a particular bond or mode of vibration.19 Knowing these various types of vibration as in the ethyl bromide molecule one is tempted to speculate as to which are the most likely ones to be involved in the chemical reaction. For example, if ethyl bromide decomposes to give a free ethyl radical and a bromine atom as the first step the action must occur by increasing the amplitude of vibration of the fundamental frequency Fi to the point of rupture. On the other hand if the molecule disrupts into ethylene and hydrobromic acid in a single internal operation the fundamental frequency Fq or F8 must be involved. These latter... 

Secondary radicals with free valence in the chain center ( CX ) participate as a rule in the reactions of macromolecular decomposition and the transfer of H atom. Decomposition is the inverse to the reaction of addition. That is why, the lower the polymerization heat, the lower the activation energy of decomposition and the stronger the probability of macromolecular destruction on free valence localization. Polymers characterized by low polymerization heat of their monomers (e.g. the polymerization heat of PMMA is about 55kJ/mole) are strongly decomposing, not only under mechanical effects, but also in other processes of radical formation. 

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