Chemistry:Hypophosphorous acid

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Short description: Chemical compound

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)
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
Related phosphorus oxoacids
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

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, elemental phosphorus reacts with alkali and alkaline earth hydroxides to give an aqueous solution of hypophosphites:

P4 + 4 OH + 4 H2O → 4 H2PO2 + 2 H2

Any phosphites produced in this step can be selectively precipitated out by treatment with calcium salts. The purified material is then treated with a strong, non-oxidizing acid (often sulfuric acid) to give the free hypophosphorous acid:

H2PO2 + 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 readily 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]


When heated hypophosphorous acid undergoes disproportionation to give phosphine and phosphoric acid.[6]



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

H3PO2 + 2 Cr2O3 → 4 CrO + H3PO4

Inorganic derivatives

Most metal-hypophosphite complexes are unstable, owing to the tendency of hypophosphites to reduce metal cations back into the bulk metal. Some examples have been characterised,[7][8] including the important nickel salt [Ni(H2O)6](H2PO2)2.[9]

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,[10] the United States Drug Enforcement Administration designated hypophosphorous acid (and its salts) as a List I precursor chemical effective November 16, 2001.[11] 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.[11][12][13]


In organic chemistry, H3PO2 can be used for the reduction of arenediazonium salts, converting ArN+2 to Ar–H.[14][15][16] 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.[17]


Hypophosphorous acid (and its salts) are used to reduce metal salts back into bulk metals. It is effective for various transition metals ions (i.e. those of: Co, Cu, Ag, Mn, Pt) but is most commonly used to reduce nickel.[18] This forms the basis of electroless nickel plating (Ni–P), which is the single largest industrial application of hypophosphites. For this application it is principally used as a salt (sodium hypophosphite).[19]


  • ChemicalLand21 Listing
  • Corbridge, D. E. C. (1995). 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.. Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X. ISBN 9780471936237. 
  • 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. Shechkov, G. T.; Pevneva, I. A.; Meshkova, O. A. (August 2003). "Thermal Disproportionation of Hypophosphorous Acid". Russian Journal of Applied Chemistry 76 (8): 1354–1355. doi:10.1023/B:RJAC.0000008318.22178.07. 
  7. Kuratieva, Natalia V.; Naumova, Marina I.; Podberezskaya, Nina V.; Naumov, Dmitry Yu. (2005-02-15). "The bivalent metal hypophosphites Sr(H 2 PO 2 ) 2 , Pb(H 2 PO 2 ) 2 and Ba(H 2 PO 2 ) 2". Acta Crystallographica Section C Crystal Structure Communications 61 (2): i14–i16. doi:10.1107/S010827010403166X. PMID 15695880. 
  8. Naumova, Marina I.; Kuratieva, Natalia V.; Podberezskaya, Nina V.; Naumov, Dmitry Yu. (2004-05-15). "The alkali hypophosphites KH 2 PO 2 , RbH 2 PO 2 and CsH 2 PO 2". Acta Crystallographica Section C Crystal Structure Communications 60 (5): i53–i55. doi:10.1107/S0108270104002409. PMID 15131359. 
  9. Kuratieva, Natalia V.; Naumova, Marina I.; Naumov, Dmitry Yu.; Podberezskaya, Nina V. (2003-01-15). "Hexaaquanickel(II) bis(hypophosphite)". Acta Crystallographica Section C Crystal Structure Communications 59 (1): i1–i3. doi:10.1107/S0108270102018541. PMID 12506208. 
  10. Gordon, P. E.; Fry, A. J.; Hicks, L. D. (23 August 2005). "Further studies on the reduction of benzylic alcohols by hypophosphorous acid/iodine". Arkivoc 2005 (vi): 393–400. ISSN 1424-6376. 
  11. 11.0 11.1 66 FR 52670—52675. 17 October 2001.
  12. "21 CFR 1309". 
  13. 21 USC, Chapter 13 (Controlled Substances Act)
  14. William H. Brown; Brent L. Iverson; Eric Anslyn; Christopher S. Foote (2013). Organic Chemistry. Cengage Learning. pp. 1003. ISBN 9781133952848. 
  15. Robison, M. M.; Robison, B. L.. "2,4,6-Tribromobenzoic acid". Organic Syntheses 36: 94. ; Collective Volume, 4 
  16. Kornblum, N. (1941). "3,3′-Dimethoxybiphenyl and 3,3′-Dimethylbiphenyl". Organic Syntheses 21: 30. doi:10.15227/orgsyn.021.0030. 
  17. 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. 
  18. Guyon, Carole; Métay, Estelle; Popowycz, Florence; Lemaire, Marc (2015). "Synthetic applications of hypophosphite derivatives in reduction". Organic & Biomolecular Chemistry 13 (29): 7879–7906. doi:10.1039/C5OB01032B. PMID 26083977. 
  19. Abrantes, L. M. (1994). "On the Mechanism of Electroless Ni–P Plating". Journal of the Electrochemical Society 141 (9): 2356–2360. doi:10.1149/1.2055125. Bibcode1994JElS..141.2356A.