Many assays used for the assessment of radical scavenging employ the artificial radicals:
2,2-diphenyl-1-picrylhydrazyl radical (dpph•) and radical cation formed from 2,2′-azino-
bis(3-ethylbenzothiazoline-6-sulfonic acid) ABTS+•. Both species are strongly electron deficient,thus,the overall kinetics for their reactions with phenolic antioxidants contain a strong component of two mechanisms: Sequential Proton-Loss Electron-Transfer1-3 and Hydrogen Atom Transfer. In non-polar solvents phenols (ArOH) react with dpph• via HAT mechanism, while in polar, protic solvents the reaction is much faster due to participation of SPLET mechanism (see Fig. 1A). In our work we measured the absolute rate constants for reaction of dpph• with series of phenols presented in Fig. 1B in methanol, and acidified methanol(in order to eliminate SPLET). We present preliminary results obtained for five phenols (Fig. 1B). Phenols 3-5 react with dpph• 105 times faster than 1 and 2. The addition of acetic acid causes a 20 to 100-fold reduction in the reaction rate constant. Our results show a strong dependence of the reaction rate on the polarity of the solvent, pH and, surprisingly, in neat methanol the order of the reaction with respect to ArOH concentration changes from 1 to 0.5 with increasing dilution of ArOH. As the SPLET mechanism dominates, the kinetic rate low is:-d[dpph•]/dt=n(kHAT[ArOH]+kET[ArO‾])[dpph•]
with increasing component kET[ArO‾], following the law of dilution:[ArO‾] = (Ka[ArOH])1/2.