Singlet Oxygen Generation in Classical Fenton Chemistry

The Fenton reaction, the Fe-catalyzed conversion of hydrogen peroxide to reactive oxygen species (ROS) was discovered more than a century ago. It occurs widely in nature because of the ubiquity of Fenton reagents, i.e., Fe and H2O2, and ROS in environmental and biological systems; however, its mechanisms and the identity of the ROS generated under varying conditions have remained controversial. The widely accepted mechanism is that of successive oxidation and reduction of Fe2+ and Fe3+ by hydrogen peroxide to form ·OH and O2-·, respectively, where ·OH is implicated as the primary oxidant. However, the formation of high-valent Fe4+=O species has also been implicated. Herein, by systematically dissecting the contributions of various ROS species generated in the classical Fenton reaction by using specific ROS traps and scavengers, we identified that singlet oxygen (1O2) is the main ROS from pH 4–7. In contrast, although ·OH is produced in measurable quantities, it was not a major contributor to the oxidation of organic molecules.