Generally, O2 (a triplet) reacts with
singlet organics by an initial single-electron transfer, forming a solvent-caged
superoxide-organic radical pair. Depending on relative rates of the
possible processes, a second electron transfer, coupled to proton transfer (s)
could occur, giving the oxidized organic and H2O2. Or, the radical
intermediate could couple, forming a peroxide which is protonated to the
hydroperoxide. Depending on the organic moiety (like an o-quinone), that
could eliminate H2O2 leaving the unsaturated organic. Or, the radical pair
could separate by diffusion, and these radicals could oxidize the reduced
organic (SET again) in a chain reaction. Another link in a chain reaction
could be the the reaction of oxidized and reduced organic to form to radicals -
especially possible with quinones. I don't know if the specifics of what
actually occurs have been addressed for o-quinones; the relative rates of these
possible reactions are likely to depend on the reduction potentials (which will
vary with substituents) and
solvent.
ciao, Bruce
Bruce A. Palfey
Associate Professor of
Biological Chemistry &
On
Oct 25, 2011, at 5:15 PM, Tapas Kar tapas.kar||usu.edu wrote:
|