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in gasoline and in ether. Toward water it is nearly as stable as hexaphosphonitrilic chloride, but is slowly attacked when exposed to atmospheric moisture. Hot dilute ammonia dissolves it very slowly, but more rapidly when alcohol is added.
This body is obviously a secondary product of the reaction of phosphorus pentachloride and ammonium chloride, as it is never found when a pure phosphonitrilic chloride is polymerized and depolymerized. It is noteworthy that no indication of other bodies of a similar nature has been observed, although no reason appears why they should not be formed at the same time. Whether it is in reality hexaphosphonitrilic chloride in which 3 chlorine atoms are replaced by one of nitrogen or not can not be decided at present.
5. THE HIGHER METAPHOSPHIMIC ACIDS.
In the preceding sections on the metaphosphimic acids it was shown that the two lower phosphonitrilic chlorides, P3N,Cl6 and P.N.Cla, give on saponification two well-defined acids, trimetaphosphimic acid, PN,0,H, and tetrametaphosphic acid, P.1,0.Hg. In a later section the existence of the higher phosphonitrilic chlorides PN,C110, P.N.Cl2, and P,N,C1,4 was shown, and it was further demonstrated that the series does not end here, but is continued through a number of members, which are incapable of separation by existing methods, and terminates with a rubber-like polymer of high molecular weight.
The work outlined in the present section was begun with the expectation of finding that each of the new phosphonitrilic chlorides would give on saponification the corresponding metaphosphimic acid of the general formula (PNO,H2).. The result, however, has not justified this anticipation. It has been found that penta- and hexaphosphonitrilic chlorides give true penta- and hexametaphosphimic acids, (PNO,H,)s and (PNO,H2), but that heptaphosphonitrilic chloride gives, not (PNO,H), but an acid (PNO,H2): + 1,0. The metaphosphimic series, therefore, appears to be limited by the acid (PNO,H,)6.
The metaphosphimic acids are the lactams of the imidophosphoric amides,' and it is therefore not surprising that there should be a certain similarity of behavior between these and certain organic oxy- and amidoacids. The y and à oxy- and amido-acids, while existing as salts in alkaline solution, pass more or less readily in the free state into the inner anhydrides, the lactones and lactams, a behavior which is not observed when the hydroxyl or amido-group is still further removed from the carboxyl. Joh. Wislicenus has shown” that the geometric configuration of the lactone-giving acids is such as to bring the reacting groups into close proximity, thus admitting of inner anhydride formation, while in other cases this is not possible, owing to their remoteness. Von Baeyer,' in his well-known “tension theory," has shown that the series of methylene-ring hydrocarbons, (CH4)n, may be expected to possess a maximum of stability in the pentamethylene ring, a ring in which there is the least “strain,” because the attraction of the carbon atoms for each other acts very nearly in the direction which the valences naturally assume, a deduction which has been confirmed by recent work on the reduction of aromatic hydrocarbons, showing the tendency of these to form pentamethylene rings when reduced.?
The imidophosphoric acids consist of chains of alternate NH and PO groups, the first being imidodiphosphoric acid, PO(OH)2.NH.PO(OH),. This and diimidotriphosphoric acid are described in the section on trimetaphosphimic acid, and triimidotetraphosphoric acid in the present section. The amides, of which the metaphosphimic acids are the lactams, may be regarded as imidophosphoric acids having one terminal hydroxyl replaced by an amnido group.
2 Räumlicho Anordnung der Atome, p. 67.
The speculations of Wislicenus and von Baeyer admit of application to the series of acids derived from the phosphonitrilic chlorides, and as far as the subject has been worked out the analogy is a complete one. The acids in the open form are amides of imidophosphoric acids; those with from 3 to 6 phosphorus atoms have the open form in alkaline solution, from which they are thrown out by silver nitrate as salts of the general formula (PNO,Ag2)n + H20, but in acid solution they spontaneously form the inner anhydrides; i. e., the metaphosphimic acids, which can be precipitated as silver salts of the general formula (PNO,HAg). The acid with 7 phosphorus atoms, on the contrary, does not form the anhydride under any conditions. Not only does the series possess an extremely marked maximum of stability in tetrametaphosphimic acid, but the higher acids, on being subjected to energetic attack, break up, yielding this body.
In previous chapters I have assumed that tri- and tetrametaphosphimic acids contain phosphorus-nitrogen rings:
We know nothing of the steric relations of phosphorus and but little of those of nitrogen, and have therefore no definite theoretical grounds for assuming the magnitude of the angle a formed by the lines joining a POOH group with two NH groups, nor of the angle formed by lines connecting a NH group with two POOH groups, in the case when these are free to assume a relation of greatest stability or least tension, as in an open chain. Neither can we assert that in a 6-sided ring, PzN.3, with alternate phosphorus and nitrogen, the angles a and b must each be 1200. We are, however, justified in assuming that the mean of a and b is 1200. Similarly in an 8-sided ring, P.14, the mean angle is 135°, even though a may be 1800 and 1 900.
Ber. Deutsch. chem. Gesell., Berlin, Vol. XVIII, p. 2277.
the angle ".
a + b
Experiment having shown that tetrametaphosphimic acid, P.N.O.Hg, is vastly more stable than any other acid of the series, we may regard
of the octagonal nucleus P.N4, which is 135o, as that
2 which most nearly coincides with the angle of least tension in the sense of von Baeyer's theory, and as approximating to that which would be assumed in an open chain, and we may expect that the stability of each ring will be less the more the angle differs from 1350. In the fol.
2 lowing table the mean angle and its difference from 1350 are shown:
a + b
It is therefore to be expected that as we ascend in the series there will be a very rapid increase of stability to a maximum, followed by a gradual decrease. This coincides with what we find. PzN30.H. is vastly less stable than P.N.O.Hg; the latter may be heated for hours with acids without complete decomposition, while the former is destroyed under the same conditions in a few minutes. P5N301,410 is much less stable than PN,0,Hg, but markedly more stable than PN,0.H., cor responding to the difference of only 90 from 135° in one case against 15° in the other. P.10,2H,2 is perceptibly less stable than P5N30,,110, and finally the ring P, N, of heptametaphosphimic acid is incapable of existence under the usual conditions, and as a result we get, instead of the ring acid P-N-014H14, the open chain P,N,013H 16.
When any of the acids above tetrametaphosphimic acid is broken up, the latter is formed in considerable amount. This may be explained in two ways: either the PN nucleus exists as such in the higher acid, or it is formed from the decomposition products. The former is highly improbable, for, considering its great stability, it should persist and be found in nearly theoretical amount, which is by no means the case. (PN,0,H10 gave 12 per cent and PN,0,2H12 30 per cent of the theoretical.) Its formation from the decomposition products is easily explained on the tension hypothesis. If a ring of POOH and NH groups be broken up by the action of a stronger acid, the molecules resulting from its decomposition will tend to assume a configuration determined by the mean angle of least tension, 135°, and this configuration in the case of the molecule of amido-triimidotetraphosphoric acid will be either a zigzag line or the form represented by
a configuration which admits of the formation of the anhydride or lactam, which is nothing else than tetrametaphosphimic acid. A theoretical yield of this can not be expected, for when a chain, PzNs, for example, is further broken up, the disruption may occur at any one of nine points, but only when one of the resulting products contains P.N. can tetrametaphosphimic acid be formed. A simple calculation shows that at most only 27 per cent of that required by the equation
P:N,0,0H10+2H20=P,NO:H+NH H,PO. can be expected. A chain PN, however, offers more chances for the formation of fragments containing P.N., and hence, as actually found, the yield of tetrametaphosphimic acid is greater, instead of less, as would be the case if the nucleus of this acid existed as such in hexametaphosphimic acid.
The first two chloronitrides, PNC, and P,N,C1, have not been obtained, and of the corresponding mono- and dimetaphosphimic acids the former is unknown and the existence of the latter questionable. Mente' has described salts of an acid to which he gives the formula of dimetaphosphimic acid, P2N,0,H,, but no proof is given that it is not some other polymer of PNO,H2, and the method by which he obtained it is one which is not likely to give a body of this molecular weight. The dimetaphosphimic ring would have a mean angle of 90°, which differs from that of tetrametaphosphimic acid by 45°. Such a ring should be much less stable than even that of heptametaphosphimic acid, and it is therefore quite possible that it is incapable of existence as such, and can exist only as the hydrate PN,0,H.. This is perhaps the acid described by Gladstone” as pyrophosphodiamic acid.
The chloronitride series presents a maximum of stability in triphosphonitrilic chloride, but this is by no means as marked as that existing in the acid series. That such a maximum should occur in one case in a ring of 6 sides, and in the other in an 8-sided ring, and that the series should be limited in one case and uplimited in the other, involves no contradiction. Triphosphonitrilic chloride is stable only in the sense that it is formed in the largest amount and that its chlorine
| Ann. Chem. (Liebig), Vol. CCXLVIII, p. 244.
Quart. Jour. Chem. Soc. London, Vol. III, pp. 135, 354; Ann. Chem. (Liebig), Vol. LXXVI, p. 79; Vol. LXXVII, p. 315; Jour. Chem. Soc. London (2), Vol. II, p. 231.
is most tenaciously held, while the term is applied to the acids as indicating the difficulty with which the ring is broken open. In the one case, the series consists of polymers of -PC1,=N- and in the other of —POOH_NA—, and the two series can not therefore be considered analogous in a stereochemical sense.
Only tri- and tetrametaphosphimic acids give characteristic salts. The salts of the acids with 5, 6, and 7 atoms of phosphorus are totally devoid of crystallizing power, the alkaline salts being precipitated from aqueous solution by alcohol as sirups, which can easily be converted into the solid form by dehydration with absolute alcohol, but which are still wholly amorphous. The tendency to diminished acidity of unneutralized hydroxyls, well known in the case of orthophosphoric acid, is here apparent, and to an increasing extent as we rise in the series. Trimetaphosphimic acid forms a salt with 3 atoms of sodium which has neutral reaction and is not decomposed by strong acetic acid; tetrametaphosphimic acid readily gives salts with 2 and 4 atoms of sodium. The 5-atom sodium salt of pentametaphosphimic acid is strongly alkaline and shows a tendency to hydrolytic dissociation, and the same tendency is still more marked in the higher acids. Owing to this and to their amorphous nature it is impossible to obtain salts of definite composition from any but the first two acids unless certain conditions are rigidly adhered to. Qualitative differences between the analogous salts of the higher acids are almost wanting, and only a quantitative study and a knowledge of their derivation serve to distinguish them. As with other phosphorus-nitrogen acids, no ammonia is evolved on boiling with alkalies.
Amides of the metaphosphimic acids.-Gerhardt' has described, under the name "phosphamide," a body of the empirical composition PN,OH 3, which he obtained by action of ammonia and water on phosphorus
NH pentachloride and to which he gave the formula PO
NH responds to the amide of a metaphosphimic acid, but its properties indicate that it is probably a substance of high molecular weight. I have attempted to obtain amides of the first three metaphosphimic acids by acting on the corresponding chloronitrides with gaseous or strong aqueous ammonia. The efforts were but partially successful. The P:N ratio is sometimes rather higher, sometimes rather lower, than is required by the formula (PN,OH).. They have none of the properties of Gerhardt's phosphamide, but are extremely soluble in water, uncrystallizable, and unstable, and have weakly acid properties, forming alkali and silver salts, which, however, are of very variable composition and of ill-defined properties. Like the metaphosphimic acids, they give off no ammonia on boiling with alkali, but are easily decomposed on treatment with acids.
I Ann. Chim. Phys. , Vol. XVIII, p. 188.