posted on 2020-04-08, 15:06authored byGraham
E. Garrett, Derek A. Pratt, J. Scott Parent
The
hydrogen atom transfer reactions that underlie peroxide-initiated
modifications of polyolefin architectures and ambient-temperature
air oxidations are examined through a combination of experiments and
quantum chemical calculations on model hydrocarbons. The regioselectivity
of H-atom from pentane, 2,4-dimethylpentane, and 2,2,4,4-tetramethylpentane
to t-BuO• generated from tert-butyl hyponitrite at 25 °C was quantified by trapping
of the alkyl radicals with a UV-active nitroxide. The data show that
hydrocarbon reactivity toward t-BuO• does follow neither well-established trends in homolytic C–H
bond dissociation energy (tertiary > secondary > primary) nor
trends
in the susceptibility of polyolefins to autoxidation. These experiments,
together with quantum chemical calculations conducted at the CBS-QB3
level of theory, reveal the importance of entropic effects in these
exergonic H-atom transfer processes. In contrast, the endergonic nature
of H-atom abstraction by t-BuOO• produces conventional hydrocarbon reactivity patterns, as enthalpic
contributions to transition-state energies are dominant. These findings
bring clarity to the structure/reactivity relationships of polyolefins
in the field of radical chemistry.