1. Preparation and availability

2. Reactions and uses

3. Organic chemistry
    DEA List I chemical status 

4. Organophosphinic acids (Phosphinates)

5. Inorganic derivatives

6. Sources

7. References

Hydroxy-λ⁵-phosphanone

Oxo-λ⁵-phosphanol

Oxo-λ⁵-phosphinous acid

Phosphonous acid (for minor tautomer)

1.22 g/cm³ (50 wt% aq. solution)

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).^[4]

The acid is prepared industrially via a two step process: Firstly, hypophosphite salts of the alkali and alkaline earth metals result from the reaction of white phosphorus with hot aqueous solution of the appropriate hydroxide, e.g. Ca(OH)₂.

P₄ + 4 OH⁻ + 4 H₂O → 4 {{chem|H|2|PO|2|−}} + 2 H₂

The salt is then treated with a strong, non-oxidizing 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 ready 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.^[5]

Reactions and uses

Its main industrial use is for electroless nickel plating (Ni–P), although it is primarily used as a salt (sodium hypophosphite).^[6]

Organic chemistry

In organic chemistry, H₃PO₂ can be used for the reduction of arenediazonium salts, converting {{chem|ArN|2|+}} to Ar–H.^[7][8][9] 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.^[10]

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,^[11] the United States Drug Enforcement Administration designated hypophosphorous acid (and its salts) as a List I precursor chemical effective November 16, 2001.^[12] 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.^[12][13][14]

Organophosphinic acids (Phosphinates)

Organophosphinic acids have the formula R₂PO₂H. The two hydrogen atoms directly bound to phosphorus in phosphinic acid are replaced by organic groups. For example, formaldehyde and H₃PO₂ react to give (HOCH₂)₂PO₂H. Similarly, phosphinic acid adds to Michael acceptors, for example with acrylamide it gives H(HO)P(O)CH₂CH₂C(O)NH₂. The Cyanex family of dialkylphosphinic acids are used in hydrometallurgy to extract metals from ores.

Inorganic derivatives

Few metal complexes have been prepared from H₃PO₂, one example is Ni(O₂PH₂)₂.

Sources

• {{Cotton&Wilkinson6th}}
• ChemicalLand21 Listing
• {{cite book|first=D. E. C. |last=Corbridge |title=Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology |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=10.1002/047084289}}
• {{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}}

References

• Lewis, Brian
• Lewis Brian Adams
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• Lewisburg High School
• Lewisburg Historic District (Lewisburg, Pennsylvania)
• Lewisburg Historic District (Lewisburg, West Virginia)
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• Lewisburg, Penn.