A Brief note on Oxoacids of Phosphorus

Phosphorus reacts with oxygen to generate a range of chemicals, and several of these are oxoacids. A phosphorus oxoacid is composed of phosphorus and oxygen, at least one hydrogen atom bonded to oxygen, and the loss of one or more protons results in the formation of an ion.

Acids that contain oxygen are called oxyacids. Phosphorus is an example of an element that can be utilized to synthesize a wide variety of oxoacids. H3PO4, H3PO3, and other frequently occurring oxyacids in phosphorus oxoacids, the phosphorus atom is tetrahedrally surrounded by other atoms. It is self-evident that these acids have at least one P–OH bond and P=O bond. Along with P=O and P–OH connections, phosphorus oxoacids have P–P and P–H bonds. The phosphorus oxidation state is less than +5 in these conditions. These acids are well-known for their ability to transition between higher and lower oxidation states. For instance, phosphorous acid decomposes into phosphoric acid and phosphine when heated.

P-H bonds in oxoacids are not ionizable, resulting in the formation of H+ ions. The H atoms linked to oxygen in the P-OH state are ionizable. As a result, we can state that basicity is a property of H atoms in combination with oxygen. Due to the presence of two P-OH bonds, phosphorus acid, H3PO3, is dibasic. Similarly, because of the presence of three P-OH bonds, phosphoric acid, H3PO4, is tribasic. Significant reducing powers are possessed by phosphorus oxoacids with P-H bonds. Hypophosphorous acid is an excellent reducing agent due to its two P-H bonds.

4AgNO3 + 2H2O + H3PO2 → 4Ag + 4HNO3 + H3PO4

Oxoacids of Phosphorus

Meta phosphoric Acid (HPO3) n–We can obtain it by heating orthophosphoric acid to a temperature of approximately 850 degrees Celsius. It does not exist as a monomer. It is a cyclic trimer, cyclic tetramer, or a polymer.

H3PO4 → HPO3 + H2O

Phosphoric acid units can be linked in rings (cyclic structures), chains (catena), or branches by condensation, which results in the formation of water with each acid addition. These structures are composed of connected molecules of metaphosphoric acid.

Orthophosphoric Acid (H3PO4)- Orthophosphoric acid is formed when P4O10 is treated with bubbled water. The structure of this acid is tribasic.

Phosphorus acid, H3PO3 -Phosphoric acid is a diprotic acid. As a result, it ionizes two protons. It is more accurately represented by the structural formula HPO(OH)2. The Phosphorous acid is obtained when the phosphorus trichloride is hydrolyzed with steam or acid.

PCl3 + 3H2O → HPO(OH)2 + 3HCl

A polyphosphorous acid may include dozens of condensed units. Regardless of the value of n, both polyphosphorous acid and polyphosphonic acid have the same chemical formula for any value of n; for example, triphosphosphonic acid is H5P3O7, whereas triphosphorous acid is H5P3O7 for n=3.Numerous OH groups exist in sufficiently large oligophosphorous acids, which can result in the condensation of a cyclophosphorous acid that lacks multiple (HPO3) metaphosphoric acid units.

However, the term cyclophosphorous acid (cyclophosphites) may refer to a misnomer in which the cyclic component is carbon-based with a phosphorous acid side chain of one or more molecules, or one or a limited number of either of the two tautomers present in the ring but in a tiny amount.The influence of ring size variation on the phosphonate-phosphite tautomerism of cyclophosphorous acids, for example, has been demonstrated.Cyclophosphorous acids, on the other hand, are biheteroorganic.

Additionally, branching can occur in oligophosphorous or polyphosphorous acid. These are referred to as ultraoligophosphorous or ultrapolyphosphorous acids, respectively, or ultraoligophosphites and ultrapolyphosphites.

Hypophosphoric Acid (H4P2O6)-Hypophosphoric acid is generated by executing a controlled oxidation of red phosphorus with sodium chlorite. Following the formation of the acid’s disodium analogue, it is passed via a cation exchanger to generate hypophosphoric acid. The acid is naturally tetrabasic.

2P + 2NaClO2 + 2H2O → Na2H2P2O6 + 2HCl

Na2H2P2O6 + 2H → H4P2O6 + 2Na

Utilization of Phosphorus Oxoacids

Among the several uses for phosphorus oxoacids are the following:

• Phosphorus is used in place of sulfuric acid in the manufacture of HI and HBr.

• It is used as a souring agent in the production of soft drinks.

• It is used to make sodium phosphate, ammonium phosphate, and potassium phosphate salts.

• It is used to make phosphatic fertilisers.

Classification–

The phosphorus oxoacids are classified according to the oxidation state(s) of the phosphorus atom(s), which can range between +1 and +5. Oxygen atoms are typically in the oxidation state -2, but may be in the oxidation state -1 if the molecule contains the peroxide groups.

1. Oxidation state +1–

Hypophosphorous acid (or phosphinic acid), H3PO2 (or H2PO(OH)), a sulfonamide monoprotic acid (meaning that only one of the hydrogen atoms is acidic). Hypophsphites or phosphinites are the names given to its salts and esters.

2. Oxidation state +3-Phosphorous acid (or phosphonic acid), H3PO3 (or HPO(OH)2), diprotic acid (with only two acidic hydrogens). The salts and esters of this compound are referred to as phosphites or phosphonates

3. Oxidation state +4 -Hypophosphoric acid, H4P2O6 (or (HO)2P–P(OH)2). Each of the four hydrogens is acidic. Hypophosphates are its salts and esters.

4. Oxidation state +5-The phosphoric acids are the most important members of this group, as each phosphorus atom is connected to four oxygen atoms, one of which has a double bond, placed as the corners of a tetrahedron. Two or more of these PO4 tetrahedral may be linked via common single-bonded oxygen to form linear or branched chains, cycles, or more complicated structures. The remaining single-bonded oxygen atoms are filled by acidic hydrogen atoms. Their generic formula is Hnx+2PnO3nx+1, where n denotes the number of phosphorus atoms in the molecule and x denotes the number of fundamental cycles in its structure.

These acids, as well as their esters and salts (“phosphates”), contain some of the most well-known and important phosphorus compounds.

CONCLUSION- 

Phosphorus oxoacid (or phosphorus acid) is a generic term that refers to any acid composed of phosphorus, oxygen, and hydrogen atoms. There are an endless number of these compounds. While some of them are unstable and have not been separated, they are present in stable salts and esters as derived anions and organic groups. The most important are the phosphoric acids, which are used in biology, geology, industry, and chemical research. Their esters and salts are phosphates.