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(PNCL)11, and a small amount of a chloronitride, P.N.Cl, not belonging to the above series. The absence of the lower members, PNCl, and (PNC12)2, is remarkable, and theoretically significant. Indications of a trace of a substance more volatile than the compound (PNC1.); and of similar but stronger odor, were observed, but there is no evidence that it consists of one of the missing bodies.
One of the most remarkable properties of the phosphonitrilic chlorides is that each member of the series is converted by heat into the rubber-like polyphosphonitrilic chloride, a body, or mixture of bodies, of very high molecular weight, which is highly elastic and insoluble in all neutral solvents, but which swells enormously in benzene, and which, on distilling at a higher temperature, breaks down into a mixture of all the lower members mentioned above, which can then be separated by appropriate means. In this way it is possible to convert any phosphonitrilic chloride quantitatively into any other by heat and distillation alone. In preparing any desired member, therefore, we are not limited to the quantity obtained from the first reaction product, but may work the residues over and over again until completely converted into the body sought after. With the exception of a few cases, in which the number of members is limited, as the aldehydes and cyanic acids, this series is therefore unique; I know of no other series of inorganic compounds in which this is possible. Polymerization takes place slowly, but perceptibly, at 2500, and is almost instantaneous at 350%, while depolymerization begins at about 350°, and is rapid at a temperature close to incipient red heat. Triphosphonitrilic chloride, P3N,Cl., is the only member which can be distilled in consid. erable amount at atmospberic pressure without considerable polymerization, and even this polymerizes almost completely on long boiling; at 760 millimeters pressure the tetra-compound, P.N.C1;, boils at 328.5°, a temperature at which polymerization occurs quite rapidly, but this, as well as the penta compound, P;N,Chio, and the hexa-compound, P.N.C112, can readily be distilled at 13 millimeters; the hepta-compound, P,NCl-, suffers marked polymerization on distilling even at this pressure, and its isolation is therefore attended with much loss. Owing to the rapid change at higher temperatures, I have been unable to isolate any of the higher members, which remain as a considerable oily residuum, and there seems to be but little probability of this being effected by any known method, unless by distilling in a nearly absolute vacuum.
The greatest difficulty in the separation of the members is caused by polymerization. It requires but a small amount of polyphosphonitrilic chloride to cause the liquid to thicken or gelatinize, and therefore to be incapable of further distillation, and some of this body is always formed in the course of a prolonged fractioning of the higher members. It was found, however, that this polymer is much more easily attacked by water than the lower members; when signs of polymerization are observed, it is only necessary to interrupt the distillation, and heat the
residue for some time with water, when the resulting oil is again in a condition to continue fractioning. The loss in this operation is small, but the tediousness of a fractional distillation is thereby extraordinarily increased.
It is noteworthy that no regular progression exists in the melting points of the phosphonitrilic chlorides, and the same is true of their solubility in the ordinary neutral solvents, but the solubility varies in the same sense as the fusibility. Of the members of known molecular weight, the second, tetraphosphonitrilic chloride, is the least soluble and has the highest melting point, while the corresponding tetrametaphosphimic acid is the least soluble and most stable of the derived acids. With respect to their stability toward water, the new members (polyphosphonitrilic chloride excepted) resemble those already described, being scarcely attacked by prolonged boiling. In ethereal solution, however, there is a perceptible decrease of stability toward water as we rise in the series, a fact already noted with regard to the first two members.
Notwithstanding the high molecular weight of the bodies isolated, no indication of isomers has been observed, although the fractioning was carried out very thoroughly up to 3000 at 13 millimeters.
A mixture (which need not be very intimate) of 4 parts perfectly dry phosphorus pentachloride and 1 part aminonium chloride, as required by the equation
PC1+NH,C1=PNCl + HCl,
is introluced into an ordinary “bomb” tube, which has previously been drawn out to a neck. It is practicable to fill the tube entirely to the neck, so that the charge for a tube of ordinary dimensions is about 125 grams, yielding 50-55 grams of chloronitrides. After sealing, the length of the neck, exclusive of the rather long capillary, should be about 10 centimeters. As the mixture liberates 55 per cent hydrochloric acid, it is necessary to regulate the heating with great care and to open the tube repeatedly. The temperature of the furnace is allowed to rise to 1500, at which the reaction begins, when the gas is at once shut off, and the tube opened at about 1000 (in the furnace!). This operation is repeated several times, the temperature being allowed to rise 10-20 bigher each time. When the evolution of hydrochloric acid has slackened and the contents of the tube are mainly liquid while hot, the temperature may be carried to 200° or higher, until little or no gas is given off. The operation requires care and judgment, but with careful working it is possible to avoid explosions, and to obtain with a four-tube furnace about 200 grams of mixed chloronitrides in sixteen hours.
The contents of the tube, after cooling, generally consists of a buttery mass or of a thick, yellow liquid filled with fine prisms and plates; if heated much above 2000, the liquid frequently separates into two layers. The crystals are soluble in gasoline, but the bulk of the product remains as an immiscible oil.
The neck of the tube is now bent down, the tube placed in an inclined combustion furnace, and by cautious heating, finally to incipient redness, the contents are distilled out. There remains in the tube a very voluminous, spongy, black residue of inconsiderable weight, due to unavoidable impurities and to the impossibility of causing complete reaction in the sense of the above equation. The distillate consists of a crystalline mass impregnated with a yellow oil, and contains about 95 per cent of the theoretical amount of phosphonitrilio chlorides, with some phosphorus pentachloride, the chloronitride P.N.Cls, and other substances of unknown nature. Before proceeding further, it is necessary to remove the pentachloride, and for this purpose the distillate is melted, poured into cold water, and the flask heated in the waterbath for about two hours, the liquids being mixed by blowing air through them. The chloronitrides are then allowed to clear under the hot water, and forced out by means of a wash-bottle arrangement; a separatory funnel can not be used, as the substance solidifies in the neck, and if allowed to solidify under the wash water it absorbs so much of this as to cause annoyance in the subsequent distillation. Special drying before distilling is unnecessary.
The product is then distilled up to 200° at 13-15 millimeters, using an Anschütz flask, as the distillate solidifies instantly on cooling. The residue, containing the members PsN,Cl, up, is set aside for later systematic fractional distillation. The distillate, about 70 per cent, consists essentially of PN,Cl and P.N.Cls, which, if desired, may be easily separated by fractional distillation in vacuo, followed by crystallization from benzene. This is more convenient than the method of separating by steam. If it is desired to convert it into the higher members, it is placed in a combustion tube bent down at about 20 centimeters from the open end, and which it should not fill more than one-half after melting. This is laid in an inclined combustion furnace and heated to gentle boiling of the contents. It is well to heat the tube somewhat strongly at a short distance above the liquid, as superheating the vapor pro.
1 This residue contains the small amount of PgN,Cl, formed as a secondary product of the original reagents, and as this is apt to cause inconvenience at a later stage by accumulating with the P.N.C1,20 it is perhaps well to remove as much as possible at this point. For this purpose the residue is allowed to stand for a day or two at the room temperature, and tho crystals removed by sucking out under a good vacuum, best in a large Gooch crucible. The filtrate is cooled for a day or two in a refrigerator, and the new crop of crystals separated in the same way, the filtering flask being allowed to stand in the icebox. The oily filtrate is set aside, and the united crystalline products distilled up to 240° at 13 millimeters, whereby most of the PsN;C10 passes over. The residue, consisting of P:N_C12, the small amount of P&N,Cly, and the adhering oil, is allowed to crystallize in the refrigerator, and the viscous mass is extracted several times with small amounts of gasoline (boiling at 50%-80%). The residue is boiled with benzone, which extracts the PN,Cly, which crystallizes on concentrating and cooling. The portion dissolved by the gasoline is worked up with the other residues.
This is the method actually employed, but I am not entirely convinced of its necessity, as it is not possible to remove all the P&N,Cl, in this way.
motes polymerization. The time required for polymerization varies greatly; pure triphosphonitrilic chloride may require two hours or more. With the above mixture the time is less, and is shorter the higher the boiling point; it is shortened by adding already gelatinized substance, which causes the liquid to thicken, and may then be but a few minutes; it is also shortened by heating under pressure. Sooner or later the liquid begins to thicken, and finally it is converted into a stiff, transparent mass, with little or no liquid, and generally discolored by traces of organic matter. The tube is then connected with a long. necked receiver, exhausted, and the depolymerization and distillation effected by heating, from the front backward, to incipient redness. This part of the operation proceeds rapidly, as it is only necessary to guard against frothing over, and to insure complete condensation, the latter being easily effected by having the limb of the tube at least 20 centimeters long. One hundred grams can be worked up at one time and the tube can be used repeatedly. The residue does not weigh more than a few milligrams. The distillation may also be made at atmospheric pressure, but the yield of higher products is thereby diminished. The distillate, which entirely resembles that first obtained, excepting in containing no phosphorus pentachloride and no P.N.Cly, is distilled as before, the washing being omitted. In this way the whole quantity of material can finally be converted into a mixture of members higher than P.N.C.
The united residues boiling above 2000 are now submitted to systematic fractional distillation at 13–15 millimeters, using an Anschütz flask, provided with a “trap," to prevent flowing back. During the first distillation polymerization generally begins when the temperature of the bath bas reached 2700, but with later distillations at a higher temperature, and the higher the purer the fractions are.
When polymerization begins, which is indicated by frothing and thickening, the operation is interrupted and the residue heated in the flask with water in the water bath until it has completely liquefied, which is assisted by agitation, the oil separated,' and the distillation continued. It has not been found practicable to continue the distillation at a higher temperature than that obtained by heating the bath to 370°, for the liquid begins to polymerize in a few moments, and but an inconsiderable distillate can be obtained. Moreover, at this temperature the polymer shows signs of breaking down into simpler bodies, and the distillate does not consist only of high-boiling members. The total amount of final residue is not very great, and, as shown below, consists likewise of phosphonitrilic chlorides of still higher molecular weight. In later distillations from 2000 upward, polymerization usually stops the process at 2600-270°, but after appropriate washing the residue may be
'In this case a separatory funnel may be used, as the mixture of higher chloronitrides is liquici below 80o.
distilled to a much higher temperature. After eight to ten distilla tions three main fractions are obtained, which are then worked up separately. As PËN,Clo, though crystalline, is extremely soluble, it is necessary to carry out the distillations with the first main fraction until a practically sharp boiling point is obtained, in which connection it may be noted that at 17–20 millimeters a change of 1 millimeter pressure causes a change of about 10 in the boiling point, and at 13 millimeters a change of about 20. PAN,C114, being liquid, must also be isolated by distillation only, but at its boiling point polymerization is so rapid that great loss ensues during a series of distillations.
P.N.Cl2 can not be separated by distillation from the P.N.Cl, which accompanies it in small amount, the latter having nearly the same boil. ing point, nor is repeated recrystallization effective. The separation is best effected by adding a crystal of pure PoN Cl2 to the supersaturated benzene solution, and filtering before the P.N,C1, begins to deposit. The latter thus concentrated is recrystalized from benzene.
Owing to many modifications introduced in developing the above method, no accurate statement of the yield can be given; the final product was about 225 grams PjN,Cljo, 110 grams P6N,C1,2, 10 grams P N,C1149 and 5 grams PN,01.
Analytical methods. With the exception of polyphosphonitrilic chloride, the chloronitrides were analyzed by decomposing in the following manner:
For phosphorus, by warming with alcohol and a little ammonia in a platinum crucible until completely dissolved, evaporating to dryness and heating to fuming for an hour with strong sulphuric acid, the crucible being kept covered.
For nitrogen, by treating as above, omitting the ammonia.
For chlorine, by heating with alcohol and ammonia. It is necessary to precipitate with silver nitrate in the presence of a large volume of 10 per cent nitric acid and to filter hot, in order to avoid the forma. tion of silver metaphosphimates, which are difficultly soluble in dilute nitric acid.
In decomposing polyphosphonitrilic chloride, which is attacked by water alone, the alcohol was omitted. The method of Carius was used for determining chlorine, as it was found that otherwise compounds insoluble in dilute nitric acid were formed. For the other chloronitrides this method offers no advantage.
Molecular weight determinations were made by the boiling point method with the apparatus of Hite,' using as solvent carefully purified and dried benzene.
I Am. Chem. Jour., Vol. XVII, p. 512. The molecular weight of PzN3Cl has been determined by the vapor-density method, Jour. Chem. Soc. London (2], Vol. II, p. 225. A series of determinations by the boiling-point method gave 346, 350, 353. Calculated 347.9.