phosphorous acid

File:Tautomerism of H3PO3.png

Solid {{chem2|HP(O)(OH)2}} has tetrahedral geometry about the central phosphorus atom, with a {{chem2|P\sH}} bond of 132 pm, one {{chem2|P\dO}} double bond of 148 pm and two longer {{chem2|P\sOH}} single bonds of 154 pm. In common with other phosphorus oxides with {{chem2|P\sH}} bonds (e.g.hypophosphorous acid and dialkyl phosphites),{{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 }} it exists in equilibrium with an extremely minor tautomer {{chem2|P(OH)3}}. (In contrast, arsenous acid's major tautomer is the trihydroxy form.) IUPAC recommends that the trihydroxy form {{chem2|P(OH)3}} be called phosphorous acid, and the dihydroxy form {{chem2|HP(O)(OH)2}} phosphonic acid.{{RedBook2005}}. Only the reduced phosphorus compounds are spelled with an "-ous" ending.

:{{chem2|P^{III}(OH)3 <-> HP^{V}(O)(OH)2}} {{spaces|5}} K = 10^10.3 (25°C, aqueous){{cite journal |doi=10.1139/v79-039|title=Tautomerization Equilibria for Phosphorous Acid and its Ethyl Esters, Free Energies of Formation of Phosphorous and Phosphonic Acids and their Ethyl Esters, and p Ka Values for Ionization of the P—H Bond in Phosphonic Acid and Phosphonic Esters |year=1979 |last1=Guthrie |first1=J. Peter |journal=Canadian Journal of Chemistry |volume=57 |issue=2 |pages=236–239 |doi-access=free }}

On an industrial scale, the acid is prepared by hydrolysis of phosphorus trichloride with water or steam:{{Ullmann|doi= 10.1002/14356007.a19_527|title= Phosphorus Compounds, Inorganic|year= 2000|last1= Bettermann|first1= Gerhard|last2= Krause|first2= Werner|last3= Riess|first3= Gerhard|last4= Hofmann|first4= Thomas}}.

:{{chem2|PCl3 + 3 H2O -> HPO(OH)2 + 3 HCl}}

{{chem2|HPO(OH)2}} could be produced by the hydrolysis of phosphorus trioxide:

:{{chem2|P4O6 + 6 H2O -> 4 HPO(OH)2}}

Phosphorous acid has a pK_a in the range 1.26–1.3.{{cite journal|first1=John W. |last1=Larson |first2=Margaret |last2=Pippin |title=Thermodynamics of ionization of hypophosphorous and phosphorous acids. Substituent effects on second row oxy acids |journal=Polyhedron |date=1989 |volume=8 |issue=4 |pages=527–530|doi=10.1016/S0277-5387(00)80751-2}}{{cite book|title=CRC Handbook of Chemistry and Physics |edition=87th |pages=8–42}}

:{{chem2|HP(O)(OH)2 -> HP(O)2(OH)- + H+}} {{quad}} pK_a = 1.3

It is a diprotic acid, the hydrogenphosphite ion, {{chem2|HP(O)2(OH)-}} is a weak acid:

:{{chem2|HP(O)2(OH)- -> HPO3(2−) + H+}} {{quad}} pK_a = 6.7

The conjugate base {{chem2|HP(O)2(OH)-}} is called hydrogen phosphite, and the second conjugate base, {{chem2|HPO3(2−)}}, is the phosphite ion.{{cite book|first=Josef |last=Novosad |date=1994 |title=Encyclopedia of Inorganic Chemistry |publisher=John Wiley and Sons |isbn=0-471-93620-0}} (Note that the IUPAC recommendations are hydrogen phosphonate and phosphonate respectively).

The hydrogen atom bonded directly to the phosphorus atom is not readily ionizable. Chemistry examinations often test students' appreciation of the fact that not all three hydrogen atoms are acidic under aqueous conditions, in contrast with {{chem2|H3PO4}}.

On heating at 200 °C, phosphorous acid disproportionates to phosphoric acid and phosphine:{{cite book|last1=Gokhale|first1=S. D.|last2=Jolly|first2=W. L.|title=Inorganic Syntheses|chapter=Phosphine|date=1967 |volume=9 |pages= 56–58 |doi=10.1002/9780470132401.ch17|isbn=9780470132401}}

:{{chem2|4 H3PO3 -> 3 H3PO4 + PH3}}

This reaction is used for laboratory-scale preparations of {{chem2|PH3}}.

Phosphorous acid slowly oxidizes in air to phosphoric acid.

Both phosphorous acid and its deprotonated forms are good reducing agents, although not necessarily quick to react. They are oxidized to phosphoric acid or its salts. It reduces solutions of noble metal cations to the metals. When phosphorous acid is treated with a cold solution of mercuric chloride, a white precipitate of mercurous chloride forms:

:{{chem2|H3PO3 + 2 HgCl2 + H2O -> Hg2Cl2 + H3PO4 + 2 HCl}}

Mercurous chloride is reduced further by phosphorous acid to mercury on heating or on standing:

:{{chem2|H3PO3 + Hg2Cl2 + H2O -> 2 Hg + H3PO4 + 2 HCl}}

File:WAJJOH.svg

Upon treatment with metals of d⁶ configuration, phosphorous acid is known to coordinate as the otherwise rare {{chem2|P(OH)3}} tautomer. Examples include {{chem2|Mo(CO)5(P(OH)3)}} and {{chem2|[Ru(NH3)4(H2O)(P(OH)3)](2+)}}.{{cite journal |title=Synthesis of molybdenum complex with novel P(OH)₃ ligand based on the one-pot reaction of Mo(CO)₆ with HP(O)(OEt)₂ and water |first1=Chanjuan |last1=Xi |first2=Yuzhou |last2=Liu |first3=Chunbo |last3=Lai |first4=Lishan |last4=Zhou |journal=Inorganic Chemistry Communications| doi=10.1016/j.inoche.2004.09.012 |year=2004 |volume=7 |pages=1202–1204 |issue=11}}{{cite journal |title=The ruthenium(II) center and the phosphite-phosphonate tautomeric equilibrium|author1=Sernaglia, R. L. |author2=Franco, D. W. |doi=10.1021/ic00317a018 |year=2005 |journal=Inorg. Chem. |volume=28 |issue=18| pages=3485–3489}}

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Heating a mixture of potassium tetrachloroplatinate and phosphorous acid gives the luminescent salt potassium diplatinum(II) tetrakispyrophosphite:{{cite book |doi=10.1002/9780470132555.ch61 |title=Inorganic Syntheses |year=2007 |last1=Alexander |first1=K. A. |last2=Bryan |first2=S. A. |last3=Dickson |first3=M. K. |last4=Hedden |first4=D. |last5=Roundhill |chapter=Potassium Tetrakis[Dihydrogen Diphosphito(2–)]Diplatinate(II) |volume=24 |pages=211–213 |isbn=9780470132555}}

:{{chem2|2 K2PtCl4 + 8 H3PO3 -> K4[Pt2(HO2POPO2H)4] + 8 HCl + 4 H2O}}

The most important use of phosphorous acid (phosphonic acid) is the production of basic lead phosphite, which is a stabilizer in PVC and related chlorinated polymers.

It is used in the production of basic lead phosphonate PVC stabilizer, aminomethylene phosphonic acid and hydroxyethane diphosphonic acid.

It is also used as a strong reducing agent and in the production of synthetic fibres, organophosphorus pesticides, and the highly efficient water treatment agent ATMP.

Ferrous materials, including steel, may be somewhat protected by promoting oxidation ("rust") and then converting the oxidation to a metalophosphate by using phosphoric acid and further protected by surface coating. (See: Passivation (chemistry)).

The IUPAC (mostly organic) name is phosphonic acid. This nomenclature is commonly reserved for substituted derivatives, that is, organic group bonded to phosphorus, not simply an ester. For example, {{chem2|(CH3)PO(OH)2}} is "methylphosphonic acid", which may of course form "methylphosphonate" esters.

{{reflist|30em}}

• {{cite book|last1=Holleman |first1=A. F. |last2=Wiberg |first2=E. |title=Inorganic Chemistry |publisher=Academic Press |location=San Diego |date=2001 |isbn=0-12-352651-5}}
• {{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|title=Concise Inorganic Chemistry |first=J.D.|last=Lee |date=3 January 2008|publisher=Oxford University Press |isbn=978-81-265-1554-7}}

{{Hydrogen compounds}}

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Category:Pnictogen oxoacids

Category:Phosphorus oxoacids

Category:Phosphonates

Category:Phosphonic acids

Category:Phosphorus(III) compounds