Hypophosphorous acid

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Hypophosphorous acid

General
Systematic name	Phosphinic acid
Molecular formula	H₃PO₂
Molar mass	66.00 g/mol
Appearance	colorless
CAS number	6303-21-5
Properties
Density and phase	1.274 g/cm³, liquid
Solubility in water	miscible
Melting point	26.5 °C (299.5 K)
Boiling point	106 °C (379 K) decomp.
Acidity (pK_a)	1.2
Viscosity	? cP at ? °C
Structure
Molecular shape	pseudo-tetrahedral
Dipole moment	? D
Hazards
MSDS	External MSDS
EU classification	not listed
NFPA 704	2 3 2
Flash point	non-flammable
RTECS number	?
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Related oxoacids	Phosphorous acid Phosphoric acid
Related compounds	Sodium hypophosphite Barium hypophosphite
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

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. 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 monobasic character (jargon for the fact that one proton can be readily deprotonated). 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.

1 Preparation and availability
2 Uses
3 Inorganic and organic derivatives
4 Sources
5 References
6 External links

[edit] 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⁻ + 4H₂O → 4H₂PO₂⁻ + 2H₂

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.

[edit] 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.^[1]. In this way, the NH₂ substituent can be removed from arenes.

[edit] 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.

[edit] Sources

Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th Edn.) New York:Wiley-Interscience. ISBN 0-471-19957-5.
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.