Hypophosphorous acid - Misplaced Pages

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.

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).

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.

Properties

The molecule displays P(═O)H to P–OH tautomerism similar to that of phosphorous acid; the P(═O) form is strongly favoured.

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.

3 H₃PO₂ → 2 H₃PO₃ + PH₃

Reactions

Inorganic

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, including the important nickel salt (H₂PO₂)₂.

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.

Organic

In organic chemistry, H₃PO₂ can be used for 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.

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.

Applications

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. 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).

Sources

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

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Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
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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.
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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.
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v t e Hydrogen compounds
H₃AsO₃ H₃AsO₄ HArF HAt HSO₃F H[BF₄] HBr HBrO HBrO₂ HBrO₃ HBrO₄ HCl HClO HClO₂ HClO₃ HClO₄ HCN HCNO H₂CrO₄/H₂Cr₂O₇ H₂CO₃ H₂CS₃ HF HFO HI HIO HIO₂ HIO₃ HIO₄ HMnO₄ H₂MnO₄ H₂MoO₄ HNC NaHCO₃ HNCO HNO HNO₂ HNO₃ H₂N₂O₂ HNO₅S H₃NSO₃ H₂O H₂O₂ H₂O₃ H₂O₄ H₂O₅ H₃PO₂ H₃PO₃ H₃PO₄ H₄P₂O₇ H₅P₃O₁₀ H₂[PtCl₆] H₂S H₂S₂ H₂Se H₂SeO₃ H₂SeO₄ H₄SiO₄ H₂[SiF₆] HSCN HNCS H₂SO₃ H₂SO₄ H₂SO₅ H₂S₂O₃ H₃O H₂S₂O₆ H₂S₂O₇ H₂S₂O₈ CF₃SO₃H H₂Te H₂TeO₃ H₆TeO₆ H₄TiO₄ H₂Po H[Co(CO)₄]

Hydrogen compounds

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