Antioxidant activities of phenols in different solvents using DPPH assay

Studies on the antioxidant activity of two model phenols containing either an electron withdrawing (p-nitrophenol) or electron donating (p-aminophenol) group and p-hydroxyacetophenone in different solvents are reported using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical assay by spectrophotometry and stopped-flow techniques. The second-order rate constants measured with p-nitrophenol were found to be (1.2-5.5) x 10(-2) dm(3) mol(-1) s(-1) but the DPPH radical reacts much faster with p-aminophenol (k = 0.5-1.1 x 10(4) dm(3) mol(-1) s(-1)). The normal kinetic solvent effect in H atom transfer was seen in the case of p-nitrophenol with the solvent independent rate constant k (o) = 0.1 dm(3) mol(-1) s(-1). The IC50 values in p-nitrophenol are similar to those measured in p-hydroxyacetophenone. On the other hand, much lower IC50 values of more than four orders of magnitude with p-aminophenol were observed. This work demonstrates that the phenol with the electron donating -NH2 substituent is a better antioxidant.