Everything about Hypophosphorous Acid totally explained
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Hypophosphorous acid is a
phosphorus oxoacid and a powerful
reducing agent with molecular formula H
3PO
2. 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 H
3PO
2, but a more descriptive presentation is HOP(O)H
2 which highlights its
monoprotic character. Salts derived from this acid are called
hypophosphites.
HOP(O)H
2 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, for example Ca(OH)
2.
» P
4 + 4OH
− + 4H
2O → 4H
2PO
2− + 2H
2
The free acid may be prepared by the action of a strong acid on these hypophosphite salts.
» H
2PO
2− + H
+ → H
3PO
2
Alternatively, H
3PO
2 arises by the oxidation of
phosphine with
iodine in water.
» PH
3 + 2I
2 + 2H
2O → H
3PO
2 + 4I
− + 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; for example deposition of metal films from solution. In organic chemistry, H
3PO
2 best known for their use in the reduction of
arenediazonium salts, converting ArN
2+ 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 H
3PO
2 react to give (HOCH
2)
2PO
2H. 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)CH
2CH
2C(O)NH
2.
Few metal complexes have been prepared from H
3PO
2, one example is Ni(O
2PH
2)
2.
Sources
-
- 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.
Further Information
Get more info on 'Hypophosphorous Acid'.
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