Acid, hypophosphorous

Hypophosphorous acid (phosphinic acid), H3PO2, mp = 26.5°C, is prepared commercially by treating white phosphorus with a boiling slnrry of calcium (or barium) hydroxide. Phosphine, hydrogen and some phosphite is also produced in a rather complicated process which can be imperfectly represented by (5.221) and (5.222). The resnlting mixture is filtered to remove insoluble calcium 

An alternative preparation is to pass phosphine into an agitated aqueous suspension of iodine until the colour has gone. This is followed by distillation under reduced pressure to remove HI and H2O, leaving pure hypophosphorous acid. 

FIGURE 5.34 Crystal structure of ammonium hypophosphite (NH4H2PO2). Small open circles = H filled circles = P. (Adapted from W.H. Zachariasen and R.C.L. Mooney, J. Chem. Phys., 2, 34,1934.) 

Most hypophosphite salts are water soluble. Neutral aqueous solutions are not oxidised by air, but, when heated strongly, the solid salts give products which include phosphites, phosphates, phosphine and red phosphorus (14.6). If they are boiled with 20% NaOH, phosphite and hydrogen are formed (5.225), but with more concentrated NaOH the reaction is (5.226), and with sulphuric acid it is (5.227). 

Hypophosphites are more powerful reducing agents than phosphites. The acid will reduce sulphur dioxide to sulphur and will precipitate Pt, Au, Ag, Hg and Bi from aqueous solutions of these salts. Hypophosphites reduce cupric salts to cuprous and dichromates to trivalent chromium salts. 

A procedure for the purification of the acid is given by Jenkins and Jones (III). 

The ion-exchange resin is again washed twice with distilled water, using 50 ml. each time it is reactivated with 5N HCl, and, after thorough washing until the eluent is acid-free, it can be used again. 

It is recommended that a batch of at least 60 g. of NaHgPOa be processed (in four portions) at one time. This procedure has the advantage that a pure solution of the free acid is obtained directly, without having to filter off the finely divided BaSO 4 as in method II. Evaporation in high vacuum assures that no decomposition of H3PO3 will occur and that the product will therefore be free of traces of HaPOg and H3PO4. 

When the temperature has risen to 105°C, the liquid is quickly filtered and the colorless filtrate is concentrated until the temperature rises to 110°C. It should not be allowed to boil. The temperature is now kept constant for 15 minutes and is then gradually raised to 130°C, again avoiding boiling. The acid simmers gently, shows no gas bubbles, and has no odor of PH3. However, some vapor is present because traces of it volatilize. By carefully heating, the temperature can be raised even to 138°C without decomposition. After the acid has been heated for about 10 minutes at 130°C, the flame is removed and the liquid is cooled and filtered into a glass-stoppered flask. 

The glass vessel is now cooled down to a few degrees below zero, and if crystallization does start, the bottom of the vessel is scratched with a glass rod and the material allowed to stand. 

Mitchell has shown that mercuric chloride oxidises this substrate to phosphorous acid 

The reaction shows a first-order dependence on substrate concentration but, except at very low concentration, is zero-order with respect to oxidant moreover, the zero-order rate coefficient is the same as that observed with oxidations by iodine, cupric chloride and silver nitrate. The reaction is acid-catalysed. The oxidation is completely analogous to the halogenation of ketones and involves a slow tautomeric equilibrium followed by rapid oxidation, viz. 

In contrast, thallic perchlorate oxidation follows a rate law d[Tl(III)] /f[Tl(III)][H3P02] 

The structure of crystalline H3PO2 is not known, but the structures of several salts have been studied. These salts MH2PO2 are not acid salts, in which there would be PO2 ions joined together by hydrogen bonds, for the closest approach of 0 atoms attached to different P atoms in NH4H2PO2 is 3-45 A. Instead the salts contain tetrahedral H2PO2 ions, though the H atoms were not directly located in the X-ray studies. The mean dimensions of the ion are as shown. 

We come now to a group of acids in which there are two or more P atoms directly bonded. 

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Hypophosphorous acid, H3PO2, H2P(0)0H. A monobasic acid. Ba(H2P02)2 is formed when white phosphorus is dissolved in Ba(OH)2 solution. H3PO2 and its salts are strong reducing agents. 

Hypophosphoric acid, H4P2O6, HP(0)(0H)(/i-0)P(0)(0H)2 although can be letrabasic (Na salt from NaOCI on red phosphorus). 

Make a thin paste of 21 5 g. of finely-powdered o-tolidine (a commercial product) with 300 ml. of water in a 1-litre beaker, add 25 g. (21 ml.) of concentrated hydrochloric acid, and warm until dissolved. Cool the solution to 10° with ice, stir mechanically, and add a further 25 g. (21 ml.) of concentrated hydrochloric acid (1) partial separation of o tolidine dihydrochloride will occur. Add a solution of 15 g, of sodium nitrite in 30 ml. of water as rapidly as possible, but keep the temperature below 15° a slight excess of nitrous acid is not harmful in this preparation. Add the clear, orange tetrazonium solution to 175 ml. of 30 per cent, hypophosphorous acid (2), and allow the mixture to stand, loosely stoppered, at room temperature for 16-18 hours. Transfer to a separatory funnel, and remove the upper red oily layer. Extract the aqueous layer with 50 ml, of benzene. Dry the combined upper layer and benzene extract with anhydrous magnesium sulphate, and remove the benzene by distillation (compare Fig. II, 13, 4) from a Widmer or similar flask (Figs. II, 24, 3-5) heat in an oil bath to 150° to ensure the removal of the last traces of benzene. Distil the residue at ca. 3 mm. pressure and a temperature of 155°. Collect the 3 3 -dimethyldiphenyl as a pale yellow liquid at 114-115°/3 mm. raise the bath temperature to about 170° when the temperature of the thermometer in the flask commences to fall. The yield is 14 g. 

If the quantity of hypophosphorous acid is doubled, the yield is increased... 

The replacement of 2-amino group by a hydrogen can be achieved by diazotization, followed by reduction with hypophosphorous acid (1-8, 13). Another method starting from 2-aminothiazole is to prepare the 2-halo-thiazole by the Sandmeyer reaction (prepared also from the 2-hydroxy-thiazole), which is then dehalogenated chemically or catalytically (1, 9, 10). 

It IS possible to replace ammo substituents on an aromatic nucleus by hydrogen by reducing a diazonium salt with hypophosphorous acid (H3PO2) or with ethanol These... 

Reductive deamination of primary arylamines The ammo substituent of an arylamine can be replaced by hydrogen by treatment of its de rived diazonium salt with ethanol or with hypophosphorous acid... 

Mercury(II) nitrate Acetylene, aromatics, ethanol, hypophosphoric acid, phosphine, unsaturated organic compounds... 

Mercury(II) oxide Chlorine, hydrazine hydrate, hydrogen peroxide, hypophosphorous acid, magnesium, phosphorus, sulfur, butadiene, hydrocarbons, methanethiol... 

Gravimetric methods more suitable for general use involve the precipitation of metallic gold from tetrachloraurate solutions by reduction with oxaUc acid, SO2, or hydroquinone. Formaldehyde, hydrazine, ferrous sulfate, and hypophosphorous acid also have been used but are considered less efficient (40). 

Complexes of Ir(III) are kineticaHy inert and undergo octahedral substitution reactions slowly. The rate constant for aquation of prBr(NH3)3] " [35884-02-7] at 298 K has been measured at 2 x 10 ° (168). In many cases, addition of a catalytic reducing agent such as hypophosphorous acid... 

Nylon-11. Nylon-11 [25035-04-5] made by the polycondensation of 11-aminoundecanoic acid [2432-99-7] was first prepared by Carothers in 1935 but was first produced commercially in 1955 in France under the trade name Kilsan (167) Kilsan is a registered trademark of Elf Atochem Company. The polymer is prepared in a continuous process using phosphoric or hypophosphoric acid as a catalyst under inert atmosphere at ambient pressure. The total extractable content is low (0.5%) compared to nylon-6 (168). The polymer is hydrophobic, with a low melt point (T = 190° C), and has excellent electrical insulating properties. The effect of formic acid on the swelling behavior of nylon-11 has been studied (169), and such a treatment is claimed to produce a hard elastic fiber (170). 

A widely used procedure for determining trace amounts of selenium involves separating selenium from solution by reduction to elemental selenium using tellurium (as a carrier) and hypophosphorous acid as reductant. The precipitated selenium, together with the carrier, are collected by filtration and the filtered soflds examined directly in the wavelength-dispersive x-ray fluorescence spectrometer (70). Numerous spectrophotometric and other methods have been pubHshed for the deterruination of trace amounts of selenium (71—88). 

Diarsines and Diarsenes. Under certain conditions, the reduction of compounds with two organic groups attached to arsenic may give rise to tetraalkyl-or tetraaryldiarsines. Thus a number of diarsines have been obtained by the reduction of arsinic acids with phosphorous or hypophosphorous acid (100). Diarsines can also be prepared by the treatment of a metal dialkyl- or diarylarsenide with iodine (101) or a 1,2-dihaloethane (102). 

A large number of polymeric substances, (RAs) or (ArAs), are also known (113). They are usually prepared by the reduction of arsonic acids with hypophosphorous acid (100,114) or sodium dithionite (115). Most of these polymers have not been well characterized. An insoluble, purple material, poly(methylarsinidene) [26403-94-1], (CH As), prepared by the interaction of methylarsine and a dihalomethylarsine, however, has been shown by an x-ray investigation to have a ladderlike polymeric stmcture in which the inter-mng distances correspond to one-electron bonds (116) ... 

The hydriodic acid employed was a 56% aqueous solution preserved with 0.8% hypophosphorous acid obtained from Fisher Scientific Co. Once a bottle is opened, the contents tend to deteriorate, becoming dark-colored in less than 2 days. However, shelf life can be extended indefinitely if the container is purged with dry nitrogen before resealing. 

Note that the value of pKa for hypophosphorous acid H3PO3 is consistent with its (correct) formulation as HPO(OH)2 rather than as P(OH)3, which would be expected to have pKa > 8. Similarly for H3PO2, which is H2PO(OH) rather than HP(OH)2. 

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