Organic Chemistry

Kinetics – The Reduction of TEMPOL by Ascorbic Acid

ESR is an invaluable tool for elucidating free radical reaction mechanisms. Although there are many radicals that live long enough to be observed directly with ESR, many important radicals, such as hydroxyl, superoxide, methyl, and other important organic intermediates, are too short lived to be observed directly. These very short lived radicals can often be observed by a technique called spin trapping.

There are many things which must be considered when choosing a spin trap, fromthe role the solvent may play to possible side reactions that create unexpected stable spin adducts- one of the pitfalls of spin-trapping.

In this laboratory exercise, students are able to observe the formation of three different radical adducts using a Fenton reaction in DMSO using PBN (t-butyl-α-phenyl nitrone). It takes about fifteen minutes for the third radical to become well established. The reaction can be observed in real time.

Spin Traps and Radical Adducts

Spin trapping is a common technique used with ESR to detect very short-lived radicals such as hydoxyl and superoxide which cannot be directly observed by ESR because their lifetimes are too short. A spin trap is a compound, commonly a nitrone or nitroso compound, which reacts quickly with a radical to form a stable nitroxide which is ESR active. This product is referred to as a spin adduct.
The ESR signature of the spin adduct provides only implicit information about the trapped radical. The hyperfine splitting from the nitrogen atom, splittings from other magnetically active heteroatoms and α hydrogen atoms all give information about the structure of the trapped radical.