Hypophosphorous acid is a phosphorus oxoacid and a powerful reducing agent with molecular formula H3PO2. Inorganic chemists refer to the free acid by this name (also as "HPA") although its IUPAC name is dihydridohydroxidooxidophosphorus, or the acceptable name of 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 + 3OH− + 3H2O → 3H2PO2− + PH3
The free acid may be prepared by the action of a strong acid on these hypophosphite salts.
- H2PO2− + H+ → H3PO2
Alternatively, H3PO2 arises by the oxidation of phosphine with iodine in water.
- PH3 + 2I2 + 2H2O → H3PO2 + 4I− + 4H+
HPA is usually supplied as a 50% aqueous solution.
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, H3
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
, 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 H3
react to give (HOCH2
H. 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)CH2
Few metal complexes have been prepared from H3PO2, one example is Ni(O2PH2)2.
- ChemicalLand21 Listing
- D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. ISBN 0-444-89307-5.
- V. V. Popik, A. G. Wright, T. A. Khan, J. A. Murphy "Hypophosphorous Acid" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.