9. PHOSPHORUS 563. Formula weight Very gradually evolves free I a with KI.

Size: px
Start display at page:

Download "9. PHOSPHORUS 563. Formula weight Very gradually evolves free I a with KI."

Transcription

1 9. PHOSPHORUS 563 A solution of g. of KH 3 PO 4, 198 g. of KOH, 120 g. of KF and g. of K 3 CrO 4 in one liter of water is prepared. A 215-ml. portion of this solution is electrolyzed in a large platinum dish (which serves as the anode), with a rapidly rotating bent Pt wire used as the cathode. The anode current density is amp./cm? The electrolyte temperature is maintained below 14 C by external cooling. The current is interrupted after three hours and the material is allowed to stand overnight at room temperature. During this period, the intermediate H S PO B decomposes, evolving O s and re-forming orthophosphate. A certain amount of K 4 P a O 8 is also formed (see above equation), while the already existing K 4 P 3 O 8 remains unchanged. Following a second electrolysis for two hours, the material is again allowed to stand overnight. Finally, the solution is electrolyzed a third time for one hour and again permitted to stand overnight. The solution is then evaporated on a water bath, with stirring, while a stream of air is directed across the upper surface so that its temperature never rises above 80 C. By recrystallizing three times, % pure K 4 P 2 O 8 is obtained. The yield is about 80%. Formula weight Very gradually evolves free I a with KI. F. Fichter and A. Rins y Miro. Helv.Chim. Acta 2, 3 (1919), F. Fichter and E. Gutzwiller. Helv. Chim. Acta 11, 323 (1928). Phosphorus Trisulfide P4S3 Phosphorus trisulfide can be synthesized either by Stock's method (I), which involves fusing red P and S and carefully purifying the crude product by recrystallization from CS 3, followed by a second recrystallization from benzene, or according to German patent 309,618 (F. C. Frary), which uses white P in a high-melting inert solvent (II). I. STOCK'S METHOD 4 P + 3 S = P 4 S The starting materials and the solvents must be carefully purified prior to use. Very pure crystalline S is powdered as finely

2 564 R. KLEMENT as possible and dried at 100 C. Red P is purified by the method given on p Carbon disulfide is shaken for 24 hours with CaCl 3 and Hg, distilled and stored in brown bottles over P 3 O S. Benzene is distilled and stored over P 3 O B. An intimate mixture of 155 g. of red P and 96 g. of S is prepared and portions (40-50 g.) are reacted in a large diameter open test tube under a stream of dry CO S. It is recommended that a pan filled with sand be placed under the test tube since the molten mass will immediately catch fire if the tube breaks. The test tube is first lightly preheated over its entire length to about 100 C and is then heated with a small fiame in one spot at the upper edge of the mixture until the onset of the reaction. As soon as the entire charge has reacted (thereby becoming molten), it is heated to the point where distillation begins and then cooled in a stream of CO 3. The cake is then pulverized and extracted with hot CS 3 (200 g. for each 100 g. of material). By evaporation of the CS 3, an almost theoretical yield of crude sulfide, which melts between 130 and 150 C, is obtained. Purification is carried out by adding hot water to the powdered product (200 ml. for each 100 g. of material) in a one-liter roundbottom flask; a strong jet stream is introduced into the mixture for one hour so that it is continually agitated. On cooling, the supernatant liquid is decanted as completely as possible and the P 4 S 3 is extracted in a separatory funnel with about 150 g. of CS S for each 100 g. of crude sulfide. The solution, which is easily separated from the aqueous layer and the undissolved impurities, is shaken for 12 hours with PsO 5 ; it is then evaporated on a steam bath until crystals form and then evaporated in aspirator vacuum until dry. The final drying may not be done over a steam bath because the sulfide is partially decomposed at this temperature. A 100-g. quantity of crude product yields about 98 g. of compound, m.p C, which still contains some CS a. This is removed by recrystallization from benzene in the following manner: A reflux condenser is connected with a ground-glass joint to the 5-cm. -diameterneck of a 750-ml. round-bottom flask; an extraction cell is suspended from the lower end of the condenser by a wire. The cell consists of a piece of glass tubing 10 cm. long and 4 cm. in diameter which has been somewhat narrowed at the lower end and fitted with several small hooks at the upper. Hard filter paper is secured over the lower opening. The cell holds about g. of the compound to be recrystallized. A 300-ml. portion of benzene is kept at a vigorous boil in the surrounding flask. In six hours, about 100 g. of P 4 S 3 is extracted from the cell. The small, uniform crystals which precipitate out of the benzene solution are carefully suction-filtered, with care not to cool too strongly and protection from atmospheric moisture. It is freed of residual solvent by passing dry H 3 over it. This is

3 9. PHOSPHORUS 565 conveniently done in an upright glass tube, not too wide, and is continued so long as the H s gives off an odor of benzene. Additional, less pure product (5-7% of the crude) can be recovered by concentrating the benzene solution. The yield of pure P 4 S 3 is about 92% of the crude product. II. PROCEDURE ACCORDING TO GERMAN PATENT 309,618 White P is dissolved in (virtually nonflammable) a-chloronaphthalene and the stoichiometric quantity of S is added. The solvent acts as a diluent controlling the rate of reaction and causes crystallization of the sulfide. When the reaction is complete, the mixture is cooled with stirring. Most ofthep 4 S 3 precipitates as a fine powder. Yellowish-green, long rhombic needles; stable in the air. M.p C, b.p. 407 C; d In the absence of O 2 and moisture, remains stable above 700 C. Decomposed by water at elevated temperatures, evolving H 3 S. Soluble in CS 3 and benzene. These solutions become turbid in air almost immediately and gradually deposit a copious, yellowish-white precipitate. I. A. Stock. Ber. dtsch. chem. Ges. 43, 150 (1910). II. F. C. Frary. German Patent 309,618, Class 12i, December 2, 1918, cited in Chem. Zentr II, 55. Phosphorus Pentasulfide P4S5 According to Boulouch, as confirmed by Treadwell and Beeli, phosphorus pentasulfide is easily prepared by exposing a solution of P 4 S 3 and S in CS 2 to light in the presence of a small amount of iodine as catalyst. P 4 S S = P 4 S A solution of 22 g. of P 4 S 3 and 7 g. of S in 200 ml. of CS 3 is prepared, and 0.2 g. of iodine is added. The mixture is exposed to light for one or two days (for three days in diffuse daylight at room temperature, according to Treadwell and Beeli). The crystals

4 566 R. KLEMENT that precipitate are washed with CS S, dried, powdered, washed again and dried in air. The crystals usually contain residual CS S, which can be removed by melting. The yield is 23 g. Sulfur-yellow crystals which, when slowly heated, melt between 170 and 220 C. d (25 C) R. Boulouch. Compt. Rend.Hebd. Seances Acad. Sci. 138, 363 (1904); W. D. Treadwell and C. Beeli. Helv. Chim.Acta 18, 1161 (1935). Phosphorus Heptasulfide P4S, According to Stock, phosphorus heptasulfide is produced in a manner analogous to that for P 4 S 3 (see p. 563). 4P + 7S = P 4 S The starting materials and the solvent must be purified as specified on p An intimate mixture of 100 g. of red P and 173 g. of S is prepared and reacted in portions (40-50 g.), after which it is heated until pronounced distillation occurs. The cooled produce is finely powdered and recrystallized from CS S in the extraction apparatus described on p The extraction cell is charged with g. of crude product; the flask is charged with about 300 ml. of CS 3, which must be maintained at a vigorous boil. Because of the low solubility of P4S7, the extraction requires about 48 hours. The sulfide, which precipitates as glittering crystals, is then recrystallized in the same manner. The crystals are suction-filtered, care being taken to exclude atmospheric moisture and not to cool too much. It is dried in a stream of hydrogen at 100 C. Almost colorless, faintly yellow monoclinic prisms. M.p. 310 C, b.p. (760 mm.) 523 C. The best solvent is CS 3. One part of P.S 7 dissolves in 3500 parts of CS 3 at 17 C and in 20,000 parts at 0 C. The very slight solubility in CS 3 distinguishes P4S7 from the other

5 9. PHOSPHORUS 567 phosphorus sulfides. Slowly decomposed by cold water, rapidly by hot water, forming H 3 S. d (17 C) REFERENCE A. Stock. Ber. dtsch. chem. Ges. 43, 414 (1910). Diphosphorus Pentasulfide According to Stock and Herscovici, the preparation of diphosphorus pentasulfide is carried out by the same synthesis as for P4S3 (p. 563). The reagents must also be purified according to the instructions given there. 2P + 5S = P 2 S An intimate mixture of 100 g. of red P and 260 g. of S (2 g. atoms of P to 5 g. atoms of S, with a 1% excess of S) is reacted in portions and heated until vaporization begins. The cooled product is coarsely ground and placed in a Pyrex tube. The tube is evacuated, using an aspirator, fused to seal it and heated for several hours at about 700 C in an iron tube packed with sand and closed with a screw cap. After cooling, the glass tube is broken; the contents are pulverized and recrystallized from boiling CS 3 in the extraction apparatus (about 300 ml. per 50 g. of sulfide). The precipitated sulfide is twice recrystallized in the same manner, and the crystals are dried at 100 C in a stream of H s. If the crude product is not heated under pressure, a pure product can only be obtained by repeated recrystallization. The yield is 60%. Light yellow, almost colorless triclinic crystals with a PAS 10 molecular lattice (Vos and Wiebenga). M.p. 276 C, b.p. 514 C; d Soluble with difficulty in boiling CS S (about 1:200). Decomposes in moist air or water forming H 3 PO 4 and H 3 S. Dissolves on heating in caustic soda to form a yellow sodium thiophosphate solution. Organic compounds containing oxygen are converted to sulfur-containing materials. A. Stock and B. Herscovici. Ber. dtsch. chem. Ges. 43, 1223 (1910). A. Vos and E. H. Wiebenga. Acta Cryst.(Copenhagen) 8, 217 (1955).