Phosphorous Acid and Phosphites

Phosphorous acid, H3P03, is the acid that forms when P4O6 reacts with water  

For preparing a small amount of the acid in a laboratory, it is more convenient to carefully hydrolyze PC13  

Phosphorous acid is a weak dibasic acid having the molecular structure 

The hydrogen atom attached to phosphorus is not acidic, so there are only two dissociation constants that have the values Kal = 5.1 x 10-2 and K.a2 = 1.8 x 10-7. Normal phosphites contain the HP032- ion that has an irregular tetrahedral structure. The acid salts contain 

Organic phosphites are relatively important because they have useful properties and function as reactive intermediates for preparing numerous other compounds. The organic phosphites have the general formulas (RO)2P(0)H and (RO)3P and the structures of these types of compounds are 

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The trioxide P406 is not nearly as important as the acids and salts (phosphorous acid and phosphites) that are derived from it. The oxide, preparable by the action of a controlled oxidation of white phosphorus, decomposes when heated to elemental phosphorus and a third oxide (P02)x. The degree of polymerization of the latter oxide (a white solid) is still unsettled. 

F. Zecchini gave for the refraction eq. 26-04 with the / -formula, and 15-17 with the /i2-formula. 0. Stelling measured the X-ray K-absorption spectrum of phosphorous acid, and of its salts and di-esters, and found that the results agree with the assumption that the phosphorus is tervalent with the solids, and tautomeric quinque- and ter-valent in soln. Phosphorous acid, the di-esters, methyl phosphorous acid, and ferric monopropyl phosphate have the structure RO)2 PO.H sodium diethyl phosphite, (C2H50)2 PO.Na silver diethyl phosphate, (C2H50)2 PO.OAg and monoacetyl phosphorous acid ... 

Sulfur-containing additives include organic mono- and polysulfides, elemental sulfur as well as a wide variety of sulfurized fats and hydrocarbons. Phosphorus containing additives include esters of phosphoric acids, derivatives of thiophosphoric acids and phosphites (Davey, 1950 Sakurai and Sato, 1970). 

Diethyl phosphite was probably first prepared in 1854 by the reaction between alcohol and phosphorus(III) chloride.1 It can also be prepared from phosphorus(III) oxide and alcohol,2 from phosphorous acid and diazoethane,3 or from lead phosphite and ethyl iodide.4... 

Oxidation in Solution.—Phosphorous acid and the phosphites are not quite such strong reducing agents as hypophosphorous add and the hypophosphites. The free acid undergoes self-oxidation and -reduction at a higher temperature than hypophosphorous acid (see p. 138). 

The oxidation of phosphites by iodine was found to proceed to completion in neutral solution 2 (see Estimation, p. 149). Over a narrow range of concentrations the reaction was found to be uni-molecular with respect to iodine and phosphorous acid.3 It is said to be catalysed by hydrogen ions which are formed as the reaction proceeds.4 A further study of the velocity constants showed that the mechanism was more complicated than had previously been supposed, and that the two tautomeric forms participated in different ways.5 A solution of iodine in potassium iodide contains the ions I- and Iij", and also molecular iodine, I2. The latter reacts directly with the normal form of phosphorous acid and this reaction is repressed by hydrogen ions. Simultaneously, the phosphorous acid changes into another form with which the I8 ion reacts. This second reaction is accelerated by hydrogen ions either directly or, more probably, because they accelerate the tautomeric change. 

The Phosphites.—Two series of phosphites are known, the primary phosphites, MH2POj, and the secondary, M2HP03, M being a univalent metal. Crystalline salts have also been prepared containing an excess of phosphorous acid. The phosphites of the alkali metals and ammonia are soluble, those of the alkaline earths sparingly soluble, while those of other metals are only very slightly soluble. They may be prepared by the usual methods —... 

The X-ray K absorption spectra of phosphorous acid and the phosphites of Na, Al, Mn, Fe", Fe , Ca, Ni and Cd were nearly the same, the head of the absorption band lying at A =5754-1 X-ray units. The band of silver diethyl phosphite was at 5760-4. The values for phosphorus in the elementary state and in different forms of combination were as follows —... 

Sodium dihydrogen phosphite, NaH2P03,2 H20, crystallizes when a solution of equimolecular proportions of phosphorous acid and sodium hydroxide is cooled to —23° C. The heat of formation of the anhydrous salt from the elements is 333 8 Cal.,8 and from the acid and sodium hydroxide in solution 14 83 Cal.10 The heat of hydration of the hydrate is 6 05 Cal. 

Rubidium phosphates.—The three phosphates are prepared by the interaction of phosphoric acid and rubidium hydroxide or carbonate in appropriate proportions.13 The primary phosphate, RbH2P04, is acidic in aqueous solution the other two are alkaline. When heated at 244° C. the primary phosphate loses water, yielding the acid pyrophosphate, RbaH2P207.14 The secondary phosphite forms a monohydrate,... 

Cupric phosphite, CuHP03,2H20.—A phosphite of this formula is obtained by the interaction of solutions of diammonium hydrogen phosphite and cupric chloride, or of phosphorous acid and cupric acetate.6 It is unstable, but admits of drying at a medium temperature. 

Flame retardants—Whereas halogen-containing Mannich bases arc mainly used as modifiers of macromolccular materials, as mentioned before, phosphorus derivatives arc employed as additives. These products arc obtained from phosphorous acid or phosphites 584 " and from other analogous phosphorus-containing compounds. N-Heterocyclcs, after reaction with melamine and formaldehyde (585), are also used as (lame retardants. -... 

The rates of reaction of hypophosphorous acid with iodine bromine ", chlorine ", iodine chlorides , iodate , selenious and tel-lurous acids, silver nitrate , cupric chloride and mercuric chloride" (all forming phosphorous acid or phosphites) have been measured, and the results of the earlier work summarized clearly" . All the data are consistent with the hypothesis that there is prior transformation to some reactive form (I). This form (I) does not discriminate very effectively between different oxidants and thus the oxidation steps are presumed to have rates close to the diffusion-controlled limit. The rates of formation of I deduced in these studies are close enough to the rates of deuterium and tritium exchange for the residual difference to represent an isotope effect. Mitchell wrote the formula H5PO3 for I. Others have supposed it to be a tautomer e.g. HPO(OH)2. Both the isotopic exchange results and the oxidation studies require that its formation and decomposition be subject to acid catalysis. For the general mechanism... 

Nucleoside phosphites. Reaction of a nucleoside with phosphorous acid and di-p-tolylcarbodiimide in pyridine affords the nucleoside phosphite in good yield." Thus... 

Kamai. G.. and Kukhtin. V.A.. Addition of complete esters of phosphorous acid and phosphonous acids to conjugated systems. Part 2. Addition of hialkyl phosphites to acrolein and crotonaldehyde, Zh. Obshch. Khim.. 27. 2376. 1956 Chem. Abstr. 52. 7127h, 1958. 

Despite wide variations of synthesis conditions, the treatment of cellulose with dimethylphosphite, monomethylphosphite and phosphorous acid failed to yield phosphites containing only one type of phosphorus-containing groups. This difficulty was overcome by using phosphoric anhydrides as the esterifying reagents. The action of mixed anhydrides of phosphorous acids and acetic acid on cellulose has yielded cellulose esters with alkyl-(methyl-)phosphorous acid (73) ... 

Optically active phosphorus(iii) acid esters PhR P (OR ) have recently been prepared for the first time by the reaction of a chiral phosphorus chloride and an alcohol or thiol in the presence of an optically active amine, in this case (-)-NN-dimethyl-(l-phenylethyl)amine. (See also Refs. 449 and 450). A kinetic investigation shows that phosphorous acid and chloral react to give 2,2,2-trichloro-l-hydroxyethyl phosphonate at a much lower rate than the corresponding reaction with dimethyl hydrogen phosphite. Phosphorus trichloride and dialkyl phosphites (R0)2P(0)H have been shown to react at low temperature in the presence of pyridine, producing tris(dialkoxyphosphoryl)phosphines [(RO)2P(0)]3P. ... 

Synthesis of Phosphoric Acids and their Derivatives.-New approaches to the preparation of dialkyl phosphorofluoridates include the treatment of a vinyl phosphate, e.g.,(l), or an irninophosphace, e.g.,(2), with triethylamine tris(hydrogen fluoride) when the alkyl group consists of a suitably protected nucleoside, the reaction between mono(crimethylsilyl) phosphite in pyridine and sulphuryl chloride fluoride at -30° has been successfully explored. Dibenzyl phosphorofluorldate, hitherto unreported, has been obtained from tecrabenzyl diphosphate and caesium fluoride, and also from the reaction between dibenzyl hydrogen phosphate and 2-fluoro-N-methylpyridinlum tosylate. ... 

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