RESUMEN
The reaction of a 2,2-diphenyl-1-picrylhydrazyl radical (dpphË) with phenols carried out in alcohols is a frequently used assay for estimation of the antiradical activity of phenolic compounds. The rates of reactions of dpphË with five phenols (ArOH: unsubstituted phenol, 4-hydroxyacetophenone, two calix[4]resorcinarenes and baicalein) measured in methanol indicate the different kinetics of the process for very diluted phenols compared to their non-diluted solutions. This effect was explained as dependent on the ratio [ArO-]/[ArOH] and for diluted ArOH corresponds to an increased contribution of much faster electron transfer (ET, ArO-/dpphË) over the Hydrogen Atom Transfer (HAT, ArOH/dpphË). Simplified analysis of the reaction kinetics resulted in estimation of k ET/k HAT ratios for each studied ArOH, and in calculation of the rate constants k ET. Described results are cautionary examples of how the concentration of a phenol might change the reaction mechanism and the overall kinetics of the observed process.