Phosphorous acid

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Phosphorous acid
Wireframe model of phosphorous acid Phosphonic-acid-2D-dimensions-vector.svg
Wireframe model of phosphorous acid
Ball and stick model of phosphorous acid Phosphonic-acid-3D-balls-A.png
Ball and stick model of phosphorous acid
Sample of H3PO3.jpg
Names
IUPAC name
Phosphonic acid
Systematic IUPAC name
Phosphorous acid
Other names
Dihydroxyphosphine oxide

Dihydroxy(oxo)-λ5-phosphane
Dihydroxy-λ5-phosphanone
Orthophosphorous acid
Oxo-λ5-phosphanediol
Oxo-λ5-phosphonous acid

Metaphosphoroic acid

Contents

Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.033.682 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 237-066-7
1619
KEGG
PubChem CID
RTECS number
  • SZ6400000
UNII
UN number 2834
  • InChI=1S/H3O3P/c1-4(2)3/h4H,(H2,1,2,3) Yes check.svgY
    Key: ABLZXFCXXLZCGV-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/H3O3P/c1-4(2)3/h4H,(H2,1,2,3)
    Key: ABLZXFCXXLZCGV-UHFFFAOYAF
  • OP(=O)O
  • OP(O)O
Properties
H3PO3
Molar mass 81.99 g/mol
Appearancewhite solid
deliquescent
Density 1.651 g/cm3 (21 °C)
Melting point 73.6 °C (164.5 °F; 346.8 K)
Boiling point 200 °C (392 °F; 473 K) (decomposes)
310 g/100 mL
Solubility soluble in ethanol
Acidity (pKa)1.1, 6.7
42.5·10−6 cm3/mol
Structure
pseudo-tetrahedral
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
skin irritant
GHS labelling: [1]
GHS-pictogram-acid.svg GHS-pictogram-exclam.svg
Danger
H302, H314
P260, P264, P270, P280, P301+P312, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P330, P363, P405, P501
NFPA 704 (fire diamond)
NFPA 704.svgHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 0: Will not burn. E.g. waterInstability 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazards (white): no code
3
0
1
Safety data sheet (SDS) Sigma-Aldrich
Related compounds
Related compounds
H3PO4 (i.e., PO(OH)3)
H3PO2 (i.e., H2PO(OH))
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Phosphorous acid (or phosphonic acid) is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in the preparation of other phosphorus compounds. Organic derivatives of phosphorous acid, compounds with the formula RPO3H2, are called phosphonic acids.

Nomenclature and tautomerism

Tautomerism of H3PO3.png

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

PIII(OH)3 ⇌ HPV(O)(OH)2     K = 1010.3 (25°C, aqueous) [4]

Preparation

On an industrial scale, the acid is prepared by hydrolysis of phosphorus trichloride with water or steam: [5]

PCl3 + 3 H2O → HPO(OH)2 + 3 HCl

HPO(OH)2 could be produced by the hydrolysis of phosphorus trioxide:

P4O6 + 6 H2O → 4 HPO(OH)2

Reactions

Acid–base properties

Phosphorous acid has a pKa in the range 1.26–1.3. [6] [7]

HP(O)(OH)2 → HP(O)2(OH) + H+     pKa = 1.3

It is a diprotic acid, the hydrogenphosphite ion, HP(O)2(OH) is a weak acid:

HP(O)2(OH) → HPO2−3 + H+     pKa = 6.7

The conjugate base HP(O)2(OH) is called hydrogen phosphite, and the second conjugate base, HPO2−3, is the phosphite ion. [8] (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 H3PO4 .

Redox properties

On heating at 200 °C, phosphorous acid disproportionates to phosphoric acid and phosphine: [9]

4 H3PO3 → 3 H3PO4 + PH3

This reaction is used for laboratory-scale preparations of PH3.

Phosphorous acid slowly oxidizes in air to phosphoric acid. [5]

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:

H3PO3 + 2 HgCl2 + H2O → Hg2Cl2 + H3PO4 + 2 HCl

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

H3PO3 + Hg2Cl2 + H2O → 2 Hg + H3PO4 + 2 HCl

As a ligand

Structure of
Mo(CO)5P(OH)3. WAJJOH.svg
Structure of Mo(CO)5P(OH)3.

Upon treatment with metals of d6 configuration, phosphorous acid is known to coordinate as the otherwise rare P(OH)3 tautomer. Examples include Mo(CO)5(P(OH)3) and [Ru(NH3)4(H2O)(P(OH)3)]2+. [10] [11]

Heating a mixture of potassium tetrachloroplatinate and phosphorous acid gives the luminescent salt potassium diplatinum(II) tetrakispyrophosphite: [12]

2 K2PtCl4 + 8 H3PO3 → K4[Pt2(HO2POPO2H)4] + 8 HCl + 4 H2O

Uses

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. [5]

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 phosphorous acid, 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)).

Organic derivatives

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, (CH3)PO(OH)2 is "methylphosphonic acid", which may of course form "methylphosphonate" esters.

Related Research Articles

<span class="mw-page-title-main">Phosphate</span> Chemical compound

In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid, a.k.a. phosphoric acid H3PO4.

<span class="mw-page-title-main">Phosphite anion</span> Ion

A phosphite anion or phosphite in inorganic chemistry usually refers to [HPO3]2− but includes [H2PO3] ([HPO2(OH)]). These anions are the conjugate bases of phosphorous acid (H3PO3). The corresponding salts, e.g. sodium phosphite (Na2HPO3) are reducing in character.

An acidic oxide is an oxide that either produces an acidic solution upon addition to water, or acts as an acceptor of hydroxide ions effectively functioning as a Lewis acid. Acidic oxides will typically have a low pKa and may be inorganic or organic. A commonly encountered acidic oxide, carbon dioxide produces an acidic solution when dissolved.

<span class="mw-page-title-main">Hydrogen bromide</span> Chemical compound

Hydrogen bromide is the inorganic compound with the formula HBr. It is a hydrogen halide consisting of hydrogen and bromine. A colorless gas, it dissolves in water, forming hydrobromic acid, which is saturated at 68.85% HBr by weight at room temperature. Aqueous solutions that are 47.6% HBr by mass form a constant-boiling azeotrope mixture that boils at 124.3 °C (255.7 °F). Boiling less concentrated solutions releases H2O until the constant-boiling mixture composition is reached.

<span class="mw-page-title-main">Phosphorus trichloride</span> Chemical compound

Phosphorus trichloride is an inorganic compound with the chemical formula PCl3. A colorless liquid when pure, it is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds. It is toxic and reacts readily with water to release hydrogen chloride.

<span class="mw-page-title-main">Phosphoric acids and phosphates</span> Class of chemical species; phosphorus oxoacids and their deprotonated derivatives

In chemistry, a phosphoric acid, in the general sense, is a phosphorus oxoacid in which each phosphorus (P) atom is in the oxidation state +5, and is bonded to four oxygen (O) atoms, one of them through a double bond, arranged as the corners of a tetrahedron. Two or more of these PO4 tetrahedra may be connected by shared single-bonded oxygens, forming linear or branched chains, cycles, or more complex structures. The single-bonded oxygen atoms that are not shared are completed with acidic hydrogen atoms. The general formula of a phosphoric acid is Hn+2−2xPnO3n+1−x, where n is the number of phosphorus atoms and x is the number of fundamental cycles in the molecule's structure, between 0 and n + 2/2.

<span class="mw-page-title-main">Phosphorus pentoxide</span> Chemical compound

Phosphorus pentoxide is a chemical compound with molecular formula P4O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydrating agent.

<span class="mw-page-title-main">Arsenous acid</span> Chemical compound

Arsenous acid (or arsenious acid) is the inorganic compound with the formula H3AsO3. It is known to occur in aqueous solutions, but it has not been isolated as a pure material, although this fact does not detract from the significance of As(OH)3.

In chemistry, phosphorus oxoacid is a generic name for any acid whose molecule consists of atoms of phosphorus, oxygen, and hydrogen. There is a potentially infinite number of such compounds. Some of them are unstable and have not been isolated, but the derived anions and organic groups are present in stable salts and esters. The most important ones—in biology, geology, industry, and chemical research—are the phosphoric acids, whose esters and salts are the phosphates.

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.

An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bonded to oxygen that can dissociate to produce the H+ cation and the anion of the acid.

Phosphinates or hypophosphites are a class of phosphorus compounds conceptually based on the structure of hypophosphorous acid. IUPAC prefers the term phosphinate in all cases, however in practice hypophosphite is usually used to describe inorganic species, while phosphinate typically refers to organophosphorus species.

<span class="mw-page-title-main">Phosphonate</span> Organic compound containing C–PO(OR)2 groups

In organic chemistry, phosphonates or phosphonic acids are organophosphorus compounds containing C−PO(OR)2 groups. Phosphonic acids, typically handled as salts, are generally nonvolatile solids that are poorly soluble in organic solvents, but soluble in water and common alcohols.

<span class="mw-page-title-main">Phosphite ester</span> Organic compound with the formula P(OR)3

In organic chemistry, a phosphite ester or organophosphite usually refers to an organophosphorous compound with the formula P(OR)3. They can be considered as esters of an unobserved tautomer phosphorous acid, H3PO3, with the simplest example being trimethylphosphite, P(OCH3)3. Some phosphites can be considered esters of the dominant tautomer of phosphorous acid (HP(O)(OH)2). The simplest representative is dimethylphosphite with the formula HP(O)(OCH3)2. Both classes of phosphites are usually colorless liquids.

Organophosphorus chemistry is the scientific study of the synthesis and properties of organophosphorus compounds, which are organic compounds containing phosphorus. They are used primarily in pest control as an alternative to chlorinated hydrocarbons that persist in the environment. Some organophosphorus compounds are highly effective insecticides, although some are extremely toxic to humans, including sarin and VX nerve agents.

<span class="mw-page-title-main">Phosphorus trioxide</span> Chemical compound

Phosphorus trioxide is the chemical compound with the molecular formula P4O6. Although the molecular formula suggests the name tetraphosphorus hexoxide, the name phosphorus trioxide preceded the knowledge of the compound's molecular structure, and its usage continues today. This colorless solid is structurally related to adamantane. It is formally the anhydride of phosphorous acid, H3PO3, but cannot be obtained by the dehydration of the acid. A white solid that melts at room temperature, it is waxy, crystalline and highly toxic, with garlic odor.

<span class="mw-page-title-main">Triethyl phosphite</span> Chemical compound

Triethyl phosphite is an organophosphorus compound, specifically a phosphite ester, with the formula P(OCH2CH3)3, often abbreviated P(OEt)3. It is a colorless, malodorous liquid. It is used as a ligand in organometallic chemistry and as a reagent in organic synthesis.

<span class="mw-page-title-main">Hypophosphoric acid</span> Chemical compound

Hypophosphoric acid is a mineral acid with the formula H4P2O6, with phosphorus in a formal oxidation state of +4. In the solid state it is present as the dihydrate, H4P2O6·2H2O. In hypophosphoric acid the phosphorus atoms are identical and joined directly with a P−P bond. Isohypophosphoric acid is a structural isomer of hypophosphoric acid in which one phosphorus has a hydrogen directedly bonded to it and that phosphorus atom is linked to the other one by an oxygen bridge to give a phosphorous acid/phosphoric acid mixed anhydride. The two phosphorus atoms are in the +3 and +5 oxidation states, respectively.

<span class="mw-page-title-main">Diethylphosphite</span> Chemical compound

Diethyl phosphite is the organophosphorus compound with the formula (C2H5O)2P(O)H. It is a popular reagent for generating other organophosphorus compounds, exploiting the high reactivity of the P-H bond. Diethyl phosphite is a colorless liquid. The molecule is tetrahedral.

<span class="mw-page-title-main">Dimethylphosphite</span> Chemical compound

Dimethylphosphite is an organophosphorus compound with the formula (CH3O)2P(O)H, known as dimethyl hydrogen phosphite (DMHP). Dimethylphosphite, is a minor tautomer of the phosphorus(V) derivative. It is a reagent for generating other organophosphorus compounds, exploiting the high reactivity of the P-H bond. The molecule is tetrahedral. It is a colorless liquid. The compounds can be prepared by methanolysis of phosphorus trichloride or by heating diethylphosphite in methanol.

References

  1. "Phosphorous acid". pubchem.ncbi.nlm.nih.gov.
  2. 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.
  3. International Union of Pure and Applied Chemistry (2005). Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSC IUPAC . ISBN   0-85404-438-8 . Electronic version..
  4. Guthrie, J. Peter (1979). "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". Canadian Journal of Chemistry. 57 (2): 236–239. doi: 10.1139/v79-039 .
  5. 1 2 3 Bettermann, Gerhard; Krause, Werner; Riess, Gerhard; Hofmann, Thomas (2000). "Phosphorus Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_527. ISBN   978-3527306732..
  6. Larson, John W.; Pippin, Margaret (1989). "Thermodynamics of ionization of hypophosphorous and phosphorous acids. Substituent effects on second row oxy acids". Polyhedron. 8 (4): 527–530. doi:10.1016/S0277-5387(00)80751-2.
  7. CRC Handbook of Chemistry and Physics (87th ed.). pp. 8–42.
  8. Novosad, Josef (1994). Encyclopedia of Inorganic Chemistry. John Wiley and Sons. ISBN   0-471-93620-0.
  9. Gokhale, S. D.; Jolly, W. L. (1967). "Phosphine". Inorganic Syntheses. Vol. 9. pp. 56–58. doi:10.1002/9780470132401.ch17. ISBN   9780470132401.
  10. 1 2 Xi, Chanjuan; Liu, Yuzhou; Lai, Chunbo; Zhou, Lishan (2004). "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". Inorganic Chemistry Communications. 7 (11): 1202–1204. doi:10.1016/j.inoche.2004.09.012.
  11. Sernaglia, R. L.; Franco, D. W. (2005). "The ruthenium(II) center and the phosphite-phosphonate tautomeric equilibrium". Inorg. Chem. 28 (18): 3485–3489. doi:10.1021/ic00317a018.
  12. Alexander, K. A.; Bryan, S. A.; Dickson, M. K.; Hedden, D.; Roundhill (2007). "Potassium Tetrakis[Dihydrogen Diphosphito(2–)]Diplatinate(II)". Inorganic Syntheses. Vol. 24. pp. 211–213. doi:10.1002/9780470132555.ch61. ISBN   9780470132555.

Further reading