Frémy's salt


Frémy's salt is a chemical compound with the formula, sometimes written as. It a bright yellowish-brown solid, but its aqueous solutions are bright violet. The related sodium salt, i.e. disodium nitrosodisulfonate is also referred to as Frémy's salt.
Regardless of the cations, the salts are distinctive because aqueous solutions contain the radical 2-.

Applications

Fremy's salt, being a long-lived free radical, is used as a standard in electron paramagnetic resonance spectroscopy, e.g. for quantitation of radicals. It intense EPR spectrum is dominated by three lines of equal intensity with a spacing of about 13 G.
The inorganic aminoxyl group is a persistent radical, akin to TEMPO.
It has been used in some oxidation reactions, e.g. for oxidation of some anilines and phenols. It can also be used as a model for peroxyl radicals in studies that examine the antioxidant mechanism of action in a wide range of natural products.

Preparation

Fremy's salt is prepared from hydroxylaminedisulfonic acid. Oxidation of the conjugate base gives the purple dianion:
The synthesis can be performed by combining nitrite and bisulfite to give the hydroxylaminedisulfonate. Oxidation is typically conducted at low-temperature, either chemically or by electrolysis.
Other reactions:

History

The Frémy's salt was discovered in 1845 by Edmond Frémy. Its use in organic synthesis was popularized by Hans Teuber, such that an oxidation using this salt is called the Teuber reaction.