Kinetics and stoichiometry of gallic acid and methyl gallate in scavenging DPPH radical as affected by the reaction solvent

Abstract

The activity and capacity of gallic acid (GA) and methyl gallate (MG) in scavenging DPPH· were determined in different solvents. Based on the bimolecular rate constants k2, both antioxidants showed highest activities in EtOH, followed by in MeOH, t-BuOH, MeCN, 2-PrOH, acetone, THF, ethyl acetate, and 1,4-dioxane. GA indicated better activities (k2 value, M−1 s−1) than MG in the alcoholic solvents (51–1939 vs. 25–1530) and in MeCN (203 vs. 187) whereas MG was of higher activities in the polar aprotic solvents (1.7–41 vs. 1.6–13). The highest stoichiometries for GA vs. MG were in 2-PrOH (6.67 vs. 5.37), followed by EtOH (5.84 vs. 4.57), MeOH (5.34 vs. 3.8) ~ acetone (5.02 vs. 4.44), MeCN (3.68 vs. 3.05) ~ t-BuOH (3.14 vs. 2.99), THF (2.34 vs. 2.2), ethyl acetate (1.2 vs. 0.93), and 1,4-dioxane (0.34 vs. 0.35).