Chain Branching via Cleavage of the Weak Vinyl and Ethynyl Peroxide Bonds

7 Chain Branching via Cleavage of the Weak Vinyl and Ethynyl Peroxide Bonds 

As discussed previously, the RCO—OH bonds are very weak in the vinyl, and ethynyl hydroperoxides. The CO—OC bonds are slightly weaker (ca. 1 - 4 kcal mol ) in the peroxides of this study. The peroxide dissociation energies range from 19.3 to 3.2 kcal mol and these species are unstable towards simple dissociation  

Reasons for this low bond strength involve the formation of the strong carbonyl bond relative to loss of a weaker H bond and the overlap of the H system. 

A chain branching process can result from addition reactions of an alkyl peroxy radical to olefins or acetylenes, where vinyl, or acetyl peroxides are formed. The initial radical formed by the addition of the peroxy radical would undergo an addition reaction with molecular oxygen, then undergo molecular elimination of HO2 to form an unsaturated peroxide. The unsaturated peroxide would then undergo rapid cleavage of the weak peroxide bonds. A mechanism that implements this chain branching results from ROO addition to olefins is illustrated by  

Standard enthalpies of formation, zW°29s are calculated using the B3LYP/6-311G(d,p) calculation level of density functional theory and isodesmic reaction schemes for cancellation of errors. The recommended a//°29s of each value is the average value of data obtained from several isodesmic reactions. Comparison of the thermodynamic properties between the peroxides and corresponding hydroperoxides shows uniformity in property trends. A set oxygenated hydrocarbon groups are determined that result in consistent prediction of thermodynamic properties for the substituted alkyl and vinyl peroxides and hydroperoxides. 

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