Abnormal solvent effects on hydrogen atom abstraction. 3. Novel kinetics in sequential proton loss electron transfer chemistry.

[reaction: see text] A prolonged search involving several dozen phenols, each in numerous solvents, for an ArOH/2,2-diphenyl-1-picrylhydrazyl (dpph(*)) reaction that is first-order in ArOH but zero-order in dpph(*) has reached a successful conclusion. These unusual kinetics are followed by 2,2'-methylene-bis(4-methyl-6-tert-butylphenol), BIS, in five solvents (acetonitrile, benzonitrile, acetone, cyclohexanone, and DMSO). In 15 other solvents the reactions were first-order in both BIS and dpph(*) (i.e., the reactions followed "normal" kinetics). The zero-order kinetics indicate that in the five named solvents the BIS/dpph(*) reaction occurs by sequential proton loss electron transfer (SPLET). This mechanism is not uncommon for ArOH/dpph(*) reactions in solvents that support ionization, and normal kinetics have always been observed previously (see Litwinienko, G.; Ingold, K. U. J. Org. Chem. 2003, 68, 3433 and Litwinienko, G.; Ingold, K. U. J. Org. Chem. 2004, 69, 5888). The zero-order kinetics found for the BIS/dpph(*) reaction in five solvents, S, imply that BIS ionization has become the rate-determining step (rds, rate constants 0.20-3.3 s(-)(1)) in the SPLET reaction sequence: S + HOAr right harpoon over left harpoon S- HOAr SH(+) + (-)OAr SH(+) + (*)OAr + dpph(-) --> S + (*)OAr + dpph-H, where ArOH = BIS. Some properties specific to BIS that may be relevant to its relatively slow ionization in the five solvents are considered.