Hypophosphorous acid
{{short description|Chemical compound}}
{{Distinguish|Hypophosphoric acid}}
{{Chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 476993494
| Reference = {{cite book|first=Ralph H.|last= Petrucci |title=General Chemistry |edition=9th |date=2007 |page= 946}}
| ImageFileL1 = Hypophosphorous-acid-2D.png
| ImageNameL1 = Wireframe model of hypophosphorous acid
| ImageFileR1 = Hypophosphorous-acid-3D-balls.png
| ImageName1 = Ball and stick model of hypophosphorous acid
| IUPACName = Phosphinic acid
| OtherNames = Hydroxy(oxo)-λ5-phosphane
Hydroxy-λ5-phosphanone
Oxo-λ5-phosphanol
Oxo-λ5-phosphinous acid
Phosphonous acid (for minor tautomer)
| Section1 = {{Chembox Identifiers
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/H3O2P/c1-3-2/h3H2,(H,1,2)
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = ACVYVLVWPXVTIT-UHFFFAOYSA-N
| CASNo = 6303-21-5
| CASNo_Ref = {{cascite|correct|CAS}}
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 8B1RL9B4ZJ
| PubChem = 3085127
| PubChem_Comment = (2H3)
| ChemSpiderID = 10449263
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID1_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID1 = 10459437
| ChemSpiderID1_Comment = (17O2)
| ChemSpiderID2_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID2 = 2342086
| ChemSpiderID2_Comment = (2H3)
| UNNumber = UN 3264
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 29031
| SMILES = O[PH2]=O
| ChEMBL_Ref = {{ebicite|changed|EBI}}
| ChEMBL = 2105054
| KEGG_Ref = {{keggcite|correct|kegg}}
| KEGG = D02334
| InChI = 1/H3O2P/c1-3-2/h3H2,(H,1,2)
| InChIKey = ACVYVLVWPXVTIT-UHFFFAOYAQ
}}
| Section2 = {{Chembox Properties
| Formula = H3PO2
| Appearance = colorless, deliquescent crystals or oily liquid
| MolarMass = 66.00 g/mol
| Density = 1.493 g/cm3Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, {{ISBN|0-07-049439-8}}
1.22 g/cm3 (50 wt% aq. solution)
| Solubility = miscible
| SolubleOther = very soluble in alcohol, ether
| MeltingPtC = 26.5
| BoilingPtC = 130
| BoilingPt_notes = decomposes
| ConjugateBase = Phosphinate
| pKa = 0.89±0.05
| Viscosity =
}}
| Section3 = {{Chembox Structure
| MolShape = pseudo-tetrahedral
| Dipole =
}}
| Section7 = {{Chembox Hazards
| ExternalSDS = [http://hazard.com/msds/mf/baker/baker/files/h8468.htm JT Baker]
| FlashPt = Non-flammable
}}
| Section8 = {{Chembox Related
| OtherFunction_label = phosphorus oxoacids
| OtherFunction = Phosphorous acid
Phosphoric acid
| OtherCompounds = Sodium hypophosphite
Barium hypophosphite
}}
}}
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.{{Greenwood&Earnshaw2nd}}
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).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) [https://babel.hathitrust.org/cgi/pt?id=ucm.5324354247;view=1up;seq=147 "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), [https://archive.org/details/elementsinorgan00grahgoog/page/n307 p. 316.]
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 {{chem|H|2|PO|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:
:{{chem|H|2|PO|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.{{Greenwood&Earnshaw2nd|page=513}}
Properties
:File:Tautomerism of H3PO2.png
The molecule displays P(═O)H to P–OH tautomerism similar to that of phosphorous acid; the P(═O) form is strongly favoured.{{cite journal | last1=Janesko | first1=Benjamin G. | last2=Fisher | first2=Henry C. | last3=Bridle | first3=Mark J. | last4=Montchamp | first4=Jean-Luc | title=P(═O)H to P–OH Tautomerism: A Theoretical and Experimental Study | journal=The Journal of Organic Chemistry | publisher=American Chemical Society (ACS) | volume=80 | issue=20 | date=2015-09-29 | issn=0022-3263 | doi=10.1021/acs.joc.5b01618 | pages=10025–10032| pmid=26372089 }}
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.{{cite journal |last1=Shechkov |first1=G. T. |last2=Pevneva |first2=I. A. |last3=Meshkova |first3=O. A. |title=Thermal Disproportionation of Hypophosphorous Acid |journal=Russian Journal of Applied Chemistry |date=August 2003 |volume=76 |issue=8 |pages=1354–1355 |doi=10.1023/B:RJAC.0000008318.22178.07|s2cid=96861842 }}
: 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,{{cite journal |last1=Kuratieva |first1=Natalia V. |last2=Naumova |first2=Marina I. |last3=Podberezskaya |first3=Nina V. |last4=Naumov |first4=Dmitry Yu. |title=The bivalent metal hypophosphites Sr(H 2 PO 2 ) 2, Pb(H 2 PO 2 ) 2 and Ba(H 2 PO 2 ) 2 |journal=Acta Crystallographica Section C Crystal Structure Communications |date=2005-02-15 |volume=61 |issue=2 |pages=i14–i16 |doi=10.1107/S010827010403166X|pmid=15695880 |bibcode=2005AcCrC..61I..14K }}{{cite journal |last1=Naumova |first1=Marina I. |last2=Kuratieva |first2=Natalia V. |last3=Podberezskaya |first3=Nina V. |last4=Naumov |first4=Dmitry Yu. |title=The alkali hypophosphites KH 2 PO 2, RbH 2 PO 2 and CsH 2 PO 2 |journal=Acta Crystallographica Section C Crystal Structure Communications |date=2004-05-15 |volume=60 |issue=5 |pages=i53–i55 |doi=10.1107/S0108270104002409|pmid=15131359 |bibcode=2004AcCrC..60I..53N }} including the important nickel salt [Ni(H2O)6](H2PO2)2.{{cite journal |last1=Kuratieva |first1=Natalia V. |last2=Naumova |first2=Marina I. |last3=Naumov |first3=Dmitry Yu. |last4=Podberezskaya |first4=Nina V. |title=Hexaaquanickel(II) bis(hypophosphite) |journal=Acta Crystallographica Section C Crystal Structure Communications |date=2003-01-15 |volume=59 |issue=1 |pages=i1–i3 |doi=10.1107/S0108270102018541|pmid=12506208 |bibcode=2003AcCrC..59I...1K }}
==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,{{cite journal|last1=Gordon |first1=P. E. |last2=Fry |first2=A. J. |last3=Hicks |first3=L. D. |title=Further studies on the reduction of benzylic alcohols by hypophosphorous acid/iodine|date= 23 August 2005 |url=http://content.arkat-usa.org/ARKIVOC/JOURNAL_CONTENT/manuscripts/2005/EJ-1559CP%20as%20published%20mainmanuscript.pdf|journal= Arkivoc |volume=2005 |issue=vi |pages=393–400|issn=1424-6376}} the United States Drug Enforcement Administration designated hypophosphorous acid (and its salts) as a List I precursor chemical effective November 16, 2001.[http://frwebgate.access.gpo.gov/cgi-bin/getdoc.cgi?dbname=2001_register&docid=01-26013-filed 66 FR 52670—52675.] 17 October 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.{{Cite web |url=http://www.access.gpo.gov/nara/cfr/waisidx_06/21cfr1309_06.html |title=21 CFR 1309 |access-date=2007-05-02 |archive-url=https://web.archive.org/web/20090503063012/http://www.access.gpo.gov/nara/cfr/waisidx_06/21cfr1309_06.html |archive-date=2009-05-03 }}[http://www.usdoj.gov/dea/pubs/csa.html 21 USC, Chapter 13 (Controlled Substances Act)]
=Organic=
In organic chemistry, H3PO2 can be used for the reduction of arenediazonium salts, converting {{chem|ArN|2|+}} to Ar–H.{{Cite book|title=Organic Chemistry|author1=William H. Brown |author2=Brent L. Iverson |author3=Eric Anslyn |author4=Christopher S. Foote |publisher=Cengage Learning|year=2013|isbn=978-1-133-95284-8|page=1003}}{{OrgSynth|last1=Robison |first1=M. M. |last2=Robison |first2=B. L. |date1956 |prep=cv4p0947 |title=2,4,6-Tribromobenzoic acid |volume=36|page=94 |collvol=4|collvolpage=947}}{{OrgSynth|last=Kornblum |first=N. |year=1941 |title=3,3′-Dimethoxybiphenyl and 3,3′-Dimethylbiphenyl|volume=21|page=30 |doi=10.15227/orgsyn.021.0030}} 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.{{cite journal|title=Palladium-Catalyzed Dehydrative Allylation of Hypophosphorous Acid with Allylic Alcohols|author=Karla Bravo-Altamirano |author2=Jean-Luc Montchamp |journal=Org. Synth.|year=2008|volume=85|page=96|doi=10.15227/orgsyn.085.0096|doi-access=free}}
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.{{cite journal |last1=Guyon |first1=Carole |last2=Métay |first2=Estelle |last3=Popowycz |first3=Florence |last4=Lemaire |first4=Marc |title=Synthetic applications of hypophosphite derivatives in reduction |journal=Organic & Biomolecular Chemistry |date=2015 |volume=13 |issue=29 |pages=7879–7906 |doi=10.1039/C5OB01032B|pmid=26083977 }} 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).{{cite journal|last1=Abrantes|first1=L. M.|title=On the Mechanism of Electroless Ni–P Plating|journal=Journal of the Electrochemical Society|date=1994|volume=141|issue=9|pages=2356–2360|doi=10.1149/1.2055125|bibcode=1994JElS..141.2356A}}
Sources
- {{Cotton&Wilkinson6th}}
- [http://www.chemicalland21.com/arokorhi/industrialchem/inorganic/HYPOPHOSPHOROUS%20ACID.htm ChemicalLand21 Listing]
- {{cite book|first=D. E. C. |last=Corbridge |title=Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology |year=1995 |edition=5th |publisher=Elsevier |location=Amsterdam |isbn=0-444-89307-5}}
- {{cite book|first1=V. V. |last1=Popik |first2=A. G. |last2=Wright |first3=T. A. |last3=Khan |first4=J. A. |last4=Murphy |chapter=Hypophosphorous Acid|title= Encyclopedia of Reagents for Organic Synthesis |editor-first=L. |editor-last=Paquette |date=2004 |publisher=J. Wiley & Sons |location=New York |doi=10.1002/047084289X|hdl=10261/236866 |isbn=978-0-471-93623-7 }}
- {{cite book|first1=D. W. |last1=Rich |first2=M. C. |last2=Smith |title=Electroless Deposition of Nickel, Cobalt & Iron |publisher=IBM Corporation |location=Poughkeepsie, NY |date=1971}}