Dinitrogen pentoxide
Chemical compound
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Through Chemical formula, Nitrogen and Oxygen
Dinitrogen pentoxide has been used for nitrations but has largely been superseded by nitronium tetrafluoroborate (NO2BF4).
Structure
Solid N2O5 is a salt, nitronium nitrate, consisting of linear nitronium cations ([NO2]+) and planar trigonal nitrate anions ([NO3]−). The N-O distances in the cation are 115 pm (1.15 Å) and 124 pm (1.24 Å) for the anion. Gaseous N2O5 is a covalently-bound molecule. It crystallizes in the space group D4
6h (C6/mmc) with Z = 2.
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).
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Physical properties
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; −80 to 89 °F), is well approximated by the formula
- ln P = 23.2348 - 7098.2/T
being about 48 Torr (6.4 kPa) at 0 °C (32 °F), 424 Torr (56.5 kPa) at 25 °C (77 °F), and 760 Torr (101 kPa) at 32 °C (90 °F), (9 °C (48 °F) below the melting point).
When gaseous N2O5 is cooled rapidly ("quenched"), one can obtain the metastable molecular form, which exothermically converts to the ionic form above −70 °C (−94 °F).
Gaseous N2O5 absorbs ultraviolet light with dissociation into the free radicals nitrogen dioxide NO•
2 and nitrogen trioxide NO•
3 (uncharged nitrate). The absorption spectrum has a broad band with maximum at wavelength 160 nm.
Preparation
N2O5 was first reported by the French chemist Henri Deville in 1840, who prepared it by treating silver nitrate (AgNO3) with chlorine.
A recommended laboratory synthesis entails dehydrating nitric acid (HNO3) with phosphorus(V) oxide (P4O10):
- 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 an acidic oxide, 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 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.
Thermal decomposition
Decomposition is negligible if the solid is kept at 0 °C (32 °F). It decomposes at room temperature into nitrogen dioxide (NO2) and O2:
- N2O5 → 2 NO2 + 0.5 O2
Its solutions in carbon tetrachloride (CCl4) decompose at 30 °C (86 °F). Both N2O5 and NO2 are soluble in CCl4 and remain in solution while oxygen is insoluble and escapes. The volume of the oxygen allows determination of the reaction kinetics:
- 2 N2O5 → 4 NO2 + O2(g)
In the presence of nitric oxide (NO), the decomposition proceeds as follows:
- N2O5 + NO → 3 NO2
Applications
Nitration of organic compounds
Dinitrogen pentoxide, for example as a solution in chloroform (CHCl3), 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 superelectrophile 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 (356 °F) (into nitryl fluoride (NO2F) and boron trifluoride (BF3)).
Dinitrogen pentoxide is relevant to the preparation of explosives.
Atmospheric occurrence
In Earth's atmosphere, dinitrogen pentoxide is an important reservoir of the NOx 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 NOx concentrations. Two important reactions of N2O5 in atmospheric aerosols are hydrolysis to form nitric acid and reaction with halide ions, particularly Cl−, to form nitryl chloride (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. Dinitrogen pentoxide can degrade to the highly toxic nitrogen dioxide gas.