Phosphine and Hypophoshite

[Anders Hoveland]

"...it is known that phosphorous acid, reduced with nascent hydrogen, yields phosphine and hypophosphorous acid

 

For the formation of phosphine from phosphorous and sodium hydroxide, heated together, the following equation is usually given:

(4)P + (3)NaOH + (3)H2O --> PH3 + (3)NaH2PO2

Without a doubt the reaction is far more complicated.

...what has not been shown, that hydrogen and at least two acids of phosphorous are formed when phosphine is generated in the usual way. ...When phosphine was obtained, by heating an aqueous solution of potassium hydroxide with phosphorous, only from 10 to 40 percent of the gas obtained was phosphine [the remainder being hydrogen]. The decomposition of phosphonium iodide, PH4I, is the only method of obtaining pure phosphine.

"investigation of sodamide and of its reaction-products with phosphorus" William Phillips Winter p42-43

 

The article goes on to say that the reaction of sodium phosphide with water generates small portions of phosphites and hydrogen gas, in addition to the main products of sodium hydroxide and phosphine.

 

Hydrogen sulfide is known to be able to reduce sulfur dioxide (actually sulfurous acid in aqueous solution) at room temperature. Perhaps phophine would similarly be able to reduce phosphorous acid (H3PO3)?

 

 

Aqueous alkali (KOH) dissolves red phopshorous, with the formation of phosphine. Interestingly, when hydrochloric acid was added to the solution, the phosphorous precipitated back into its elemental form.

Journal of the Chemical Society, Volume 75 (Great Britain) p.976

It seems apparent that hypophosphorous acid, H3PO2, is reduced by phosphine, PH3, to elemental phosphorous. The selective oxidation of PH3 by aqueous iodine also produces H3PO2. (in a similar reaction, iodine reduces thiosulfate, S2O3[-2], to tetrathionate, S4O6[-2] )

 

The fact that H3PO2 even exists suggests that the chemistry of the P-O bond is more similar to the C-O bond than the S-O bond, which is to say that H3PO3 is essentially not an oxidizer at ordinary temperatures, like SO2 can be. However, one paper mentions that PH3 can reduce 1-naphthol to naphthalene: "Phosphine as a Reducing Agent"

SHELDON A. BUCKLER, LOIS DOLL, FRANK K. LIND, MARTIN EPSTEIN. J. Org. Chem., 1962, 27 (3), pp 794–798

So the idea that PH3 could reduce H3PO3 is quite plausible.

 

Whether PH3 reacts with H3PO3 in aqeous solution is uncertain.

 

(the paper also describes the reaction of phosphine with nitrobenzene. There was no reaction at neutral conditions, but when sodium hydroxide was added then azoxybenzene was produced in high yield)

 

When heated by itself, hypophosphorous acid is resolved into phosphoric acid and phosphine. Phosphorous acid on heating at 200°C converts to phosphoric acid and phosphine.

 

It should be obvious that PH3 would not reduce H3PO4, just as H2S does not reduce aqueous H2SO4.

 

When boiled in alkaline liquids [aqueous KOH] they [hypophosphites] are decomposed into phosphates and hydrogen.

KPH2O2 + (2)KHO --> K3PO4 + H4

 

The dry salts decompose when heated, giving phosphoretted hydrogen [PH3] (hence they are very flammable) and leaving a residue of pyrophosphate.

(2)BaP2H4O4 --> (2)PH3 + H2O + Ba2P2O7

"A dictionary of chemistry and the allied branches of other sciences, Volume 4", Henry Watts, p524

 

Various Other Hydrides of Phosphorous

 

P2H4, first obtained by P. Thenard (Comptes rendus, 1844, 18, p. 652) by decomposing calcium phosphide with warm water, the products of reaction being then passed through a U tube surrounded by a freezing mixture (see also L. Gattermann, Ber., 1890, 23, p. 1174). It is a colourless liquid which boils at 57°-58° C. It is insoluble in water, but soluble in alcohol and ether. It is very unstable, being readily decomposed by heat or light. By passing the products of the decomposition of calcium phosphide with water over granular calcium chloride, the P2H4 gives a new hydride, P12H6 and phosphine, the former being an odourless, canary-yellow, amorphous powder. When heated in a vacuum it evolves phosphine, and leaves an orange-red residue of a second new hydride, P9H2 (A. Stock, W. Bottcher, and W. Lenger, Ber., 9 9, 4 39, 47, 2853).

 

P4H2, first obtained by Le Verrier, is formed by the action of phosphorus trichloride on gaseous phosphine (Besson, Comptes rendus, 111, p. 972); by the action of water on phosphorus di-iodide and by the decomposition of calcium phosphide with hot concentrated hydrochloric acid. It is a yellow solid, which is insoluble in water. It burns when heated to about 200° C. When warmed with alcoholic potash it yields gaseous phosphine, hydrogen and a hypophosphite.

 

Interesting Non-Reactivity of Phosphorus:

Elemental phosphorous can apparently be dissolved in liquid sulfur dichloride without reaction.

Edited December 2, 2011 by Anders Hoveland

[Anders Hoveland]

Precipitated cuprous oxide [Cu2O] is acted on rapidly by phosphine at ordinary temperature, forming a grey white mass and water, the grey substance is quite insoluble in water, if air be excluded; it is [PCu3]. It melts at a red heat, is rapidly dissolved by nitric acid or bromine water, and is attacked by hot sulfuric acid, with the formation of sulfur dioxide and phosphine. It does not reduce potassium permanganate.

 

While phosphine acts differently on the aqueous solutions of different cupric salts, its action of the ammoniacal solutions of all of them (chloride, sulfate, nitrate, acetate, formate, hydroxide) is the same. Copper phosphide is formed in amount corresponding to two-thirds of the phosphine which disappears, while the phopshorous of the other third is founf in the liquid, as phosphoruc and hypophosphorous acids in molecular proportions-

 

(6)PH3 + (12)CuCl2 + (6)H2O + (x)NH3 --> (4)PCu3 + PO4H3 + PO2H3 + (24)HCl + (x)NH3

 

If, after the absorption of the phosphine, oxygen be admitted, the precipitate redissolves completely, using up a volume of oxygen double that of the phosphine absorbed in the first place.

 

Journal of the Society of Chemical Industry, Volume 18, p716