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Oxoacids of Phosphorus

Module by: Andrew R. Barron. E-mail the author

Phosphorous pentoxide (P2O5) is an excellent drying agent, and its action is a result of the formation of a range of oxoacids.

Hypophosphorous acid

Hypophosphorous acid, H3PO2, is easily prepared pure buy the reaction of white phosphorous with base, followed by acidification, Equation 1. The pure acid is a solid (Mp = 27 °C) and very soluble in water.

graphics1.jpg
(1)

The structure of H3PO2 is determined by X-ray crystallography to be tetrahedral with two hydride ligands and a hydroxide (Figure 1a). The presence of two hydrides is confirmed by NMR spectroscopy. The 1H NMR resonance shows an OH line and a doublet from the P-H with a large one-bond coupling constant to the 31P nucleus. The non-decoupled 31P NMR spectrum shows a triplet (δ = 13 ppm, JP-H = 530 Hz) due to the two hydrides.

Figure 1: Oxoacids of phosphorous: (a) hypophosphorous acid, (b), phosphorous acid, and (c) phosphoric acid.
Figure 1 (graphics2.jpg)

In water hypophosphorous acid is a monobasic acid (pK = 1.2), Equation 2, and it forms a wide range of salts. It is also a powerful reducing agent, but its reaction kinetics is slow.

graphics3.jpg
(2)

Phosphorous acid

The reaction of P4O6 or PCl3 with water yields phosphorous acid, H3PO3; which like hypophosphorous acid is a solid (Mp = 70.1 °C) and very soluble in water. The structure is shown by X-ray crystallography to be comprised of a tetrahedral phosphorus with one hydride and two hydroxides (Figure 1b). 31P NMR spectroscopy demonstrates the presence of a single hydride by the presence of a doublet as a consequence of the phosphorous center being split by a single hydride (δ = 4 ppm, JP-H = 700 Hz). The 1H NMR spectrum shows a doublet for the hydride and a single resonance of twice the intensity for the hydroxide.

As expected, in water phosphorous acid is dibasic, Equation 3. The acid (and the anions) are strong reducing agents, yielding phosphoric acid. They actually react very slowly, and it is thought that this may be due to the reaction being in the tautomeric form, P(OH)3. Although this has not been isolated, the trialkyl derivatives exist in both forms, i.e., esters of phosphoric acids (Figure 2a) and trialkyl phosphates (Figure 2b).

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(3)
Figure 2: Structures of two tautomeric forms; (a) an ester of phosphoric acid and (b) a trialkyl phosphate.
Figure 2 (graphics5.jpg)

Ortho phosphoric acid

Orthophosphoric acid (H3PO4) is the most common oxoacid of phosphorus (Figure 1c). The term acid phosphoric acid is commonly used. It is made from the reaction of phosphates with sulfuric acid (H2SO4), or from the hydrolysis of P4O10. The pure acid is a colorless crystalline solid (Mp = 42.35 °C) with extensive hydrogen bonding. The inter-phosphoric acid hydrogen bonding is partially maintained in aqueous solutions above 50% solutions.

Phosphoric acid is very stable and shows no oxidation chemistry below 350 °C. As expected, phosphoric acid is tribasic, Equation 4. The anions H2PO4- and HPO42- have particular names, dihydrogen phosphate and monohydrogen phosphate, respectively.

graphics6.jpg
(4)

Many salts of are known for all three anions; those with phosphate (PO42-) are often insoluble in water. Many coordination complexes are known, especially with M3+ and M4+ ions.

Phosphorous esters

The alkyl and aryl homologs of phosphoric acid (i.e., the phosphate triester O=P(OR)3) are prepared by the reaction of P4O10 with the appropriate alcohol. The triesters are good solvents and Lewis basic ligands with coordination via the oxide moiety. The diesters and monoesters can also be made, and they are important in biochemical processes.

Phosphite triesters, P(OR)3, may be made by the reaction of PCl3 with alcohols or phenols in the presence of an organic base as the acceptor for the HCl formed. Alternatively, they can be prepared directly from white phosphorous, Equation 5.

graphics7.jpg
(5)

Phosphite triesters are readily oxidized to the appropriate phosphate triester, Equation 6. They also react with alkyl halides to form the dialkyl phosphonate via the Michaelis-Arbusov reaction, Equation 7.

graphics8.jpg
(6)
graphics9.jpg
(7)

Polyphosphates

Polyphosphates contain the PO4 unit, and the simplest example (pyrophosphate or diphosphate, Figure 3a) can be considered as condensation products of monohydrogen phosphate, Equation 8. Longer chains can be formed, e.g., the triphosphate P3O105- (Figure 3b). The formation and reverse (hydrolysis) reaction are slow, but are readily catalyzed, e.g., by enzymes.

graphics10.jpg
(8)
Figure 3: The structures of the (a) diphosphate and (b) triphosphate anions.
Figure 3 (graphics11.jpg)

The monoesters of the diphosphate and triphosphate are very important in biological processes. In particular the conversion of the triphosphate ATP (adenosine triphosphate) to the diphosphate ADP (adenosine diphosphate) by the transfer of a phosphate group, Equation 9, is important in the energetics of biological reactions.

graphics12.jpg
(9)

Cyclic polyphosphates

Also known as meta phosphates, cyclic phosphates are also made from fused PO4 units. The simplest, P3O93-, is shown in Figure 4. Slow addition of water to P4O10 results in the formation of the tetrameric polyphosphate, P4O124- (Figure 5), which is known as calgon due to its ability to complex Ca2+ as well as other metals.

Figure 4: The structure of P3O93-. Phosphorus atoms are represented by the large spheres, and oxygen atoms by the small spheres (Copyright: Karl Harrison, 2005).
Figure 4 (P3O9.jpg)
Figure 5: The structure of P4O124-calgon. Phosphorus atoms are represented by the large spheres, and oxygen atoms by the small spheres (Copyright: Karl Harrison, 2005).
Figure 5 (graphics14.jpg)

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