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DINITROGEN PENTOXIDE
356
NITROGEN: K. JONES
to the presence of nitrite in solution, which can also form the basis of a colorimetric method of analysis572. The application of titrimetric methods is somewhat similar to those described for nitric oxide. Nitrogen dioxide can be oxidized with excess cerium (IV) which is back-titrated with sodium oxalate: N 0 2 + Ce4+ + H 2 0 -> N O " + Ce3+ + 2 H +
Reference should also be made, however, to the analysis of nitrous fumes. This usually refers to mixtures of nitric oxide, nitrogen dioxide and other oxides that exist in equilibrium with them (principally N2O3 and N2O4). Their toxicity has directed considerable attention to the detection and estimation of small amounts of such materials573. Thus in this connection, and frequently in other situations, only the total nitrogen oxide content is required, and this can be obtained by oxidation with alkaline hydrogen peroxide to nitrate which can be determined colorimetrically by the phenoldisulphonic acid method. This can be preceded by absorption in Griess-Ilosvay reagent to obtain a separate estimation of N 0 2 content if required. For full analysis of such nitrogen oxide mixtures, mass spectrometry provides the quickest most reliable method. However, more recently, gas Chromatographie techniques have been developed which are capable of separating mixtures containing oxides of both nitrogen and carbon as well as elemental nitrogen using helium as carrier gas and thermal conductivity cell detectors574.
14. DINITROGEN PENTOXIDE 14.1. PREPARATION
Dinitrogen pentoxide is usually prepared in the laboratory by the dehydration of concentrated nitric acid with phosphorus pentoxide 575 · 576 : 2 H N O 3 + P 2 O 5 -> N2O5 + 2HPO3
Considerable care must be exercised during this reaction, as the procedure577 involves either the dropwise addition of concentrated HNO3 (d 1.525g/cm3) onto a large excess of P2O5 at — 10°C followed by slow distillation at about 35°C, or by adding P2O5 in one portion to nitric acid previously cooled to — 78 °C then allowing the mixture to attain room temperature slowly578. The principal alternative method is by the oxidation of nitrogen dioxide with ozone: 2 Ν 0 2 + θ 3 - * Ν 2 θ 5 + θ2 573 Methods for the Detection of Toxic Substances in Air, Booklet N o . 5, Nitrous Fumes, Ministry o f Employment and Productivity, H M Factory Inspectorate, H M S O , London (1969). 574 j . Janak and M. Rusek, Chemické Listy 48 (1954) 397. 5751. R. Beattie, in ref. 571, pp. 269-77. 576 p . w . Schenk, in Handbook of Preparative Inorganic Chemistry, Vol. I (ed. G. Brauer), Academic Press, N e w York (1963), p. 489. 577 N . S. Gruenhut, M. Goldfrank, M. L. Cushing and G. V. Caesar, Inorganic Syntheses, Vol. I l l (ed. L. F. Andrieth), McGraw-Hill, N e w York (1950), p. 78. ^78 G. V. Caesar and M. Goldfrank, / . Am. Chem. Soc. 68 (1962) 372.
STRUCTURE AND BONDING OF N 2 0 5
357
The difficulties encountered when attempting to dry NO2 make it preferable579 to oxidize dry NO with dry O2 first, and when this reaction is complete then passing Ο2-Ο3 through. Another variation is bubbling ozonized oxygen through liquid N2O4. N2O5 is also formed in the Birkland-Eyde arc process for the fixation of nitrogen and also when chlorine, phosphoric oxychloride or nitryl chloride reacts with silver nitrate575 : N O 2 C I + A g N 0 3 -> Ν2Ο5 + A g C l
The electrolysis of N2O4 dissolved in nitric acid also affords oxidation575 to N2O5: 2NOy + N204 -* 2N205 + 2
-i
-i
*/ " v N
_ .1.
O
N
_.i.
F I G . 37. Non-pairing valence bond configuration for molecular N2O5.
Valence bond structures for the NOJ ion (also isoelectronic and isostructural with N J and CNf," ions) together with Linnett non-pairing structures583 are illustrated in Fig. 38. Alternatively, in terms of molecular orbital theory, the ground state configuration is (1 σ ,)2(1 σΗ )2(2 σ< ,)2(2 σΐί )2(1 πΜ )4(1 π9 )4
in which the sixteen electrons are contained in filled bonding and non-bonding orbitals while the anti-bonding levels remain empty583. 579 R. E. Nightingale, A . R. D o w n i e , D . L. Rothenberg, B. Crawford and R. A. Ogg, Jr., / . Phys. Chem. 58 (1954) 1047. 580 E. Grison, K. Eriks and J. L. de Vries, Acta Cryst. 3 (1950) 290. 581 R. F. Barrow and A . J. Merer, in ref. 571, p. 520. 582 L , E. Sutton, Tables of Interatomic Distances and Configurations in Molecules and Ions, Chemical Society, London (1958). 583 M. Green, in Developments in Inorganic Nitrogen Chemistry, Vol. I (ed. C. B. Colburn), Elsevier, Amsterdam (1966), p. 40.
358
NITROGEN : K. JONES
+ ·Ό= :N = =o:
+ :0
h
:0-
-Ä-
-O:
2
+ N = 0 :
-f-
2
:0-^Ν=:0:
+ :0=N
+ O:
:Ô=N
0:
FIG. 38. Valence bond configuration for NOJ.
14.3. M O L E C U L A R
CONSTANTS
A few molecular constants for N2O5 in the crystalline phase (NO\ NO 3 ) and the molecular form are included in Table 75.
TABLE 75. MOLECULAR CONSTANTS OF N2O5
Value
Property X-ray diffraction data a on N O J N O j Space group Lattice constants N2O5 structure b Terminal N - O Central N - O Dipole moment0
Dlh-C6/mmc a = 5.41 Â, c = 6.57 Â 1.2 Â 1.3-1.4 A 1.40/) N0 2 f
N-O bond length Raman spectrumd
1.154Â 1400 cm-1 538 cm » 2375 cm 1
NO3 1.24 1050 cm -J 824 cm-i 1413 cm-i 722 cm I
a
Ref. 580. Ref. 582. A. L. McClellan, Tables of Experimental Dipole Moments, W. H. Freeman, San Francisco (1963). d Ref. 581.
b
c
14.4. P H Y S I C A L P R O P E R T I E S
Some values for the physical constants of N2O5 are listed in Table 76, principally thermodynamic data584. Spectroscopic measurements have been recorded for N2O5 in both gaseous and solid phases and also in solutionssi. 584 D . D. Wagman, W. H. Evans, V. B. Barker, I. Halow, S. M. Bailey and R. H. Schumm, Selected Values of Chemical Thermodynamic Properties, NBS Technical Note 270-3, Washington (1968).
REACTIONS OF N 2 0 5
359
TABLE 76. PHYSICAL PROPERTIES* OF N 2 0 5 Temperature (K)
Property
Sublimation temperature Density (calculated) Thermodynamic functions5 N 2 0 5 (cryst.) AH (enthalpy of sublimation) S (entropy of sublimation) AH} (enthalpy of formation) Δ G° (Gibbs energy of formation) 5° (entropy) Cp (heat capacity) N2O5 (g) i\H°f (enthalpy of formation) &H°f (enthalpy of formation) H2n-Ho (enthalpy) Δ G} (Gibbs energy of formation) S° (entropy) C° (heat capacity) Vapour pressure equation
a
Ref. 575.
b
\
Value
305.6 78 213 288
32.4°C 2.175 g/cm3 2.14 g/cm3 2.05 g/cm3
305.6 305.6 298 298 298 298
13.6 kcal/mol 44.5 cal/deg mol - 1 0 . 3 kcal/mol 27.2 kcal/mol 42.6 cal/deg mol 34.2 cal/deg mol
0 5.7 kcal/mol 298 2.7 kcal/mol 298 4.237 kcal/mol 298 27.5 kcal/mol 298 85.0 cal/deg mol 298 20.2 cal/deg mol logioPmm = 1244/Γ+34.1 log 10 Γ-85.929 236.4 1 mm 10 mm I 256.5 40 mm 270.3 100 mm 280.6 400 mm 297.6 305.6 760 mm
Ref. 584. 14.5. REACTIONS
Although N2O5 is colourless, absorption occurs at ~ 3 8 0 0 Â in the ultraviolet region, and this has been assigned to photodecomposition according to the equation 5 8 1 N 2 05^N 2 04+0(3i>) The thermal decomposition of N2O5 has played an important rôle in the theory of reaction rates^s : 2N205-^2N204+02 The decomposition rate is first order: - N 0 2 + 0 2 + N O NO + N 2 0 5 ->3N0 2 585 R. A. Ogg, Jr., / . Chem. Phys. 15 (1947) 337.
360
NITROGEN: K. JONES
Labelled NO2 undergoes rapid exchange 586 with N2O5: "N2O5 + 1 5 N 0 2 ^ i 5 N 2 0 5 +
14
N02
and many gas phase reactions of N2O5 depend on initial dissociation to NO2 and NO3 with the latter then further acting as an oxidizing agent. Dinitrogen pentoxide is the formal anhydride of nitric acid and being deliquescent it is very readily hydrated: N 2 0 5 + H 2 0-*2HN0 3 With hydrogen peroxide, pernitric acid is also formed 5 8 7 : N 2 0 5 + H 2 0 2 -> HNO3+HNO4 In anhydrous solvents like nitric, perchloric, sulphuric and phosphoric acids, N2O5 dissociates providing a convenient source of nitronium ions and a suitable preparative route to nitronium salts 5 8 8 : N 2 0 5 + 3H2S04 -> 2NOJ + H 3 0 + + 3HSO4 N 2 0 5 + 3HC104 -> 2NOÎ + H 3 0 + + 3C104 2N 2 0 5 + H30+C104 -> NO£C10; + 3HN0 3 N 2 0 5 + 2S0 3 -> [NOJ]2S20?N 2 0 5 + FS03H -> N0JFS0I + HN0 3 The strong oxidizing action of N2O5 can cause violent reactions with reducing agents including metals and organic substances to give nitrates and/or oxides 575 : N 2 O s + Na -> NaN0 3 + N0 2 N 2 0 5 + NaF -^ NaN0 3 + FN0 2 N 2 0 5 + I 2 ->I 2 0 5 + N2
15. N I T R O G E N T R I O X I D E The existence of the nitrogen trioxide radical NO3 has for long been assumed judging by its frequent postulation in nitrogen oxide reaction mechanisms. For example, dinitrogen pentoxide undergoes thermal decomposition by a homogeneous first-order gas-phase reaction (see section 14). However, the reaction is not unimolecular as was initially thought, and one mechanism which is consistent with the data involves the preliminary equilibrium 589 followed by
N205^N02+N03 N0 3 + N0 2 -► N0 2 + 0 2 + NO N0 3 + N 0 ^ 2 N 0 2
586 A . R . Amell a n d F . Daniels, / . Am. Chem. Soc. 74 (1952) 6209. 587 R . Schwartz, Z . anorg. allgem. Chem. 256 (1947) 3. 588 D . R . G o d d a r d , E . D . H u g h e s a n d C . K . Ingold, / . Chem. Soc. (1950) 25«Q 589 R . A . Ogg, J r . , / . Chem. Phys. 5 (1937) 8 7 \
NITROGEN: K. JONES
to the presence of nitrite in solution, which can also form the basis of a colorimetric method of analysis572. The application of titrimetric methods is somewhat similar to those described for nitric oxide. Nitrogen dioxide can be oxidized with excess cerium (IV) which is back-titrated with sodium oxalate: N 0 2 + Ce4+ + H 2 0 -> N O " + Ce3+ + 2 H +
Reference should also be made, however, to the analysis of nitrous fumes. This usually refers to mixtures of nitric oxide, nitrogen dioxide and other oxides that exist in equilibrium with them (principally N2O3 and N2O4). Their toxicity has directed considerable attention to the detection and estimation of small amounts of such materials573. Thus in this connection, and frequently in other situations, only the total nitrogen oxide content is required, and this can be obtained by oxidation with alkaline hydrogen peroxide to nitrate which can be determined colorimetrically by the phenoldisulphonic acid method. This can be preceded by absorption in Griess-Ilosvay reagent to obtain a separate estimation of N 0 2 content if required. For full analysis of such nitrogen oxide mixtures, mass spectrometry provides the quickest most reliable method. However, more recently, gas Chromatographie techniques have been developed which are capable of separating mixtures containing oxides of both nitrogen and carbon as well as elemental nitrogen using helium as carrier gas and thermal conductivity cell detectors574.
14. DINITROGEN PENTOXIDE 14.1. PREPARATION
Dinitrogen pentoxide is usually prepared in the laboratory by the dehydration of concentrated nitric acid with phosphorus pentoxide 575 · 576 : 2 H N O 3 + P 2 O 5 -> N2O5 + 2HPO3
Considerable care must be exercised during this reaction, as the procedure577 involves either the dropwise addition of concentrated HNO3 (d 1.525g/cm3) onto a large excess of P2O5 at — 10°C followed by slow distillation at about 35°C, or by adding P2O5 in one portion to nitric acid previously cooled to — 78 °C then allowing the mixture to attain room temperature slowly578. The principal alternative method is by the oxidation of nitrogen dioxide with ozone: 2 Ν 0 2 + θ 3 - * Ν 2 θ 5 + θ2 573 Methods for the Detection of Toxic Substances in Air, Booklet N o . 5, Nitrous Fumes, Ministry o f Employment and Productivity, H M Factory Inspectorate, H M S O , London (1969). 574 j . Janak and M. Rusek, Chemické Listy 48 (1954) 397. 5751. R. Beattie, in ref. 571, pp. 269-77. 576 p . w . Schenk, in Handbook of Preparative Inorganic Chemistry, Vol. I (ed. G. Brauer), Academic Press, N e w York (1963), p. 489. 577 N . S. Gruenhut, M. Goldfrank, M. L. Cushing and G. V. Caesar, Inorganic Syntheses, Vol. I l l (ed. L. F. Andrieth), McGraw-Hill, N e w York (1950), p. 78. ^78 G. V. Caesar and M. Goldfrank, / . Am. Chem. Soc. 68 (1962) 372.
STRUCTURE AND BONDING OF N 2 0 5
357
The difficulties encountered when attempting to dry NO2 make it preferable579 to oxidize dry NO with dry O2 first, and when this reaction is complete then passing Ο2-Ο3 through. Another variation is bubbling ozonized oxygen through liquid N2O4. N2O5 is also formed in the Birkland-Eyde arc process for the fixation of nitrogen and also when chlorine, phosphoric oxychloride or nitryl chloride reacts with silver nitrate575 : N O 2 C I + A g N 0 3 -> Ν2Ο5 + A g C l
The electrolysis of N2O4 dissolved in nitric acid also affords oxidation575 to N2O5: 2NOy + N204 -* 2N205 + 2
-i
-i
*/ " v N
_ .1.
O
N
_.i.
F I G . 37. Non-pairing valence bond configuration for molecular N2O5.
Valence bond structures for the NOJ ion (also isoelectronic and isostructural with N J and CNf," ions) together with Linnett non-pairing structures583 are illustrated in Fig. 38. Alternatively, in terms of molecular orbital theory, the ground state configuration is (1 σ ,)2(1 σΗ )2(2 σ< ,)2(2 σΐί )2(1 πΜ )4(1 π9 )4
in which the sixteen electrons are contained in filled bonding and non-bonding orbitals while the anti-bonding levels remain empty583. 579 R. E. Nightingale, A . R. D o w n i e , D . L. Rothenberg, B. Crawford and R. A. Ogg, Jr., / . Phys. Chem. 58 (1954) 1047. 580 E. Grison, K. Eriks and J. L. de Vries, Acta Cryst. 3 (1950) 290. 581 R. F. Barrow and A . J. Merer, in ref. 571, p. 520. 582 L , E. Sutton, Tables of Interatomic Distances and Configurations in Molecules and Ions, Chemical Society, London (1958). 583 M. Green, in Developments in Inorganic Nitrogen Chemistry, Vol. I (ed. C. B. Colburn), Elsevier, Amsterdam (1966), p. 40.
358
NITROGEN : K. JONES
+ ·Ό= :N = =o:
+ :0
h
:0-
-Ä-
-O:
2
+ N = 0 :
-f-
2
:0-^Ν=:0:
+ :0=N
+ O:
:Ô=N
0:
FIG. 38. Valence bond configuration for NOJ.
14.3. M O L E C U L A R
CONSTANTS
A few molecular constants for N2O5 in the crystalline phase (NO\ NO 3 ) and the molecular form are included in Table 75.
TABLE 75. MOLECULAR CONSTANTS OF N2O5
Value
Property X-ray diffraction data a on N O J N O j Space group Lattice constants N2O5 structure b Terminal N - O Central N - O Dipole moment0
Dlh-C6/mmc a = 5.41 Â, c = 6.57 Â 1.2 Â 1.3-1.4 A 1.40/) N0 2 f
N-O bond length Raman spectrumd
1.154Â 1400 cm-1 538 cm » 2375 cm 1
NO3 1.24 1050 cm -J 824 cm-i 1413 cm-i 722 cm I
a
Ref. 580. Ref. 582. A. L. McClellan, Tables of Experimental Dipole Moments, W. H. Freeman, San Francisco (1963). d Ref. 581.
b
c
14.4. P H Y S I C A L P R O P E R T I E S
Some values for the physical constants of N2O5 are listed in Table 76, principally thermodynamic data584. Spectroscopic measurements have been recorded for N2O5 in both gaseous and solid phases and also in solutionssi. 584 D . D. Wagman, W. H. Evans, V. B. Barker, I. Halow, S. M. Bailey and R. H. Schumm, Selected Values of Chemical Thermodynamic Properties, NBS Technical Note 270-3, Washington (1968).
REACTIONS OF N 2 0 5
359
TABLE 76. PHYSICAL PROPERTIES* OF N 2 0 5 Temperature (K)
Property
Sublimation temperature Density (calculated) Thermodynamic functions5 N 2 0 5 (cryst.) AH (enthalpy of sublimation) S (entropy of sublimation) AH} (enthalpy of formation) Δ G° (Gibbs energy of formation) 5° (entropy) Cp (heat capacity) N2O5 (g) i\H°f (enthalpy of formation) &H°f (enthalpy of formation) H2n-Ho (enthalpy) Δ G} (Gibbs energy of formation) S° (entropy) C° (heat capacity) Vapour pressure equation
a
Ref. 575.
b
\
Value
305.6 78 213 288
32.4°C 2.175 g/cm3 2.14 g/cm3 2.05 g/cm3
305.6 305.6 298 298 298 298
13.6 kcal/mol 44.5 cal/deg mol - 1 0 . 3 kcal/mol 27.2 kcal/mol 42.6 cal/deg mol 34.2 cal/deg mol
0 5.7 kcal/mol 298 2.7 kcal/mol 298 4.237 kcal/mol 298 27.5 kcal/mol 298 85.0 cal/deg mol 298 20.2 cal/deg mol logioPmm = 1244/Γ+34.1 log 10 Γ-85.929 236.4 1 mm 10 mm I 256.5 40 mm 270.3 100 mm 280.6 400 mm 297.6 305.6 760 mm
Ref. 584. 14.5. REACTIONS
Although N2O5 is colourless, absorption occurs at ~ 3 8 0 0 Â in the ultraviolet region, and this has been assigned to photodecomposition according to the equation 5 8 1 N 2 05^N 2 04+0(3i>) The thermal decomposition of N2O5 has played an important rôle in the theory of reaction rates^s : 2N205-^2N204+02 The decomposition rate is first order: - N 0 2 + 0 2 + N O NO + N 2 0 5 ->3N0 2 585 R. A. Ogg, Jr., / . Chem. Phys. 15 (1947) 337.
360
NITROGEN: K. JONES
Labelled NO2 undergoes rapid exchange 586 with N2O5: "N2O5 + 1 5 N 0 2 ^ i 5 N 2 0 5 +
14
N02
and many gas phase reactions of N2O5 depend on initial dissociation to NO2 and NO3 with the latter then further acting as an oxidizing agent. Dinitrogen pentoxide is the formal anhydride of nitric acid and being deliquescent it is very readily hydrated: N 2 0 5 + H 2 0-*2HN0 3 With hydrogen peroxide, pernitric acid is also formed 5 8 7 : N 2 0 5 + H 2 0 2 -> HNO3+HNO4 In anhydrous solvents like nitric, perchloric, sulphuric and phosphoric acids, N2O5 dissociates providing a convenient source of nitronium ions and a suitable preparative route to nitronium salts 5 8 8 : N 2 0 5 + 3H2S04 -> 2NOJ + H 3 0 + + 3HSO4 N 2 0 5 + 3HC104 -> 2NOÎ + H 3 0 + + 3C104 2N 2 0 5 + H30+C104 -> NO£C10; + 3HN0 3 N 2 0 5 + 2S0 3 -> [NOJ]2S20?N 2 0 5 + FS03H -> N0JFS0I + HN0 3 The strong oxidizing action of N2O5 can cause violent reactions with reducing agents including metals and organic substances to give nitrates and/or oxides 575 : N 2 O s + Na -> NaN0 3 + N0 2 N 2 0 5 + NaF -^ NaN0 3 + FN0 2 N 2 0 5 + I 2 ->I 2 0 5 + N2
15. N I T R O G E N T R I O X I D E The existence of the nitrogen trioxide radical NO3 has for long been assumed judging by its frequent postulation in nitrogen oxide reaction mechanisms. For example, dinitrogen pentoxide undergoes thermal decomposition by a homogeneous first-order gas-phase reaction (see section 14). However, the reaction is not unimolecular as was initially thought, and one mechanism which is consistent with the data involves the preliminary equilibrium 589 followed by
N205^N02+N03 N0 3 + N0 2 -► N0 2 + 0 2 + NO N0 3 + N 0 ^ 2 N 0 2
586 A . R . Amell a n d F . Daniels, / . Am. Chem. Soc. 74 (1952) 6209. 587 R . Schwartz, Z . anorg. allgem. Chem. 256 (1947) 3. 588 D . R . G o d d a r d , E . D . H u g h e s a n d C . K . Ingold, / . Chem. Soc. (1950) 25«Q 589 R . A . Ogg, J r . , / . Chem. Phys. 5 (1937) 8 7 \