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]
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| Names | |||
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| IUPAC name
Phosphinic acid
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| Other names
Hydroxy(oxo)-λ5-phosphane
Hydroxy-λ5-phosphanone | |||
| Identifiers | |||
3D model (JSmol)
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| ChemSpider | |||
| ECHA InfoCard | 100.026.001 | ||
| KEGG | |||
PubChem CID
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| UNII | |||
| UN number | UN 3264 | ||
CompTox Dashboard (EPA)
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| Properties | |||
| H3PO2 | |||
| 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 | ||
| miscible | |||
| Solubility | very soluble in alcohol, ether | ||
| Acidity (pKa) | 1.2 | ||
| Conjugate base | Phosphinate | ||
| Structure | |||
| pseudo-tetrahedral | |||
| Hazards | |||
| Flash point | Non-flammable | ||
| Safety data sheet (SDS) | JT Baker | ||
| Related compounds | |||
Related phosphorus oxoacids
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Phosphorous acid Phosphoric acid | ||
Related compounds
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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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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
editHypophosphorous 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 white phosphorus reacts with alkali and alkaline earth hydroxides to give an aqueous solution of hypophosphites:
- P4 + 4 OH− + 4 H2O → 4 H
2PO−
2 + 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:
- H
2PO−
2 + 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]
Properties
edit
The molecule displays P(═O)H to P–OH tautomerism similar to that of phosphorous acid; the P(═O) form is strongly favoured.[6]
HPA is usually supplied as a 50% aqueous solution and heating at low temperatures (up to about 90 °C) prompts it to react with water to form phosphorous acid and hydrogen gas.
- H3PO2 + H2O → H3PO3 + H2
Heating above 110 °C causes hypophosphorous acid to undergo disproportionation to give phosphorous acid and phosphine.[7]
- 3 H3PO2 → 2 H3PO3 + PH3
Reactions
editInorganic
editHypophosphorous acid can reduce chromium(III) oxide to chromium(II) oxide:
- H3PO2 + 2 Cr2O3 → 4 CrO + H3PO4
Inorganic derivatives
editMost 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,[8][9] including the important nickel salt [Ni(H2O)6](H2PO2)2.[10]
DEA List I chemical status
editBecause 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]
Organic
editIn organic chemistry, H3PO2 can be used for the reduction of arenediazonium salts, converting ArN+
2 to Ar–H.[15][16][17] 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.[18]
Applications
editHypophosphorous 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.[19] 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).[20]
Sources
edit- 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. (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. (ed.). Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X. hdl:10261/236866. ISBN 978-0-471-93623-7.
- Rich, D. W.; Smith, M. C. (1971). Electroless Deposition of Nickel, Cobalt & Iron. Poughkeepsie, NY: IBM Corporation.
References
edit- ^ Petrucci, Ralph H. (2007). General Chemistry (9th ed.). p. 946.
- ^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
- ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
- ^ 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:
- Dulong (1816) "Extrait d'un mémoire sur les combinaisons du phosphore avec l'oxigène" (Extract from a memoir on the compounds of phosphorus with oxygen), Annales de Chimie et de Physique, 2 : 141–150. [in French]
- Graham, Thomas, Elements of Inorganic Chemistry, 2nd ed. (Philadelphia, Pennsylvania: Blanchard and Lea, 1858), p. 316.
- ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 513. ISBN 978-0-08-037941-8.
- ^ Janesko, Benjamin G.; Fisher, Henry C.; Bridle, Mark J.; Montchamp, Jean-Luc (2015-09-29). "P(═O)H to P–OH Tautomerism: A Theoretical and Experimental Study". The Journal of Organic Chemistry. 80 (20). American Chemical Society (ACS): 10025–10032. doi:10.1021/acs.joc.5b01618. ISSN 0022-3263. PMID 26372089.
- ^ 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. S2CID 96861842.
- ^ 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. Bibcode:2005AcCrC..61I..14K. doi:10.1107/S010827010403166X. PMID 15695880.
- ^ 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. Bibcode:2004AcCrC..60I..53N. doi:10.1107/S0108270104002409. PMID 15131359.
- ^ 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. Bibcode:2003AcCrC..59I...1K. doi:10.1107/S0108270102018541. PMID 12506208.
- ^ 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.
- ^ a b 66 FR 52670—52675. 17 October 2001.
- ^ "21 CFR 1309". Archived from the original on 2009-05-03. Retrieved 2007-05-02.
- ^ 21 USC, Chapter 13 (Controlled Substances Act)
- ^ William H. Brown; Brent L. Iverson; Eric Anslyn; Christopher S. Foote (2013). Organic Chemistry. Cengage Learning. p. 1003. ISBN 978-1-133-95284-8.
- ^ Robison, M. M.; Robison, B. L. "2,4,6-Tribromobenzoic acid". Organic Syntheses. 36: 94; Collected Volumes, vol. 4.
- ^ Kornblum, N. (1941). "3,3′-Dimethoxybiphenyl and 3,3′-Dimethylbiphenyl". Organic Syntheses. 21: 30. doi:10.15227/orgsyn.021.0030.
- ^ 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.
- ^ 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.
- ^ Abrantes, L. M. (1994). "On the Mechanism of Electroless Ni–P Plating". Journal of the Electrochemical Society. 141 (9): 2356–2360. Bibcode:1994JElS..141.2356A. doi:10.1149/1.2055125.