Related compounds IUPAC ID Dinitrogen pentoxide Melting point 41 °C Boiling point 47 °C | Formula N2O5 Molar mass 108.01 g/mol Density 1.64 g/cm³ Appearance white solid | |
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Dinitrogen pentoxide preparation properties and uses
Dinitrogen pentoxide is the chemical compound with the formula N2O5. Also known as nitrogen pentoxide, N2O5 is one of the binary nitrogen oxides, a family of compounds that only contain nitrogen and oxygen. It 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dangerous oxidizer that once was used as a reagent when dissolved in chloroform for nitrations but has largely been superseded by NO2BF4 (nitronium tetrafluoroborate).
Contents
- Dinitrogen pentoxide preparation properties and uses
- Dinitrogen pentoxide meaning
- Syntheses and properties
- Structure
- Reactions and applications
- Hazards
- References
N2O5 is a rare example of a compound that adopts two structures depending on the conditions: most commonly it is a salt, but under some conditions it is a polar molecule:
[NO2+][NO3−] ⇌ N2O5Dinitrogen pentoxide meaning
Syntheses and properties
N2O5 was first reported by Deville in 1840, who prepared it by treating AgNO3 with Cl2. A recommended laboratory synthesis entails dehydrating nitric acid (HNO3) with phosphorus(V) oxide:
P4O10 + 12 HNO3 → 4 H3PO4 + 6 N2O5In the reverse process, N2O5 reacts with water (hydrolyses) to produce nitric acid. Thus, dinitrogen pentoxide is the anhydride of nitric acid:
N2O5 + H2O → 2 HNO3N2O5 exists as colourless crystals that sublime slightly above room temperature. The salt eventually decomposes at room temperature into NO2 and O2.
Structure
Solid N2O5 is a salt, consisting of separated anions and cations. The cation is the linear nitronium ion NO2+ and the anion is the planar nitrate NO3− ion. Thus, the solid could be called nitronium nitrate. Both nitrogen centers have oxidation state +5.
The intact molecule O2N–O–NO2 exists in the gas phase (obtained by subliming N2O5) and when the solid is extracted into nonpolar solvents such as CCl4. In the gas phase, the O–N–O angle is 133° and the N–O–N angle is 114°. When gaseous N2O5 is cooled rapidly ("quenched"), one can obtain the metastable molecular form, which exothermically converts to the ionic form above −70 °C.
Reactions and applications
Dinitrogen pentoxide, for example as a solution in chloroform, has been used as a reagent to introduce the NO2 functionality. This nitration reaction is represented as follows:
N2O5 + Ar–H → HNO3 + Ar–NO2where Ar represents an arene moiety.
For this use, dinitrogen pentoxide has been largely replaced by nitronium tetrafluoroborate [NO
2]+[BF
4]−. This salt retains the high reactivity of NO2+, but it is thermally stable, decomposing at about 180 °C (into NO2F and BF3). The reactivity of the NO2+ can be further enhanced with strong acids that generate the "super-electrophile" HNO22+.
Dinitrogen pentoxide is relevant to the preparation of explosives.
In the 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.
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.