Hypophosphorous acid is a
phosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate rocks. Elemental phosphorus exists in two major forms - white phosphorus and red phosphorus...
oxoacidAn oxoacid is an acid which contains oxygen. More specifically, it is an acid which:#contains oxygen;#contains at least one other element;#has at least one hydrogen atom bound to oxygen; and#forms an ion by the loss of one or more protons....
and a powerful reducing agent with molecular formula H
3PO
2. Inorganic chemists refer to the free acid by this name (also as "HPA") although its
IUPAC nameThe IUPAC nomenclature of inorganic chemistry is a systematic method of naming inorganic chemical compounds as recommended by the International Union of Pure and Applied Chemistry . Ideally, every inorganic compound should have a name from which an unambiguous formula can be determined...
is dihydridohydroxidooxidophosphorus, or the acceptable name of
phosphinic acid. It is a colorless low-melting compound, which is soluble in water, dioxane, and alcohols. The formula for hypophosphorous acid is generally written H
3PO
2, but a more descriptive presentation is HOP(O)H
2 which highlights its monoprotic character.
Hypophosphorous acid is a
phosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate rocks. Elemental phosphorus exists in two major forms - white phosphorus and red phosphorus...
oxoacidAn oxoacid is an acid which contains oxygen. More specifically, it is an acid which:#contains oxygen;#contains at least one other element;#has at least one hydrogen atom bound to oxygen; and#forms an ion by the loss of one or more protons....
and a powerful reducing agent with molecular formula H
3PO
2. Inorganic chemists refer to the free acid by this name (also as "HPA") although its
IUPAC nameThe IUPAC nomenclature of inorganic chemistry is a systematic method of naming inorganic chemical compounds as recommended by the International Union of Pure and Applied Chemistry . Ideally, every inorganic compound should have a name from which an unambiguous formula can be determined...
is dihydridohydroxidooxidophosphorus, or the acceptable name of
phosphinic acid. It is a colorless low-melting compound, which is soluble in water, dioxane, and alcohols. The formula for hypophosphorous acid is generally written H
3PO
2, but a more descriptive presentation is HOP(O)H
2 which highlights its monoprotic character. Salts derived from this acid are called
phosphinates (hypophosphites).
HOP(O)H
2 exists in
equilibriumIn a chemical process, chemical equilibrium is the state in which the chemical activities or concentrations of the reactants and products have no net change over time. Usually, this would be the state that results when the forward chemical process proceeds at the same rate as their reverse reaction...
with the minor
tautomerTautomers are isomers of organic compounds that readily interconvert by a chemical reaction called tautomerization. It is common that this reaction results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond. In solutions in which...
HP(OH)
2. Sometimes the minor tautomer is called hypophosphorous acid and the major tautomer is called phosphinic acid.
Preparation and availability
The acid is prepared industrially via a two step process. Hypophosphite salts of the
alkaliThe alkali metals are a series of chemical elements forming Group 1 of the periodic table: lithium , sodium , potassium , rubidium , caesium , and francium ....
and
alkaline earth metalThe alkaline earth metals are a series of elements comprising Group 2 of the periodic table: beryllium , magnesium , calcium , strontium , barium and radium . This specific group in the periodic table owes its name to their oxides that simply give basic alkaline solutions...
s result from treatment of
white phosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate rocks. Elemental phosphorus exists in two major forms - white phosphorus and red phosphorus...
with hot aqueous solution of the appropriate
hydroxideIn chemistry, hydroxide is the name for the diatomic anion OH−, consisting of oxygen and hydrogen atoms, usually derived from the dissociation of a base. It is one of the simplest diatomic ions known....
, e.g. Ca(OH)
2.
- P4 + 3OH− + 3H2O → 3H2PO2− + PH3
The free acid may be prepared by the action of a strong acid on these hypophosphite salts.
- H2PO2− + H+ → H3PO2
Alternatively, H
3PO
2 arises by the oxidation of
phosphinePhosphine is the common name for phosphorus trihydride , also known by the IUPAC name phosphane and, occasionally, phosphamine. It is a colorless, flammable gas with a boiling point of −88 °C at standard pressure...
with
iodineIodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
in water.
- PH3 + 2I2 + 2H2O → H3PO2 + 4I− + 4H+
HPA is usually supplied as a 50% aqueous solution.
Uses
Hypophosphorous acid is used in the formulation of pharmaceuticals, discoloration of polymers, water treatment, retrieval of precious or non-ferrous metals. Its main use is for electroless plating; i.e. deposition of select metal films from solution on a sensitized surface. In organic chemistry, H
3PO
2 best known for their use in the reduction of arenediazonium salts, converting ArN
2+ to Ar-H. When diazotized in a concentrated solution of hypophosphorous acid, an
amineAmines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
substituent can be removed from arenes, selectively over alkyl amines.
DEA List I chemical status
Because hypophosphorous acid can reduce elemental
iodineIodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
to form hydroiodic acid, which is a reagent effective for reducing
ephedrineEphedrine is a sympathomimetic amine commonly used as a stimulant, appetite suppressant, concentration aid, decongestant, and to treat hypotension associated with anaesthesia. Ephedrine is similar in structure to the synthetic derivatives amphetamine and methamphetamine...
or
pseudoephedrinePseudoephedrine is a sympathomimetic amine commonly used as a decongestant. The salts pseudoephedrine hydrochloride and pseudoephedrine sulfate are found in many over-the-counter preparations either as single-ingredient preparations, or more commonly in combination with antihistamines, paracetamol...
to
methamphetamineMethamphetamine also known as metamfetamine , dextromethamphetamine, methylamphetamine, N-methylamphetamine, and desoxyephedrine) is a psychostimulant and sympathomimetic drug. Methamphetamine enters the brain and triggers a cascading release of dopamine, serotonin and norepinephrine...
, the
United States Drug Enforcement AdministrationThe Drug Enforcement Administration is a federal law enforcement agency under the United States Department of Justice, tasked with combating drug smuggling and use within the U.S...
designated hypophosphorous acid (and its salts) as a
List I precursor chemicalThe United States Drug Enforcement Administration maintains lists regarding not only the classification of illicit drugs . It also maintains List I of chemicals and List II of chemicals, which contain chemicals which are used to manufacture the controlled substances/illicit drugs...
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 ActThe Controlled Substances Act was enacted into law by the Congress of the United States as Title II of the Comprehensive Drug Abuse Prevention and Control Act of 1970. The CSA is the federal U.S. drug policy under which the manufacture, importation, possession, use and distribution of certain...
and 21
CFRThe Code of Federal Regulations is the codification of the general and permanent rules and regulations published in the Federal Register by the executive departments and agencies of the Federal Government of the United States...
§§ 1309 and 1310.
Inorganic and organic derivatives
Numerous derivatives are known in which the two
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
atoms directly bound to phosphorus are replaced by organic groups. These derivatives are known as
phosphinic acids, and their salts as
phosphinates. For example, formaldehyde and H
3PO
2 react to give (HOCH
2)
2PO
2H. The reaction is akin to the addition of thiols and HCN to aldehydes. Similarly, it adds to
Michael acceptorsThe Michael reaction or Michael addition is the nucleophilic addition of a carbanion to an alpha, beta unsaturated carbonyl compound. It belongs to the larger class of conjugate additions. This is one of the most useful methods for the mild formation of C-C bonds...
, for example with acrylamide it gives H(HO)P(O)CH
2CH
2C(O)NH
2.
Few metal complexes have been prepared from H
3PO
2, one example is Ni(O
2PH
2)
2.
Sources
- ChemicalLand21 Listing
- D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. ISBN 0-444-89307-5.
- V. V. Popik, A. G. Wright, T. A. Khan, J. A. Murphy "Hypophosphorous Acid" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
- D.W.Rich, M.C. Smith, Electroless Deposition of Nickel, Cobalt & Iron. IBM Corporation, Poughkeepsie, NY(1971)