Hypophosphorous acid (HPA), orphosphinic acid, is aphosphorusoxyacid and a powerfulreducing agent withmolecular 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 itsmonoprotic character. Salts derived from this acid are calledhypophosphites.[3]
HOP(O)H2 exists inequilibrium with the minortautomer HP(OH)2. Sometimes the minor tautomer is called hypophosphorous acid and the major tautomer is called phosphinic acid.
Hypophosphorous acid was first prepared in 1816 by the French chemistPierre Louis Dulong (1785–1838).[4]
The acid is prepared industrially via a two step process: Firstly, elementalwhite phosphorus reacts withalkali andalkaline earthhydroxides to give an aqueous solution of hypophosphites:
P4 + 4 OH− + 4 H2O → 4 H 2PO− 2 + 2 H2
Anyphosphites produced in this step can be selectively precipitated out by treatment withcalcium salts. The purified material is then treated with a strong, non-oxidizing acid (oftensulfuric 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 tophosphorous acid andphosphoric acid and alsodisproportionates to phosphorous acid andphosphine. Pure anhydrous hypophosphorous acid can be formed by the continuous extraction of aqueous solutions withdiethyl ether.[5]
The molecule displays P(═O)H to P–OHtautomerism similar to that ofphosphorous 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 formphosphorous acid and hydrogen gas.
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,[8][9] including the important nickel salt [Ni(H2O)6](H2PO2)2.[10]
In organic chemistry, H3PO2 can be used for the reduction ofarenediazonium salts, convertingAr−+N≡N toAr−H.[15][16][17] Whendiazotized in a concentrated solution of hypophosphorous acid, anamine 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 inFischer esterification reactions, where it prevents the formation of colored impurities.
It is used to prepare phosphinic acid derivatives.[18]
Hypophosphorous acid (and its salts) are used to reduce metal salts back into bulk metals. It is effective for varioustransition metals ions (i.e. those of: Co, Cu, Ag, Mn, Pt) but is most commonly used to reducenickel.[19] This forms the basis ofelectroless 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]
Corbridge, D. E. C. (1995).Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology (5th ed.). Amsterdam: Elsevier.ISBN0-444-89307-5.
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.ISBN978-0-471-93623-7.
Rich, D. W.; Smith, M. C. (1971).Electroless Deposition of Nickel, Cobalt & Iron. Poughkeepsie, NY: IBM Corporation.
^Dulong preparedacide hypo-phosphoreux by adding barium phosphide (Ba3P2) to water, which yieldedphosphine 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:
^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.ISSN0022-3263.PMID26372089.
^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.S2CID96861842.
^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.PMID15695880.
^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.PMID15131359.
