Acylperoxy groups

The present work deals with the intramolecular interaction of free radicals with peroxide initiators. Our original concept was to examine suitably constructed systems in which it was anticipated that the geometries of the interacting free radical and peroxidic sites could be accurately defined so that the factors which affect these interactions could be isolated. Intramolecular attack of a free-radical on an atomic center with displacement of a radical constitutes an SHi process, a subclass of the SH2 reaction, or homo-lytic substitution ( ). The emphasis in our work is on such displacements on acylperoxy groups, but for a broader understanding of the phenomena ordinary ester groups are being examined as well. 

The intramolecular reactivity of free radicals with acylperoxy groups as a function of distance between the two centers has been delineated by a number of investigations. These radicals were generated by homolysis, free radical addition, or atom abstraction to form the desired system. Radical sites a to acylperoxy groups lead to a-lactone formation as the characteristic reaction (eq. 1), whereas 3-radicals lead to elimination (eq. 2-6). 

Radicals in y or 6 positions relative to acylperoxy groups also lead to lactone formation (eq. 9-12). In these lactone-forming reactions homolytic attack could occur either at carbonyl or peroxidic oxygen (eq. 13) but apparently this has yet to be determined for the examples noted in eq. 9-12. 

In summary the presence of an acylperoxy group and a free radical center in an intermediate may lead to intramolcular induced decomposition by one of several routes, or induced decomposition may not occur. Many interesting examples in which such processes may occur still await study. 

Reaction of butanal with all three reactive species leads primarily to the formation of the butanoyl radical by abstraction of the aldehydic H-atom. Papagni et al. (2000) noted an increase in the rate coefficient of OH with butanal, compared with that with propanal, which is compatible with the sensitization of the C—H bond to the carbonyl group proposed by Kwok and Atkinson (1995). Application of their structure-activity relations (SARs) gives 17% abstraction at this site with a channel yield of 78% for production of the acyl radical. Reaction of the acylperoxy radical with NO2 forms peroxybutanoyl nitrate, while reaction with NO forms CO2 and the -propyl radical, which then reacts with O2 to form the n-propylperoxy radical, whose main atmospheric fate is conversion to propanal. 

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