Dinitrogen pentoxide

Dinitrogen pentoxide

Names
IUPAC name Dinitrogen pentoxide
Other names Nitric anhydride Nitronium nitrate Nitryl nitrate DNPO Anhydrous nitric acid
Identifiers
CAS Number	10102-03-1 Y
3D model (JSmol)	gas phase: Interactive image solid phase: Interactive image
ChEBI	CHEBI:29802 Y
ChemSpider	59627 Y
ECHA InfoCard	100.030.227
EC Number	233-264-2
PubChem CID	66242
UNII	6XB659ZX2W N
CompTox Dashboard (EPA)	DTXSID90143672
InChI InChI=1S/N2O5/c3-1(4)7-2(5)6 Y Key: ZWWCURLKEXEFQT-UHFFFAOYSA-N Y InChI=1/N2O5/c3-1(4)7-2(5)6 Key: ZWWCURLKEXEFQT-UHFFFAOYAN
SMILES gas phase: [O-][N+](=O)O[N+]([O-])=O solid phase: [O]=[N+]=[O].[N+](=O)([O-])[O-]
Properties
Chemical formula	N2O5
Molar mass	108.01 g/mol
Appearance	white solid
Density	2.0 g/cm3
Boiling point	33 °C (91 °F; 306 K) sublimes
Solubility in water	reacts to give HNO3
Solubility	soluble in chloroform negligible in CCl4
Magnetic susceptibility (χ)	−35.6×10−6 cm3 mol−1 (aq)
Dipole moment	1.39 D
Structure
Crystal structure	Hexagonal, hP14
Space group	P63/mmc No. 194
Lattice constant	a = 0.54019 nm, c = 0.65268 nm
Formula units (Z)	2
Molecular shape	planar, C2v (approx. D2h) N–O–N ≈ 180°
Thermochemistry
Heat capacity (C)	143.1 J K−1 mol−1 (s) 95.3 J K−1 mol−1 (g)
Std molar entropy (S⦵298)	178.2 J K−1 mol−1 (s) 355.7 J K−1 mol−1 (g)
Std enthalpy of formation (ΔfH⦵298)	−43.1 kJ/mol (s) +13.3 kJ/mol (g)
Gibbs free energy (ΔfG⦵)	113.9 kJ/mol (s) +117.1 kJ/mol (g)
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	strong oxidizer, forms strong acid in contact with water
NFPA 704 (fire diamond)	4 0 2 W OX
Flash point	Non-flammable
Related compounds
Related nitrogen oxides	Nitrous oxide Nitric oxide Dinitrogen trioxide Nitrogen dioxide Dinitrogen tetroxide
Related compounds	Nitric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Dinitrogen pentoxide (also known as nitrogen pentoxide or nitric anhydride) is the chemical compound with the formula N₂O₅. It is one of the binary nitrogen oxides, a family of compounds that contain only nitrogen and oxygen. It exists as colourless crystals that sublime slightly above room temperature, yielding a colorless gas.

Dinitrogen pentoxide is an unstable and potentially dangerous oxidizer that once was used as a reagent when dissolved in chloroform for nitrations but has largely been superseded by nitronium tetrafluoroborate (NO₂BF₄).

N₂O₅ is a rare example of a compound that adopts two structures depending on the conditions. The solid is a salt, nitronium nitrate, consisting of separate nitronium cations [NO₂]⁺ and nitrate anions [NO₃]⁻; but in the gas phase and under some other conditions it is a covalently-bound molecule.

History

N₂O₅ was first reported by Deville in 1840, who prepared it by treating silver nitrate (AgNO₃) with chlorine.

Structure and physical properties

Pure solid N₂O₅ is a salt, consisting of separated linear nitronium ions NO+2 and planar trigonal nitrate anions NO−3. Both nitrogen centers have oxidation state +5. It crystallizes in the space group D⁴
_6h (C6/mmc) with Z = 2, with the NO−3 anions in the D_3h sites and the NO+2 cations in D_3d sites.

The vapor pressure P (in atm) as a function of temperature T (in kelvin), in the range 211 to 305 K (−62 to 32 °C), is well approximated by the formula

$\ln P=23.2348-{\frac {7098.2}{T}}$

being about 48 torr at 0 °C, 424 torr at 25 °C, and 760 torr at 32 °C (9 °C below the melting point).

In the gas phase, or when dissolved in nonpolar solvents such as carbon tetrachloride, the compound exists as covalently-bonded molecules O₂N−O−NO₂. In the gas phase, theoretical calculations for the minimum-energy configuration indicate that the O−N−O angle in each −NO₂ wing is about 134° and the N−O−N angle is about 112°. In that configuration, the two −NO₂ groups are rotated about 35° around the bonds to the central oxygen, away from the N−O−N plane. The molecule thus has a propeller shape, with one axis of 180° rotational symmetry (C₂)

When gaseous N₂O₅ is cooled rapidly ("quenched"), one can obtain the metastable molecular form, which exothermically converts to the ionic form above −70 °C.

Gaseous N₂O₅ absorbs ultraviolet light with dissociation into the free radicals nitrogen dioxide NO₂^• and nitrogen trioxide NO₃^• (uncharged nitrate). The absorption spectrum has a broad band with maximum at wavelength 160 nm.

Preparation

A recommended laboratory synthesis entails dehydrating nitric acid (HNO₃) with phosphorus(V) oxide:

P₄O₁₀ + 12 HNO₃ → 4 H₃PO₄ + 6 N₂O₅

Another laboratory process is the reaction of lithium nitrate LiNO₃ and bromine pentafluoride BrF₅, in the ratio exceeding 3:1. The reaction first forms nitryl fluoride FNO₂ that reacts further with the lithium nitrate:

BrF₅ + 3 LiNO₃ → 3 LiF + BrONO₂ + O₂ + 2 FNO₂

FNO₂ + LiNO₃ → LiF + N₂O₅

The compound can also be created in the gas phase by reacting nitrogen dioxide NO₂ or N₂O₄ with ozone:

2 NO₂ + O₃ → N₂O₅ + O₂

However, the product catalyzes the rapid decomposition of ozone:

2 O₃ + N₂O₅ → 3 O₂ + N₂O₅

Dinitrogen pentoxide is also formed when a mixture of oxygen and nitrogen is passed through an electric discharge. Another route is the reactions of Phosphoryl chloride POCl₃ or nitryl chloride NO₂Cl with silver nitrate AgNO₃

Reactions

Dinitrogen pentoxide reacts with water (hydrolyses) to produce nitric acid HNO₃. Thus, dinitrogen pentoxide is the anhydride of nitric acid:

N₂O₅ + H₂O → 2 HNO₃

Solutions of dinitrogen pentoxide in nitric acid can be seen as nitric acid with more than 100% concentration. The phase diagram of the system H₂O−N₂O₅ shows the well-known negative azeotrope at 60% N₂O₅ (that is, 70% HNO₃), a positive azeotrope at 85.7% N₂O₅ (100% HNO₃), and another negative one at 87.5% N₂O₅ ("102% HNO₃").

The reaction with hydrogen chloride HCl also gives nitric acid and nitryl chloride NO₂Cl:

N₂O₅ + HCl → HNO₃ + NO₂Cl

Dinitrogen pentoxide eventually decomposes at room temperature into NO₂ and O₂. Decomposition is negligible if the solid is kept at 0 °C, in suitably inert containers.

Dinitrogen pentoxide reacts with ammonia NH₃ to give several products, including nitrous oxide N₂O, ammonium nitrate NH₄NO₃, nitramide NH₂NO₂ and ammonium dinitramide NH₄N(NO₂)₂, depending on reaction conditions.

Decomposition of dinitrogen pentoxide at high temperatures

Dinitrogen pentoxide between high temperatures of 600 and 1,100 K (327–827 °C), is decomposed in two successive stoichiometric steps:

N₂O₅ → NO₂ + NO₃

2 NO₃ → 2 NO₂ + O₂

In the shock wave, N₂O₅ has decomposed stoichiometrically into nitrogen dioxide and oxygen. At temperatures of 600 K and higher, nitrogen dioxide is unstable with respect to nitrogen oxide NO and oxygen. The thermal decomposition of 0.1 mM nitrogen dioxide at 1000 K is known to require about two seconds.

Decomposition of dinitrogen pentoxide in carbon tetrachloride at 30 °C

Apart from the decomposition of N₂O₅ at high temperatures, it can also be decomposed in carbon tetrachloride CCl₄ at 30 °C (303 K). Both N₂O₅ and NO₂ are soluble in CCl₄ and remain in solution while oxygen is insoluble and escapes. The volume of the oxygen formed in the reaction can be measured in a gas burette. After this step we can proceed with the decomposition, measuring the quantity of O₂ that is produced over time because the only form to obtain O₂ is with the N₂O₅ decomposition. The equation below refers to the decomposition of N₂O₅ in CCl₄:

2 N₂O₅ → 4 NO₂ + O₂(g)

And this reaction follows the first order rate law that says:

$-{\frac {d[\mathrm {A} ]}{dt}}=k[\mathrm {A} ]$

Decomposition of nitrogen pentoxide in the presence of nitric oxide

N₂O₅ can also be decomposed in the presence of nitric oxide NO:

N₂O₅ + NO → 3 NO₂

The rate of the initial reaction between dinitrogen pentoxide and nitric oxide of the elementary unimolecular decomposition.

Applications

Nitration of organic compounds

Dinitrogen pentoxide, for example as a solution in chloroform, has been used as a reagent to introduce the −NO₂ functionality in organic compounds. This nitration reaction is represented as follows:

N₂O₅ + Ar−H → HNO₃ + Ar−NO₂

where Ar represents an arene moiety. The reactivity of the NO+2 can be further enhanced with strong acids that generate the "super-electrophile" HNO2+2.

In this use, N₂O₅ has been largely replaced by nitronium tetrafluoroborate [NO₂]⁺[BF₄]⁻. This salt retains the high reactivity of NO+2, but it is thermally stable, decomposing at about 180 °C (into NO₂F and BF₃).

Dinitrogen pentoxide is relevant to the preparation of explosives.

Atmospheric occurrence

In the atmosphere, dinitrogen pentoxide is an important reservoir of the NO_x species that are responsible for ozone depletion: its formation provides a null cycle with which NO and NO₂ are temporarily held in an unreactive state. Mixing ratios of several parts per billion by volume have been observed in polluted regions of the nighttime troposphere. Dinitrogen pentoxide has also been observed in the stratosphere at similar levels, the reservoir formation having been postulated in considering the puzzling observations of a sudden drop in stratospheric NO₂ levels above 50 °N, the so-called 'Noxon cliff'.

Variations in N₂O₅ reactivity in aerosols can result in significant losses in tropospheric ozone, hydroxyl radicals, and NO_x concentrations. Two important reactions of N₂O₅ in atmospheric aerosols are hydrolysis to form nitric acid and reaction with halide ions, particularly Cl⁻, to form ClNO₂ molecules which may serve as precursors to reactive chlorine atoms in the atmosphere.

Hazards

N₂O₅ is a strong oxidizer that forms explosive mixtures with organic compounds and ammonium salts. The decomposition of dinitrogen pentoxide produces the highly toxic nitrogen dioxide gas.

Cited sources

• Haynes, William M., ed. (2016). CRC Handbook of Chemistry and Physics (97th ed.). CRC Press. ISBN 9781498754293.