Dinitrogen pentoxide
IUPAC name	Dinitrogen pentaoxide
Other names	Nitric anhydride dnpo
Identifiers
CAS number	10102-03-1 Y
PubChem	66242
Properties
Molecular formula	N2O5
Molar mass	108.01 g/mol
Appearance	white solid
Density	1.642 g/cm3 (18 °C)
Melting point	30
Boiling point	47 °C subl.
Solubility in water	reacts to give HNO3
Solubility	soluble in chloroform
Structure
Molecular shape	planar, C2v (approx. D2h) N–O–N ≈ 180º
Thermochemistry
Std enthalpy of formation ΔfHo298	−43.1 kJ/mol (s) +11.3 kJ/mol (g)
Standard molar entropy So298	178.2 J K−1 mol−1 (s) 355.6 J K−1 mol−1 (g)
Hazards
EU Index	Not listed
Main hazards	strong oxidizer, forms strong acid in contact with water
NFPA 704	0 3 0 OX
Flash point	Non-flammable
Related compounds
Related nitrogen oxides	Nitrous oxide Nitric oxide Dinitrogen trioxide Nitrogen dioxide Dinitrogen tetroxide
Related compounds	Nitric acid
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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is an unstable and potentially dangerous oxidizer that once was used as a reagent for nitrations but has largely been superseded by NO₂BF₄ (nitronium tetrafluoroborate).

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 nonpolar molecule:

N₂O₅ ⇌ [NO₂⁺][NO₃⁻]

Contents 1 Syntheses and properties 2 Structure 3 Reactions and applications 4 NO2BF4 5 Hazards 6 References

[edit] Syntheses and properties

N₂O₅ was first reported by Deville in 1840, who prepared it by treating AgNO₃ with Cl₂.^[1] A recommended laboratory synthesis entails dehydrating nitric acid (HNO₃) with phosphorus(V) oxide:^[2]

P₄O₁₀ + 12 HNO₃ → 4 H₃PO₄ + 6 N₂O₅

In the reverse process, N₂O₅ reacts with water (hydrolyses) to produce nitric acid. Thus, nitrogen pentoxide is the anhydride of nitric acid:

N₂O₅ + H₂O → 2 HNO₃

N₂O₅ exists as colourless crystals that sublime slightly above room temperature. The salt eventually decomposes at room temperature into NO₂ and O₂. ^[3]

[edit] 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 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.^[2]

[edit] 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–NO₂

where Ar represents an arene moiety.

N₂O₅ is of interest for the preparation of explosives.^[4]

[edit] NO₂BF₄

Replacement of the NO₃⁻ portion of N₂O₅ with BF₄⁻ gives NO₂BF₄ (CAS#13826-86-3). This salt retains the high reactivity of NO₂⁺, but it is thermally stable, decomposing at ca. 180°C (into NO₂F and BF₃). NO₂BF₄ has been used to nitrate a variety of organic compounds, especially arenes and heterocycles. Interestingly, the reactivity of the NO₂⁺ can be further enhanced with strong acids that generate the "super-electrophile" HNO₂²⁺.

[edit] 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.

[edit] References