CCL: o-hydroquinone to o-quinone reaction mechanism

 Sent to CCL by: "Kalju Kahn" [kalju+/-chem.ucsb.edu]
 Kar,
 Bruce's reply reminded me of an old theoretical work where I looked how
 triplet O2 may react with urate (hydroquinone in disguise) to yield
 something called dehydroisourate (quinone in disguise).  Reactivity in
 this system was a strong function of the ionic form (as one would guess,
 anion was better e-donor than acid) and solvent (radical-ion pair
 stabilized by polar solvent).  The work was published in Bioorganic
 Chemistry, 27(5), 351-362, 1999.
 There are couple of newer studies of the same reaction, that explore some
 of alternative mechanisms.  Other systems with similar chemistry (but with
 possibly more stable radicals) are tetrahydropterin and the reduced
 flavin.
 Best wishes,
 Kalju
 > 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 &
 > Associate Director, Program in Chemical Biology
 > Department of Biological Chemistry
 > University of Michigan Medical School
 > 5220E MSRB III
 > 1150 W. Medical Center Drive
 > Ann Arbor, MI 48109-0606
 > (734) 615-2452
 > brupalf^^umich.edu
 >
 > http://www.biochem.med.umich.edu/?q=palfey
 > http://www.chembio.umich.edu/people/palfey.html
 >
 >
 > On Oct 25, 2011, at 5:15 PM, Tapas Kar tapas.kar||usu.edu wrote:
 >
 >>
 >> Sent to CCL by: "Tapas  Kar" [tapas.kar()usu.edu]
 >> Hello,
 >>
 >> I am looking for o-hydroquinone to o-quinone reaction mechanism in
 >> O2/H2O. I checked literature but could not found any paper describing
 >> step-by-step reaction mechanism and kinetics. Is there any
 >> theoretical/computation study reported on this reaction? I am
 interested
 >> to know how this reaction takes place  I guess some oxygen based
 radical
 >> is involved but what kind? Is singlet O2 or triplet O2 facilitated the
 >> reaction?
 >>
 >>
 >> Thanks
 >>
 >> TK>> To recover the email address of the author of the message,
 please
 >> change>> > > >
 >>
 >>
 >>
 >
 >
 >
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Dr. Kalju Kahn
 Department of Chemistry and Biochemistry
 UC Santa Barbara, CA 93106