Hypophosphorous acid


Hypophosphorous acid, 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 HOPH2, which highlights its monoprotic character. Salts derived from this acid are called hypophosphites.
HOPH2 exists in equilibrium with the minor tautomer HP2. 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.
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. Ca2.
The salt is then treated with a strong, non-oxidizing acid to give the free hypophosphorous acid:
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.

Reactions and uses

Its main industrial use is for electroless nickel plating, although it is primarily used as a salt.
Hypophosphorous acid can reduce chromium oxide to chromium oxide:
H3PO2 + 2 Cr2O3 → 4 CrO + H3PO4

Organic chemistry

In organic chemistry, H3PO2 can be used for the reduction of arenediazonium salts, converting to Ar–H. 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.

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, the United States Drug Enforcement Administration designated hypophosphorous acid as a List I precursor chemical effective November 16, 2001. 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.

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 2PO2H. Similarly, phosphinic acid adds to Michael acceptors, for example with acrylamide it gives HPCH2CH2CNH2. 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 Ni2.