Hypophosphorous acid

Editor-In-Chief: Henry A. Hoff

Overview
Hypophosphorous acid is a phosphorus oxoacid and a powerful reducing agent. Inorganic chemists refer to the free acid by this name (also as "HPA") although its official IUPAC name is phosphinic acid. See Phosphinic acid. It is a colorless low-melting compound, which is soluble in water, dioxane, and alcohols. The formula for hypophosphorous acid is generally written H3PO2, but a more descriptive presentation is HOP(O)H2 which highlights its monoprotic character. Salts derived from this acid are called hypophosphites.

HOP(O)H2 exists in equilibrium with the minor tautomer HP(OH)2. Sometimes the minor tautomer is called hypophosphorous acid and the major tautomer is called phosphinic acid.

Preparation and availability
The acid is prepared industrially via a two step process. Hypophosphite salts of the alkali and alkaline earth metals result from treatment of white phosphorus with hot aqueous solution of the appropriate hydroxide, e.g. Ca(OH)2.
 * P4 + 4OH&minus; + 4H2O → 4H2PO2&minus; + 2H2

The free acid may be prepared by the action of a strong acid on these hypophosphite salts.
 * H2PO2&minus; + H+ → H3PO2

Alternatively, H3PO2 arises by the oxidation of phosphine with iodine in water.
 * PH3 + 2I2 + 2H2O → H3PO2 + 4I&minus; + 4H+

HPA is usually supplied as a 50% aqueous solution.

Uses
Hypophosphorous acid is used in the formulation of pharmaceuticals, discoloration of polymers, water treatment, retrieval of precious or non-ferrous metals. Its main use is for electroless plating, i.e. deposition of metal films from solution. In organic chemistry, H3PO2 best known for their use in the reduction of arenediazonium salts, converting ArN2+ to Ar-H. When diazotized in a concentrated solution of hypophosphorous acid, an amine substituent can be removed from arenes, selectively over alkyl amines.

DEA List I chemical status
Because hypophosphorous acid can reduce elemental iodine to form hydroiodic acid, which is a reagent effective for reducing ephedrine or pseudoephedrine to methamphetamine, the United States Drug Enforcement Administration designated hypophosphorous acid (and its salts) as a List I precursor chemical effective November 16, 2001. Accordingly, handlers of hypophosphorous acid or its salts in the United States are subject to stringent regulatory controls including registration, recordkeeping, reporting, and import/export requirements pursuant to the Controlled Substances Act and 21 CFR §§ 1309 and 1310.

Inorganic and organic derivatives
Numerous derivatives are known in which the two hydrogen atoms directly bound to phosphorus are replaced by organic groups. These derivatives are known as phosphinic acids, and their salts as phosphinates. For example, formaldehyde and H3PO2 react to give (HOCH2)2PO2H. The reaction is akin to the addition of thiols and HCN to aldehydes. Similarly, it adds to Michael acceptors, for example with acrylamide it gives H(HO)P(O)CH2CH2C(O)NH2.

Few metal complexes have been prepared from H3PO2, one example is Ni(O2PH2)2.