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The principal results of the preceding investigations may be summed up as follows:
1. There exists a series of bodies of the general formula (PNCl)n, beginning with Liebig's chloronitride of phosphorus, and extending indefinitely upward.
2. Each of these bodies yields, on saponification, an acid with the same number of phosphorus and nitrogen atoms.
3. The first four of these acids exist in two forms—the lactam form, (PNO,H2). (metaphosphimic acids), where n is either 3, 4, 5, or 6, which is formed only in neutral or acid solution, and the open-chain form (PNO,H2)+H20, formed under the influence of alkalies.
4. The acid derived from the fifth chloronitride, P,N,Cl-, and possibly those from the higher members, do not form lactams, but persist in the open form under all circumstances.
5. The properties of these acids with respect to stability, power of forming lactams, and nature of decomposition products may be explained by stereochemical considerations, analogous to those of von Baeyer on the methylene hydrocarbons and of Joh. Wislicenus on the lactones.
EXPERIMENTAL PART. Saponification of the phosphonitrilic chlorides.-As pointed out in the preceding papers, triphosphonitrilic chloride is conveniently saponified by shaking its ethereal solution with an aqueous solution of sodium acetate, and tetraphosphonitrilic chloride in a similar manner with water, the trifling quantities of secondary decomposition products being easily removed. These methods can not be applied to the higher chlorides, for the secondary products formed by the liberated acetic or hydrochloric acid can not be separated from the main product owing to the uncrystallizable nature of the latter. A smooth saponification without secondary products may be effected by using sodium hydroxide in sufficient amount to keep the solution always strongly alkaline.
Four parts chloride are dissolved in about 20 parts alcohol-free ether and shaken with a solution of 5 parts pure sodium hydroxide in 20 parts water. A bottle with a carefully paraffined glass stopper must be used, corks being inadmissible, as they give rise to colored prod. ucts which can not be removed. The shaking is conveniently effected by a rotator, moved by a small electric motor, and about fifty hours are sufficient for complete saponification. As soon as the ether is found to be practically free from chloronitrides, the alkaline solution is drawn off and precipitated by 2 or 3 volumes of alcohol. This throws down the sodium salt as a thick sirup, which is repeatedly washed by stirring with 60 per cent alcohol, dissolved in water, reprecipitated by alcohol, and again washed in the same way until it is free from sodium chloride. It is then dehydrated by stirring, kneading, and finally pulverizing under renewed portions of absoluto alcohol. After standing several hours under absolute alcohol it iş filtered off and dried in vacuo over sulphuric acid. The yield in each case is about 90 per cent of the theoretical, the remainder being lost in the alcoholic solution.
This is the lactam of amidotetrimidopentaphosphoric acid,
It exists in the lactam form in the 5-atom silver salt and apparently in the solution of its acid and normal salts; in alkaline solutions, however, it has probably the open form, as the silver salt prepared from such a solution has a composition corresponding to this.
The free acid may be obtained, somewhat contaminated with decomposition products, by decomposing the silver salt under water by hydrogen sulphide, care being taken to keep the liquid cool. The solution has an acid, somewhat astringent, taste, and is imperfectly precipitated by alcohol in a gelatinous form, resembling precipitated alumina. This shows the reaction of the salts, but is impure, as caustic alkali causes the evolution of little ammonia.
Sodium salts.-As all the sodium salts are amorphous and precipitated by alcohol, in a sirupy or viscous form, it is not easy to obtain any of them pure. The salt prepared in the above manner contains about 5.4 atoms of sodium (analyses 1 and 2) and forms a white, sandy, amorphous powder, which is not hygroscopic, but which slowly absorbs carbon dioxide. Under water it first becomes pasty and then slowly dissolves, with considerable evolution of heat. Salts with 4 or more atoms of sodium invariably contain 2 molecules of water which are retained at 1000,
The penta-sodium salt, PzN;0,,H.Nas +24,0, may be obtained by: adding to the solution of a salt containing a known excess of sodium nearly enough nitric acid to neutralize the excess and precipitating by alcohol, or by nearly neutralizing with acetic acid, with phenolphthalein as indicator, and precipitating by alcohol. In the latter case, however, there is a slight deficiency of sodium (analyses 3 and 4). The normal salt has a strongly alkaline reaction and loses some alkali by repeated precipitation by alcohol.
1 and 2 were precipitated directly from alkaline solution. 1. P:N:Na=5:5.02:5.42. 2. P:N:Na=5:5.03:5.40. 3 and 4 were precipitated from a solution neutralized with acetic acid with
phenolphthalein as indicator. 3. P:N: Na=5:5.03: 4.75. 4. P:N: Na=5: 5.02:4.80.
The tetra-sodium salt, P5N301,H5.Na H+27,0, is obtained by dissolving the crude salt in water with its own weight of 80 per cent acetic acid and precipitating twice by alcohol. It resembles the normal salt, but has nentral reaction.
It gave after drying at 1000:
Salts with still less alkali can be obtained by precipitating the crude salt by alcohol from a nitric acid solution. This precipitate is viscous rather than sirupy, as with the salts with 4 and 5 atoms of sodium. Salts with 5 or more atoms of sodium can be kept indefivitely without alteration and do not evolve ammonia with alkalies, but those with less gradually decompose.
Barium salt.-A solution of the sodium salt gives, with barium chloride, a voluminous precipitate of unknown composition, insoluble in water and acetic acid.
Magnesium salts.- Pentametaphosphimic acid forms several salts with magnesium alone, as well as double salts with magnesium and other metals.
A solution of the sodium salt, strongly acidified with acetic acid, gives, with magnesium salts, a voluminous, amorphous precipitate, nearly insoluble in water and but slightly more soluble in strong acetic acid. The composition of this, after drying at 100°, approximates to PN,OH,Mg,H+5HO.
A considerable portion of the acid remains in solution, however, even in the presence of a large excess of the precipitant, in combination with less than 2 atoms of magnesium, and can be precipitated by alco. hol as a salt soluble in water. If this insoluble magnesium salt be dissolved in dilute nitric acid and ammonia added to incipient precipitation, the solution, after filtering, contains essentially the primary salt (P:1,0,H,)2Mg; from this solution silver nitrate throws down an amorphous magnesium silver salt with a varying amount of silver. The primary salt is easily soluble in water and is remarkably stable, giving no precipitate with ammonia, sodium hydroxide or carbonate, even on boiling. The magnesium can be removed only by adding to its solution ammonia and an alkaline phosphate. On boiling in neutral or acetic acid solution, however, a precipitate of the 2-atom salt at once forms. Even the latter dissolves in boiling sodium carbonate solution.
Indications of an intermediate salt were observed, but it could not be isolated in pure condition.
Silver salts.—The silver salts of the phosphorus nitrogen acids are invariably free from water, and it is upon these, therefore, that the formulas of the acids themselves are based. There is no difficulty in obtaining normal silver tri- and tetrametaphosphimates of theoretical composition, and their crystalline nature affords a guaranty of their homogeneity. The same difficulty is encountered with the silver pentametaphosphimates, however, as with the sodium salts; they are amorphous flocculent precipitates, the composition of which corresponds to a definite formula only when they are formed under special conditions. In this case, as in others in this paper, the actual percentage composition expresses very little, if compared with the calculated composition of a definite salt. It is therefore better to express the results of the analysis in a molecular formula based on the atomic ratios of phosphorus, nitrogen, and silver, as actually determined; a comparison of the percentage composition found, with that calculated for salts of the lactam and open chain acids containing phosphorus and silver in the same ratio, then shows at once to which of these acids the salt is to be referred.
The composition of the precipitates depends altogether on the relative amounts of the reacting bodies, and even the free acid can be almost completely precipitated, provided a sufficient excess of silver nitrate be used. The preparations analyzed were made by precipitating a solution of 1 gram sodium salt in 50 cubic centimeters water with 55 cubic centimeters one-fifth normal silver nitrate solution. To the sodium salt was added enough nitric acid to produce a salt of known composition. Under these conditions it was found that a salt with 4 atoms of sodium gives very nearly the normal salt, P5N50,,H;Ags (analyses 3, 4, 6), the number of equivalents of silver in the precipitače always exceeding the number of equivalents of sodium. Up to 5 atoms of silver the precipitates are white; with more silver they become more and more yellow in proportion to the amount of silver they contain. An excess of 0.3 atom over the normal imparts a perceptible yellow tint. The salts are unaffected by light or by heating at 100°, and are decomposed by cold caustic alkalies with separation of silver oxide.
In the following table the molecular formulas given are based on the ratio of phosphorus to silver actually determined and referred to the acids P3N301.410 and PsH,02H2. A comparison shows that the salts are derivatives of the former, a true metaphosphimic acid. The salts were dried at 1000.
Tri- and tetrametaphosphimic acids, when precipitated from ammo. niacal solution by excess of silver nitrate, gave amorphous yellow salts
IP2N206H,Ag, is scarcely affected by boiling with strong caustic potash and P9N.0,H, Ag. is scarcely affected in the cold, but is at once decomposed on boiling.
Bull. 167- -10