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PHOSPHORUS OXIDES
440
PHOSPHORUS : A R T H U R D . F. TOY
PS(NCO) 3 is obtained by the addition of sulfur to P(NCO) 3 at 140° 217. i t is stable when formed but polymerizes to a solid on heating. The compound PS(NCS) 3 is prepared by the reaction of PSC1 3 with N H 4 S C N in S 0 2 solvent at - 3 0 ° 21s, S P C I 3 H- 3 N H 4 S C N
S P ( N C S ) 3+ 3 N H 4C 1
219
or in C H 3 C N as solvent at room temperature . The thiophosphoryl fluoride diisothiocyanate, SPF(NCS) 2, is obtained by the reaction of C1 2P(S)F with KSCN (b.p. 46-47° at 1 mm, 1.4827, ,ig> 1.6390). It is a colorless liquid with a sharp odor and powerful lachrymatory properties. It decomposes gradually 220 on storage and hydrolyzes slowly in w a t e r . The compounds SPF(NCS) 2 and SPF 2(NCS) 22 are prepared by the fluorination of SP(NCS) 3 with S b F 3 under reduced pressure *. Liquid exchange reactions between SPBr 3 and SP(NCS) 3 result in the formation of 2 SPBr 2(NCS) and SPBr(NCS) 2 as indicated by the new lines in the N M R spectra **. The equilibrium state for this ligand exchange reaction is reached after heating at 150° for 156 hr. Compositions with various ratios of phosphorus-sulfur-halogen, other than those described, have also been reported in the literature. Many of these compounds are as yet not well characterized. The properties of some thiophosphoryl halides and thiophosphoryl pseudohalides are listed in Table 9.
7. P H O S P H O R U S
OXIDES
The oxidation of white phosphorus takes place by a chain reaction between a lower and upper limit of oxygen pressures. At the lower limit, the oxygen chain carriers are destroyed by impact with the walls and at the upper limit triple collision occurs and 0 3 222 is f o r m e d . The net result is the formation of many intermediate oxidation products as represented by the many oxides reported in the literature. Of the oxides, phosphorus pentoxide only is commercially important. Phosphorus trioxide has been the subject of many recent investigations. Much less is known, however, on phosphorus tetroxide and other oxides such as P4O7, P4O8, and P4O9.
7.1. P H O S P H O R U S M O N O X I D E
Phosphorus forms a diatomic molecule, PO, with oxygen. It is unstable under ordinary 2 23 conditions and is detected in the vapor state only in spectral bands. Cordes and W a r k e h r 2 2 2 2 showed that the systems correspond t o Σ - * Χ π and π Χ π transitions. They reported the dissociation energy of P O as 5.30 eV. Other workers calculated the PO 2 Π Η . H. Anderson, Silicon, Sulfur, Phosphates, pp. 235-7, IUPAC Colloquium, Münster, 2-6 Sept. 1954. 218 E. Ruck, H. Binder and F. L. Goldmann, Ζ. anorg. u. allgem. Chem. 338(1-2) (1965) 58-62. 219 B. S. Green, D. B. Sowerby and K. J. Wihksne, Chem. & Ind. (1960) 1306-7. 220 ZH. M. Ivanova, Ε. A. Stukalo and G. I. Derkach, Zhurnal. Obshchei Khimii, 37(5) (1967) 1144-7 221 H. W. Roesky and Z. Muller, Z. anorg. u. allgem. Chem. 3 5 3 (1967) 266-9. 222 H . Cordes and W. Witschel, Z . Physik. Chem. 46(1/2) (1965) 35-48. 223 H. Cordes and E. Warkehr, Ζ . Physik. Chem. 46(1/2) (1965) 26-34.
PHOSPHORUS 2 24
TRIOXIDE
441
225
dissociation energy as 6.2 e V and 6.8 e V . The P - O distance is 1.447 Â and the excitation energy for the molecule is 143 kcal/mole. The dissociation is to oxygen and an 226 excited phosphorus a t o m . A polymeric (PO) n has also been reported. It is formed by the electrolysis of POCl 3 in (C2H 5)3N.HC1 at 0° or by the reaction of POBr 3 with magnesium in C 2 H 5 O H solution. The product is a brownish amorphous-crystalline substance insoluble and stable in water but decomposes in an aqueous alkali solution with liberation of P H 3 . It oxidizes to P 2 0 5 2 2 7 in an oxygen atmosphere at 300° .
7.2. P H O S P H O R U S
TRIOXIDE
Phosphorus trioxide, P 2 0 3 or P4O6, is prepared by the controlled oxidation of phos226 phorus at a pressure of 90 mm with air enriched to contain 75% of total oxygen . A more recent method is by the oxidation of phosphorus with N 2 0 at 550-600° under 70 torr. 228 A yield of 50% of P4O6 is obtained . The usual contamination of 1 to 2% of dissolved white phosphorus is removed by conversion to the less soluble and less volatile red phosphorus by u.v. radiation followed by distillation in vacuo. Phosphorus trioxide has a tetrahedral symmetry of the point group Td. This is confirmed -1 by two strong absorption bands at 639 and 911 c m together with seven Raman 229 frequencies . The phosphorus atoms are located at the corners of a regular tetrahedron and each of the six oxygens located between two phosphorus atoms in the plane of symmetry 2 30 passing through the trigonal a x e s :
o—Ρ—Ο
The P - P distance is 2.95 ±0.03 A. Phosphorus trioxide is a colorless liquid at ordinary temperature and has a m.p. of 23.9° and b.p. of 175.4°. The standard enthalpy of formation AH} of P 4 0 6 (crystal) is calculated to be - 3 9 2 kcal/mole [taking AH} P4O10 (crystal) as - 7 1 3 kcal/mole]. The heat of sublimation is 16 kcal/mole and the entropy of vaporization = P 4 O 6 (liq.)-*P4C>6 231 (gas): Δ 5 4 4 8 . 5 6 = 21.0 cal mole/deg . The bond energy = (P-O) is calculated to be 232 86 kcal/mole . The dielectric constant is 3.2 at 22° and the surface tension is 36.6 dynes/cm at 34.3°. The compound hydrolyzes in an excess of water forming H 3 P 0 3 . Upon heating to 224 J. Berkowitz, / . Chem. Phys. 30 (1959) 858-60. 225 c. V. V. S. Ν. K. Santharam and P. Tiruvenganna Rao, Indian J. Phys. 37 (1963) 14-17. 226 j . R. van Wazer, Phosphorus and Its Compounds, Vol. I, pp. 266-86, Interscience Publishers Inc., N.Y. (1958). 227 H. Spandau and A. Beyer, Naturwissenschaften, 46 (1959) 400. 228 Ε. Thilo and D. Heinz, Ger. 1,172,241, 18 June 1964, to Deutsche Akademie der Wissenschaften zu Berlin. 229 T. A. Sidorov, Trudy Fiz. Inst. im. P.N. Lebedeva, Akad. Nauk. SSSR 12 (1960) 225-73. 230 D. E. C. Corbridge, Topics in Phosphorus Chemistry, 3 (1966) 71-81. 231 T. D. Farr, Phosphorus, Properties of the Element and Some of Its Compounds, Chemical Engineering Report No. 8, pp. 18-25, T.V.A., Wilson Dam, Alabama (1950). 232 s. B. Hartley and J. C. McCoubrey, Nature, 1 9 8 (1963) 476.
442
P H O S P H O R U S : A R T H U R D . F. T O Y
above 210°, it disproportionates to red phosphorus and a phosphorus oxide of the approxi2 3 1 mate composition of P n 0 2 n . It displaces CO from Ni(CO)4 forming successively coordinates containing one to four Ni(CO)3 groups. The compound P4O6 [Ni(CO) 3] 4 is crystalline. Reaction of P4O6 with B2H6 results in the symmetrical cleavage of B2H6 2 3 3 5 with the formation of Η 3 Β Ρ 4 θ 6 Β Η 3 " .
7.3. P H O S P H O R U S
PENTOXIDE
Preparation, Structure and Properties When phosphorus burns in the presence of an excess of oxygen in air, the product formed is P4O10, commonly called phosphorus pentoxide. One commercial process for collecting the fine white smoke is by condensing it continuously on a fluidized bed of 236 granular P4O10 at 129° into free flowing dustless small b e a d s . The P4O10 thus obtained is the hexagonal or " H " form, the product of commerce. The free energy of formation of P4O10 is - 6 4 4 . 8 kcal/mole and the standard entropy at 298.15°K is 54.66±0.1 cal/mole/deg. The heat capacity at 298.15°K Cp = 50.60 cal/deg. o Other molar thermodynamic properties at 298.15°K are (H-H 0) = SUl cal, (H-H°0)IT = 23 27.22 cal/deg and -(G-H°0)IT= 27.47 cal/deg ?. The value of enthalpy of formation 238 of hexagonal P4O10 at 25° has also been reported as - 7 1 3 . 2 kcal/mole . The mean bond energy E(P-0) and dissociation energy S ( P - O ) were calculated as 86 and 138 232 kcal/mole respectively . The P4O10 molecule in the vapor state with a Td symmetry has the arrangement of 230 atoms as shown in the following d i a g r a m . It has been estimated that there is essentially
no 7Γ bonding between the phosphorus and the shared oxygen and about 1.5-2.0 π-bond 226 between the phosphorus and the unshared oxygen in addition to the δ-bond . The crystals are uniaxial positive with the indexes of refraction in sodium light: nw= 1.469 23 and nB = 1.471, both values±0.002 i. It has a density of 2.30. There are two other crystalline modifications of phosphorus oxide : the O-form and a stable orthorhombic form designated as the O'-form. The O-form is prepared by heating the Η-form in a closed system for 2 hr at 400°. It is built of rings of ten PO4 tetrahedra linking together to form a continuous three233 j . G . Riess and J. R. van Wazer, / . Am. Chem. Soc. 8 8 (10) (1966) 2166-70. 234 G . Kodama and H. Kondo, / . Am. Chem. Soc. 88(9) (1966) 2045-6. 235 j . G . Riess and J. R. van Wazer, / . Am. Chem. Soc. 88(10) (1966) 2339-40. 236 G . I. Klein, R. E. Newby and L. B. Post, U.S. 3,100,693, 13 August 1963, to Stauffer Chemical Co. 237 R. j . L . Andon, J. F . Counsell, H. McKerrell and J. F . Martin, Trans. Faraday Soc. 59(492) (1963) 2702-5. 238 w . S. Holmes, Trans. Faraday Soc. 5 8 (1962) 1916-25.
PHOSPHORUS PENTOXIDE
443
230
dimensional structure . The unit cell is orthorhombic with < z = 1 6 . 3 Â , o = 8.12Â, and c = 5.25 Â. The indexes of refraction in sodium light are na = 1.545, n0 = 1.578, 231 ny = 1.589, all values ±0.002. The density is 2 . 7 2 . The O'-form which is also orthorhombic is prepared by heating the Η-form in a closed system for 24 hr at 450°. The product consists of horny aggregates of relatively large orthorhombic crystals with a structure composed of infinite corrugated sheets parallel to [100] plane. The indices of refraction in sodium light are nw = 1.599 and ne = 1.624, both values±0.002. The density calculated from the indices of refraction is 2.89. It melts slowly at 580°±5° to give a very viscous liquid with a vapor pressure of 720 mm at 2 3 1 600° . The conversion of Η-form to the O- or O'-form is facilitated by condensing the H-form vapor on a seed bed of either the O- or O'-form, in the presence or absence of 0.2 to 0.3% 239 2 4 moisture . ° . The Η-form absorbs moisture at a fast rate and dissolves in water with a hissing sound. The O'-form reacts less rapidly with water but more rapidly than the O-form. The final product in the presence of excess water in all cases is orthophosphoric acid. Under controlled 241 conditions, the Η-form hydrolyzes to form principally tetrametaphosphoric a c i d . The reaction of P4O10 with 100% H2O2 gives peroxomonophosphoric acid and lower polyphosphoric acids but no tetrametaphosphoric acid since cyclic phosphates are rapidly 2 4 2 decomposed by water in the presence of much H2O2 . Applications The largest use for phosphoric pentoxide is the direct hydration into phosphoric acid. A very important use is as the intermediate for the preparation of triethyl phosphate. This is accomplished via the very elegant reaction of P4O10 with ethyl ether to form ethyl polyphosphates which, on subsequent pyrolysis and distillation, disproportionate into 243 triethyl p h o s p h a t e . . Phosphorus pentoxide is used also for the synthesis of the mixed mono- and dialkyl-phosphoiic acids by direct reaction with alcohol. This reaction involves the cleavage of the POP bonds with R O H :
P4O10+6ROH
Ο Ο II II • 2ROP(OH) 2+2(RO) 2POH
For this reaction, the generation of P4O10 in situ by blowing air through a solution of 244 white phosphorus in the alcohol at 150°F has been claimed in the patent literature . 2 45 Phosphorus pentoxide is also used as a desiccant and as a dehydrating agent in organic reactions. For the latter purpose, phosphorus pentoxide was, until recently, employed extensively in the synthesis of methyl methacrylate. Phosphorus pentoxide reacts with ammonia to give a white powdery solid of unknown structure containing both P - N - P as well as P O N H 4 nitrogen. This product has been employed in flameproofing compositions. Phosphorus pentoxide is used as catalyst in 239 240 241 242 243 244 245
F. McCollough, Jr., U.S. 3,034,860, 15 May 1962, to Stauffer Chemical Co. w . F. Tucker, U.S. 2,907,635, 5 October 1959, to Monsanto Chemical Co. M. Shima, K. Hamamoto and S. Utsumi, Bull. Chem. Soc. Japan, 33 (1960) 1386-9. p. w. Schenk and K. Dommain, Ζ. anorg. u. allgem. Chem. 326(3-^) (1963) 139-51. D. C. Hull and J. R. Snodgrass, U.S. 2,407,279, 1946, to Eastman Kodak Co. R. w. Malone, U.S. 3,167,577, 26 January 1965, to Esso Research and Engineering Co. F. Trusell and H. Diehl, Anal. Chem. 35 (1963) 674-7.
444
P H O S P H O R U S : A R T H U R D . F. T O Y 2 46
the air blowing of asphalt and when supported on kieselguhr acts as catalyst for the 247 polymerization of isobutylenes .
7.4.
PHOSPHORUS
TETROXIDE
AND
OTHER
OXIDES
Phosphorus tetroxide (PC>2)n with a molecular weight ranging from 293 to 721 is best prepared by the heating of P4O6 in an evacuated tube at 200-250° for several days. It is separated from the by-product, red phosphorus, by fractional sublimation. The product is a lustrous transparent crystal with a density of 2.54 at 23°. It is deliquescent in air and 226 very soluble in w a t e r . 2 48 Thilo, Heinz, and J o s t in their study of the thermal decomposition of P4O6 obtained two phosphorus (IH/V) oxides, rhombohedral mixed crystals of P4O9 and P4O8 (α-form) and monoclinic mixed crystals of P4O8 and P4O7 molecules (/3-form). The P4O9, P4O8, and P4O7 molecules are structurally similar to P4O10 but lack respectively one, two or three of the terminal oxygens. These mixed oxides can also be obtained by reaction between P4O10 and elemental phosphorus in an inert gas atmosphere. The relationship 2 49 of the structure of these oxides with that of P4O10 and P4O6 is shown b e l o w : ο II
o1
ç/f
Ιο Ο^' I
I
^ O
Ιο °^pJ
p=o
1°
I /P=0
°^P 0
ο
ο
Wo
4°9
P
<
ο I
ο
I
I Λ T o ο ρ.ο 0
o7 I? L
/A-
TABLE 10. PHYSICAL PROPERTIES OF PHOSPHORUS OXIDES
Compound
M.p. °C
Ρ4θιο[Η]
po
23.8 420
Ρ4θ 1 0(Ο] Ρ 4 θ 1 0[ Ο Ί [ P 0 2] 2
562 580 sublimes
4
6
B.p. °C 175.4 340 360 (sublime) 605 605 —
Heat of vaporization (kcal/mole)
Heat of fusion (kcal/mole)
—
—
Heat of sublimation (kcal/mole) —
16.2
6.5
22.7
18.7 18.7
7.7 5.2
36.4 33.9
—
Density
Ref. a
2.135 at 21° 2.3
a,
b
2.72 2.89 2.54 at 23°
a, a, a
b b
a
J. R. van Wazer, Phosphorus and Its Compounds, Vol. I, pp. 266-86. Interscience Publishers Inc., Newb York (1958). T. D. Fair, Phosphorus, Properties of the Element and Some of Its Compounds, Chemical Engineering Report No. 8, pp. 18-25, TVA, Wilson Dam (1950). 246 Ε. K. Brown and R. A. Bürge, U.S. 2,886,506, 12 May 1959, to Standard Oil of Indiana. T. A. Kolesnikova, Ο. I. Lapitskaya and T. N. Lanina, Tr. Boshkirsk. Nauchn. Issled. Inst, po Pereabotke Nefte. No. 5 (1960) 176-80. 24 » E. Thilo, D. Heinz and Κ. H. Jost, Angew. Chem. Intern. Ed. 3(3) (1964) 232. 24 <> D. Heinz, Z. anorg. u. allgem. Chem. 3 3 6 (1965) 137. 2 47
P H O S P H O R U S SESQUJSULFIDE, 250
445
P 4S 3
2 51
Schenk and Vietzke » obtained a mixture of two peroxides by exposing a mixture of P4O10 and oxygen to a corona discharge. One product is violet in color and radicallike. It is stable at room temperature; on heating it is converted to the colorless P4O11 which is stable u p to 130° and hydrolyzes according to : • H4P2O8+H4P2O7
P 40 i i + 4 H 20
8. P H O S P H O R U S S U L F I D E S , O X Y S U L F I D E S A N D RELATED
COMPOUNDS
Phosphorus forms with sulfur four well-characterized crystalline compounds, P4S10, P4S7, P4S5 and P4S3. Of these, P4S10 and P4S3 are produced commercially. The melting
point diagram for the sulfur-phosphorus system is shown in Fig. 7
2 5 2
.
A t o m - % of phosphorus
FIG. 7. Phosphorus-sulfur system.
2 52
(Reprinted by permission of John Wiley & Sons Inc.)
The structures of these sulfides may be treated as insertion and addition of the correct 2 53 amount of sulfur to the phosphorus tetrahedral. They are s h o w n in Fig. 8.
8 . 1 . P H O S P H O R U S S E S Q U I S U L F I D E , P4S3
Phosphorus sesquisulfide is prepared by heating a stoichiometric mixture of white phosphorus and sulfur at above 180° under an inert atmosphere. It is purified by distillation at 420° under ordinary pressure or by recrystallization from toluene. In an industrial scale, to moderate the highly exothermic reaction, phosphorus is added continuously to sulfur. The final purified product is a yellow rhombic crystal which dissolves 31.2 g at 17° and 15.4 g at 111° in 100 g of toluene. The structure of phosphorus sesquisulfide 2
50 P. W. Schenk and H. Vietzke, Angew. Chem. 74 (1962) 75. 51 P. W. Schenk and H. Vietzke, Z. anorg. u. allgem. Chem. 326(3-4) (1963) 152-69. J. R. van Wazer, Phosphorus and Its Compounds, Vol. I, pp. 289-309, Interscience Publishers Inc., New2 York (1959). " D. E. C. Corbridge, Topics in Phosphorus Chemistry, 3 (1966) 81-5. 2
2 52
PHOSPHORUS : A R T H U R D . F. TOY
PS(NCO) 3 is obtained by the addition of sulfur to P(NCO) 3 at 140° 217. i t is stable when formed but polymerizes to a solid on heating. The compound PS(NCS) 3 is prepared by the reaction of PSC1 3 with N H 4 S C N in S 0 2 solvent at - 3 0 ° 21s, S P C I 3 H- 3 N H 4 S C N
S P ( N C S ) 3+ 3 N H 4C 1
219
or in C H 3 C N as solvent at room temperature . The thiophosphoryl fluoride diisothiocyanate, SPF(NCS) 2, is obtained by the reaction of C1 2P(S)F with KSCN (b.p. 46-47° at 1 mm, 1.4827, ,ig> 1.6390). It is a colorless liquid with a sharp odor and powerful lachrymatory properties. It decomposes gradually 220 on storage and hydrolyzes slowly in w a t e r . The compounds SPF(NCS) 2 and SPF 2(NCS) 22 are prepared by the fluorination of SP(NCS) 3 with S b F 3 under reduced pressure *. Liquid exchange reactions between SPBr 3 and SP(NCS) 3 result in the formation of 2 SPBr 2(NCS) and SPBr(NCS) 2 as indicated by the new lines in the N M R spectra **. The equilibrium state for this ligand exchange reaction is reached after heating at 150° for 156 hr. Compositions with various ratios of phosphorus-sulfur-halogen, other than those described, have also been reported in the literature. Many of these compounds are as yet not well characterized. The properties of some thiophosphoryl halides and thiophosphoryl pseudohalides are listed in Table 9.
7. P H O S P H O R U S
OXIDES
The oxidation of white phosphorus takes place by a chain reaction between a lower and upper limit of oxygen pressures. At the lower limit, the oxygen chain carriers are destroyed by impact with the walls and at the upper limit triple collision occurs and 0 3 222 is f o r m e d . The net result is the formation of many intermediate oxidation products as represented by the many oxides reported in the literature. Of the oxides, phosphorus pentoxide only is commercially important. Phosphorus trioxide has been the subject of many recent investigations. Much less is known, however, on phosphorus tetroxide and other oxides such as P4O7, P4O8, and P4O9.
7.1. P H O S P H O R U S M O N O X I D E
Phosphorus forms a diatomic molecule, PO, with oxygen. It is unstable under ordinary 2 23 conditions and is detected in the vapor state only in spectral bands. Cordes and W a r k e h r 2 2 2 2 showed that the systems correspond t o Σ - * Χ π and π Χ π transitions. They reported the dissociation energy of P O as 5.30 eV. Other workers calculated the PO 2 Π Η . H. Anderson, Silicon, Sulfur, Phosphates, pp. 235-7, IUPAC Colloquium, Münster, 2-6 Sept. 1954. 218 E. Ruck, H. Binder and F. L. Goldmann, Ζ. anorg. u. allgem. Chem. 338(1-2) (1965) 58-62. 219 B. S. Green, D. B. Sowerby and K. J. Wihksne, Chem. & Ind. (1960) 1306-7. 220 ZH. M. Ivanova, Ε. A. Stukalo and G. I. Derkach, Zhurnal. Obshchei Khimii, 37(5) (1967) 1144-7 221 H. W. Roesky and Z. Muller, Z. anorg. u. allgem. Chem. 3 5 3 (1967) 266-9. 222 H . Cordes and W. Witschel, Z . Physik. Chem. 46(1/2) (1965) 35-48. 223 H. Cordes and E. Warkehr, Ζ . Physik. Chem. 46(1/2) (1965) 26-34.
PHOSPHORUS 2 24
TRIOXIDE
441
225
dissociation energy as 6.2 e V and 6.8 e V . The P - O distance is 1.447 Â and the excitation energy for the molecule is 143 kcal/mole. The dissociation is to oxygen and an 226 excited phosphorus a t o m . A polymeric (PO) n has also been reported. It is formed by the electrolysis of POCl 3 in (C2H 5)3N.HC1 at 0° or by the reaction of POBr 3 with magnesium in C 2 H 5 O H solution. The product is a brownish amorphous-crystalline substance insoluble and stable in water but decomposes in an aqueous alkali solution with liberation of P H 3 . It oxidizes to P 2 0 5 2 2 7 in an oxygen atmosphere at 300° .
7.2. P H O S P H O R U S
TRIOXIDE
Phosphorus trioxide, P 2 0 3 or P4O6, is prepared by the controlled oxidation of phos226 phorus at a pressure of 90 mm with air enriched to contain 75% of total oxygen . A more recent method is by the oxidation of phosphorus with N 2 0 at 550-600° under 70 torr. 228 A yield of 50% of P4O6 is obtained . The usual contamination of 1 to 2% of dissolved white phosphorus is removed by conversion to the less soluble and less volatile red phosphorus by u.v. radiation followed by distillation in vacuo. Phosphorus trioxide has a tetrahedral symmetry of the point group Td. This is confirmed -1 by two strong absorption bands at 639 and 911 c m together with seven Raman 229 frequencies . The phosphorus atoms are located at the corners of a regular tetrahedron and each of the six oxygens located between two phosphorus atoms in the plane of symmetry 2 30 passing through the trigonal a x e s :
o—Ρ—Ο
The P - P distance is 2.95 ±0.03 A. Phosphorus trioxide is a colorless liquid at ordinary temperature and has a m.p. of 23.9° and b.p. of 175.4°. The standard enthalpy of formation AH} of P 4 0 6 (crystal) is calculated to be - 3 9 2 kcal/mole [taking AH} P4O10 (crystal) as - 7 1 3 kcal/mole]. The heat of sublimation is 16 kcal/mole and the entropy of vaporization = P 4 O 6 (liq.)-*P4C>6 231 (gas): Δ 5 4 4 8 . 5 6 = 21.0 cal mole/deg . The bond energy = (P-O) is calculated to be 232 86 kcal/mole . The dielectric constant is 3.2 at 22° and the surface tension is 36.6 dynes/cm at 34.3°. The compound hydrolyzes in an excess of water forming H 3 P 0 3 . Upon heating to 224 J. Berkowitz, / . Chem. Phys. 30 (1959) 858-60. 225 c. V. V. S. Ν. K. Santharam and P. Tiruvenganna Rao, Indian J. Phys. 37 (1963) 14-17. 226 j . R. van Wazer, Phosphorus and Its Compounds, Vol. I, pp. 266-86, Interscience Publishers Inc., N.Y. (1958). 227 H. Spandau and A. Beyer, Naturwissenschaften, 46 (1959) 400. 228 Ε. Thilo and D. Heinz, Ger. 1,172,241, 18 June 1964, to Deutsche Akademie der Wissenschaften zu Berlin. 229 T. A. Sidorov, Trudy Fiz. Inst. im. P.N. Lebedeva, Akad. Nauk. SSSR 12 (1960) 225-73. 230 D. E. C. Corbridge, Topics in Phosphorus Chemistry, 3 (1966) 71-81. 231 T. D. Farr, Phosphorus, Properties of the Element and Some of Its Compounds, Chemical Engineering Report No. 8, pp. 18-25, T.V.A., Wilson Dam, Alabama (1950). 232 s. B. Hartley and J. C. McCoubrey, Nature, 1 9 8 (1963) 476.
442
P H O S P H O R U S : A R T H U R D . F. T O Y
above 210°, it disproportionates to red phosphorus and a phosphorus oxide of the approxi2 3 1 mate composition of P n 0 2 n . It displaces CO from Ni(CO)4 forming successively coordinates containing one to four Ni(CO)3 groups. The compound P4O6 [Ni(CO) 3] 4 is crystalline. Reaction of P4O6 with B2H6 results in the symmetrical cleavage of B2H6 2 3 3 5 with the formation of Η 3 Β Ρ 4 θ 6 Β Η 3 " .
7.3. P H O S P H O R U S
PENTOXIDE
Preparation, Structure and Properties When phosphorus burns in the presence of an excess of oxygen in air, the product formed is P4O10, commonly called phosphorus pentoxide. One commercial process for collecting the fine white smoke is by condensing it continuously on a fluidized bed of 236 granular P4O10 at 129° into free flowing dustless small b e a d s . The P4O10 thus obtained is the hexagonal or " H " form, the product of commerce. The free energy of formation of P4O10 is - 6 4 4 . 8 kcal/mole and the standard entropy at 298.15°K is 54.66±0.1 cal/mole/deg. The heat capacity at 298.15°K Cp = 50.60 cal/deg. o Other molar thermodynamic properties at 298.15°K are (H-H 0) = SUl cal, (H-H°0)IT = 23 27.22 cal/deg and -(G-H°0)IT= 27.47 cal/deg ?. The value of enthalpy of formation 238 of hexagonal P4O10 at 25° has also been reported as - 7 1 3 . 2 kcal/mole . The mean bond energy E(P-0) and dissociation energy S ( P - O ) were calculated as 86 and 138 232 kcal/mole respectively . The P4O10 molecule in the vapor state with a Td symmetry has the arrangement of 230 atoms as shown in the following d i a g r a m . It has been estimated that there is essentially
no 7Γ bonding between the phosphorus and the shared oxygen and about 1.5-2.0 π-bond 226 between the phosphorus and the unshared oxygen in addition to the δ-bond . The crystals are uniaxial positive with the indexes of refraction in sodium light: nw= 1.469 23 and nB = 1.471, both values±0.002 i. It has a density of 2.30. There are two other crystalline modifications of phosphorus oxide : the O-form and a stable orthorhombic form designated as the O'-form. The O-form is prepared by heating the Η-form in a closed system for 2 hr at 400°. It is built of rings of ten PO4 tetrahedra linking together to form a continuous three233 j . G . Riess and J. R. van Wazer, / . Am. Chem. Soc. 8 8 (10) (1966) 2166-70. 234 G . Kodama and H. Kondo, / . Am. Chem. Soc. 88(9) (1966) 2045-6. 235 j . G . Riess and J. R. van Wazer, / . Am. Chem. Soc. 88(10) (1966) 2339-40. 236 G . I. Klein, R. E. Newby and L. B. Post, U.S. 3,100,693, 13 August 1963, to Stauffer Chemical Co. 237 R. j . L . Andon, J. F . Counsell, H. McKerrell and J. F . Martin, Trans. Faraday Soc. 59(492) (1963) 2702-5. 238 w . S. Holmes, Trans. Faraday Soc. 5 8 (1962) 1916-25.
PHOSPHORUS PENTOXIDE
443
230
dimensional structure . The unit cell is orthorhombic with < z = 1 6 . 3 Â , o = 8.12Â, and c = 5.25 Â. The indexes of refraction in sodium light are na = 1.545, n0 = 1.578, 231 ny = 1.589, all values ±0.002. The density is 2 . 7 2 . The O'-form which is also orthorhombic is prepared by heating the Η-form in a closed system for 24 hr at 450°. The product consists of horny aggregates of relatively large orthorhombic crystals with a structure composed of infinite corrugated sheets parallel to [100] plane. The indices of refraction in sodium light are nw = 1.599 and ne = 1.624, both values±0.002. The density calculated from the indices of refraction is 2.89. It melts slowly at 580°±5° to give a very viscous liquid with a vapor pressure of 720 mm at 2 3 1 600° . The conversion of Η-form to the O- or O'-form is facilitated by condensing the H-form vapor on a seed bed of either the O- or O'-form, in the presence or absence of 0.2 to 0.3% 239 2 4 moisture . ° . The Η-form absorbs moisture at a fast rate and dissolves in water with a hissing sound. The O'-form reacts less rapidly with water but more rapidly than the O-form. The final product in the presence of excess water in all cases is orthophosphoric acid. Under controlled 241 conditions, the Η-form hydrolyzes to form principally tetrametaphosphoric a c i d . The reaction of P4O10 with 100% H2O2 gives peroxomonophosphoric acid and lower polyphosphoric acids but no tetrametaphosphoric acid since cyclic phosphates are rapidly 2 4 2 decomposed by water in the presence of much H2O2 . Applications The largest use for phosphoric pentoxide is the direct hydration into phosphoric acid. A very important use is as the intermediate for the preparation of triethyl phosphate. This is accomplished via the very elegant reaction of P4O10 with ethyl ether to form ethyl polyphosphates which, on subsequent pyrolysis and distillation, disproportionate into 243 triethyl p h o s p h a t e . . Phosphorus pentoxide is used also for the synthesis of the mixed mono- and dialkyl-phosphoiic acids by direct reaction with alcohol. This reaction involves the cleavage of the POP bonds with R O H :
P4O10+6ROH
Ο Ο II II • 2ROP(OH) 2+2(RO) 2POH
For this reaction, the generation of P4O10 in situ by blowing air through a solution of 244 white phosphorus in the alcohol at 150°F has been claimed in the patent literature . 2 45 Phosphorus pentoxide is also used as a desiccant and as a dehydrating agent in organic reactions. For the latter purpose, phosphorus pentoxide was, until recently, employed extensively in the synthesis of methyl methacrylate. Phosphorus pentoxide reacts with ammonia to give a white powdery solid of unknown structure containing both P - N - P as well as P O N H 4 nitrogen. This product has been employed in flameproofing compositions. Phosphorus pentoxide is used as catalyst in 239 240 241 242 243 244 245
F. McCollough, Jr., U.S. 3,034,860, 15 May 1962, to Stauffer Chemical Co. w . F. Tucker, U.S. 2,907,635, 5 October 1959, to Monsanto Chemical Co. M. Shima, K. Hamamoto and S. Utsumi, Bull. Chem. Soc. Japan, 33 (1960) 1386-9. p. w. Schenk and K. Dommain, Ζ. anorg. u. allgem. Chem. 326(3-^) (1963) 139-51. D. C. Hull and J. R. Snodgrass, U.S. 2,407,279, 1946, to Eastman Kodak Co. R. w. Malone, U.S. 3,167,577, 26 January 1965, to Esso Research and Engineering Co. F. Trusell and H. Diehl, Anal. Chem. 35 (1963) 674-7.
444
P H O S P H O R U S : A R T H U R D . F. T O Y 2 46
the air blowing of asphalt and when supported on kieselguhr acts as catalyst for the 247 polymerization of isobutylenes .
7.4.
PHOSPHORUS
TETROXIDE
AND
OTHER
OXIDES
Phosphorus tetroxide (PC>2)n with a molecular weight ranging from 293 to 721 is best prepared by the heating of P4O6 in an evacuated tube at 200-250° for several days. It is separated from the by-product, red phosphorus, by fractional sublimation. The product is a lustrous transparent crystal with a density of 2.54 at 23°. It is deliquescent in air and 226 very soluble in w a t e r . 2 48 Thilo, Heinz, and J o s t in their study of the thermal decomposition of P4O6 obtained two phosphorus (IH/V) oxides, rhombohedral mixed crystals of P4O9 and P4O8 (α-form) and monoclinic mixed crystals of P4O8 and P4O7 molecules (/3-form). The P4O9, P4O8, and P4O7 molecules are structurally similar to P4O10 but lack respectively one, two or three of the terminal oxygens. These mixed oxides can also be obtained by reaction between P4O10 and elemental phosphorus in an inert gas atmosphere. The relationship 2 49 of the structure of these oxides with that of P4O10 and P4O6 is shown b e l o w : ο II
o1
ç/f
Ιο Ο^' I
I
^ O
Ιο °^pJ
p=o
1°
I /P=0
°^P 0
ο
ο
Wo
4°9
P
<
ο I
ο
I
I Λ T o ο ρ.ο 0
o7 I? L
/A-
TABLE 10. PHYSICAL PROPERTIES OF PHOSPHORUS OXIDES
Compound
M.p. °C
Ρ4θιο[Η]
po
23.8 420
Ρ4θ 1 0(Ο] Ρ 4 θ 1 0[ Ο Ί [ P 0 2] 2
562 580 sublimes
4
6
B.p. °C 175.4 340 360 (sublime) 605 605 —
Heat of vaporization (kcal/mole)
Heat of fusion (kcal/mole)
—
—
Heat of sublimation (kcal/mole) —
16.2
6.5
22.7
18.7 18.7
7.7 5.2
36.4 33.9
—
Density
Ref. a
2.135 at 21° 2.3
a,
b
2.72 2.89 2.54 at 23°
a, a, a
b b
a
J. R. van Wazer, Phosphorus and Its Compounds, Vol. I, pp. 266-86. Interscience Publishers Inc., Newb York (1958). T. D. Fair, Phosphorus, Properties of the Element and Some of Its Compounds, Chemical Engineering Report No. 8, pp. 18-25, TVA, Wilson Dam (1950). 246 Ε. K. Brown and R. A. Bürge, U.S. 2,886,506, 12 May 1959, to Standard Oil of Indiana. T. A. Kolesnikova, Ο. I. Lapitskaya and T. N. Lanina, Tr. Boshkirsk. Nauchn. Issled. Inst, po Pereabotke Nefte. No. 5 (1960) 176-80. 24 » E. Thilo, D. Heinz and Κ. H. Jost, Angew. Chem. Intern. Ed. 3(3) (1964) 232. 24 <> D. Heinz, Z. anorg. u. allgem. Chem. 3 3 6 (1965) 137. 2 47
P H O S P H O R U S SESQUJSULFIDE, 250
445
P 4S 3
2 51
Schenk and Vietzke » obtained a mixture of two peroxides by exposing a mixture of P4O10 and oxygen to a corona discharge. One product is violet in color and radicallike. It is stable at room temperature; on heating it is converted to the colorless P4O11 which is stable u p to 130° and hydrolyzes according to : • H4P2O8+H4P2O7
P 40 i i + 4 H 20
8. P H O S P H O R U S S U L F I D E S , O X Y S U L F I D E S A N D RELATED
COMPOUNDS
Phosphorus forms with sulfur four well-characterized crystalline compounds, P4S10, P4S7, P4S5 and P4S3. Of these, P4S10 and P4S3 are produced commercially. The melting
point diagram for the sulfur-phosphorus system is shown in Fig. 7
2 5 2
.
A t o m - % of phosphorus
FIG. 7. Phosphorus-sulfur system.
2 52
(Reprinted by permission of John Wiley & Sons Inc.)
The structures of these sulfides may be treated as insertion and addition of the correct 2 53 amount of sulfur to the phosphorus tetrahedral. They are s h o w n in Fig. 8.
8 . 1 . P H O S P H O R U S S E S Q U I S U L F I D E , P4S3
Phosphorus sesquisulfide is prepared by heating a stoichiometric mixture of white phosphorus and sulfur at above 180° under an inert atmosphere. It is purified by distillation at 420° under ordinary pressure or by recrystallization from toluene. In an industrial scale, to moderate the highly exothermic reaction, phosphorus is added continuously to sulfur. The final purified product is a yellow rhombic crystal which dissolves 31.2 g at 17° and 15.4 g at 111° in 100 g of toluene. The structure of phosphorus sesquisulfide 2
50 P. W. Schenk and H. Vietzke, Angew. Chem. 74 (1962) 75. 51 P. W. Schenk and H. Vietzke, Z. anorg. u. allgem. Chem. 326(3-4) (1963) 152-69. J. R. van Wazer, Phosphorus and Its Compounds, Vol. I, pp. 289-309, Interscience Publishers Inc., New2 York (1959). " D. E. C. Corbridge, Topics in Phosphorus Chemistry, 3 (1966) 81-5. 2
2 52