Hypophosphorous acid

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Hypophosphorous acid[1]
Wireframe model of hypophosphorous acid
IUPAC name
Phosphinic acid
Other names

Oxo-λ5-phosphinous acid

Phosphonous acid (for minor tautomer)
3D model (JSmol)
ECHA InfoCard 100.026.001
UN number UN 3264
Molar mass 66.00 g/mol
Appearance colorless, deliquescent crystals or oily liquid
Density 1.493 g/cm3[2]

1.22 g/cm3 (50 wt% aq. solution)

Melting point 26.5 °C (79.7 °F; 299.6 K)
Boiling point 130 °C (266 °F; 403 K) decomposes
Solubility very soluble in alcohol, ether
Acidity (pKa) 1.2
Conjugate base Phosphinate
Safety data sheet JT Baker
Flash point Non-flammable
Related compounds
Phosphorous acid
Phosphoric acid
Related compounds
Sodium hypophosphite
Barium hypophosphite
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Hypophosphorous acid (HPA), or phosphinic acid, is a phosphorus oxyacid and a powerful reducing agent with molecular formula H3PO2. It is a colorless low-melting compound, which is soluble in water, dioxane, and alcohols. The formula for this 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.[3]

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[edit]

Hypophosphorous acid was first prepared in 1816 by the French chemist Pierre Louis Dulong (1785–1838).[4]

The acid is prepared industrially via a two step process: Firstly, hypophosphite salts of the alkali and alkaline earth metals result from the reaction of white phosphorus with hot aqueous solution of the appropriate hydroxide, e.g. Ca(OH)2.

P4 + 4 OH + 4 H2O → 4 H
+ 2 H2

The salt is then treated with a strong, non-oxidizing acid to give the free hypophosphorous acid:

+ H+ → H3PO2

HPA is usually supplied as a 50% aqueous solution. Anhydrous acid cannot be obtained by simple evaporation of the water, as the acid ready oxidises to phosphorous acid and phosphoric acid and also disproportionates to phosphorous acid and phosphine. Pure anhydrous hypophosphorous acid can be formed by the continuous extraction of aqueous solutions with diethyl ether.[5]

Reactions and uses[edit]

Its main industrial use is for electroless nickel plating (Ni–P), although it is primarily used as a salt (sodium hypophosphite).[6]

Hypophosphorous acid can reduce chromium(III) oxide to chromium(II) oxide:

H3PO2 + 2 Cr2O3 → 4 CrO + H3PO4

Organic chemistry[edit]

In organic chemistry, H3PO2 can be used for the reduction of arenediazonium salts, converting ArN+
to Ar–H.[7][8][9] When diazotized in a concentrated solution of hypophosphorous acid, an amine substituent can be removed from arenes.

Owing to its ability to function as a mild reducing agent and oxygen scavenger it is sometimes used as an additive in Fischer esterification reactions, where it prevents the formation of colored impurities.

It is used to prepare phosphinic acid derivatives.[10]

DEA List I chemical status[edit]

Because hypophosphorous acid can reduce elemental iodine to form hydroiodic acid, which is a reagent effective for reducing ephedrine or pseudoephedrine to methamphetamine,[11] the United States Drug Enforcement Administration designated hypophosphorous acid (and its salts) as a List I precursor chemical effective November 16, 2001.[12] 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.[12][13][14]

Organophosphinic acids (Phosphinates)[edit]

Organophosphinic acids have the formula R2PO2H. The two hydrogen atoms directly bound to phosphorus in phosphinic acid are replaced by organic groups. For example, formaldehyde and H3PO2 react to give (HOCH2)2PO2H. Similarly, phosphinic acid adds to Michael acceptors, for example with acrylamide it gives H(HO)P(O)CH2CH2C(O)NH2. The Cyanex family of dialkylphosphinic acids are used in hydrometallurgy to extract metals from ores.

Inorganic derivatives[edit]

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


  • Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5
  • ChemicalLand21 Listing
  • Corbridge, D. E. C. Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology (5th ed.). Amsterdam: Elsevier. ISBN 0-444-89307-5.
  • Popik, V. V.; Wright, A. G.; Khan, T. A.; Murphy, J. A. (2004). "Hypophosphorous Acid". In Paquette, L. (ed.). Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289.
  • Rich, D. W.; Smith, M. C. (1971). Electroless Deposition of Nickel, Cobalt & Iron. Poughkeepsie, NY: IBM Corporation.


  1. ^ Petrucci, Ralph H. (2007). General Chemistry (9th ed.). p. 946.
  2. ^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  3. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  4. ^ Dulong prepared acide hypo-phosphoreux by adding barium phosphide (Ba3P2) to water, which yielded phosphine gas (PH3), barium phosphate, and barium hypophosphite. Since the phosphine gas left the solution and the barium phosphate precipitated, only the barium hypophosphite remained in solution. Hypophosphorous acid could then be obtained from the filtrate by adding sulfuric acid, which precipitated barium sulfate, leaving hypophosphorous acid in solution. See:
  5. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 513. ISBN 978-0-08-037941-8.
  6. ^ Abrantes, L. M. (1994). "On the Mechanism of Electroless Ni–P Plating". Journal of the Electrochemical Society. 141 (9): 2356. doi:10.1149/1.2055125.
  7. ^ William H. Brown; Brent L. Iverson; Eric Anslyn; Christopher S. Foote (2013). Organic Chemistry. Cengage Learning. p. 1003. ISBN 9781133952848.
  8. ^ Robison, M. M.; Robison, B. L. "2,4,6-Tribromobenzoic acid". Organic Syntheses. 36: 94.; Collective Volume, 4
  9. ^ Kornblum, N. (1941). "3,3′-Dimethoxybiphenyl and 3,3′-Dimethylbiphenyl". Organic Syntheses. 21: 30. doi:10.15227/orgsyn.021.0030.
  10. ^ Karla Bravo-Altamirano, Jean-Luc Montchamp (2008). "Palladium-Catalyzed Dehydrative Allylation of Hypophosphorous Acid with Allylic Alcohols". Org. Synth. 85: 96. doi:10.15227/orgsyn.085.0096.CS1 maint: uses authors parameter (link)
  11. ^ Gordon, P. E.; Fry, A. J.; Hicks, L. D. (23 August 2005). "Further studies on the reduction of benzylic alcohols by hypophosphorous acid/iodine" (PDF). ARKIVOC. 2005 (vi): 393–400. ISSN 1424-6376.
  12. ^ a b 66 FR 52670—52675. 17 October 2001.
  13. ^ "21 CFR 1309". Archived from the original on 2009-05-03. Retrieved 2007-05-02.
  14. ^ 21 USC, Chapter 13 (Controlled Substances Act)