Dinitrogen pentoxide

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is an unstable and potentially dangerous oxidizer that once was used as a reagent for nitrations but has largely been superseded by NO2BF4.

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−]

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  &rarr;  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  &rarr;  2 HNO3

N2O5 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 planar NO3&minus; ions. Thus, the solid could be called nitronium nitrate. Both nitrogen centers have oxidation states V.

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.
 * N2O5 Lewis Structure:
 * [[Image:n2o5.gif|Structure of the (rare) molecular form of dinitrogen pentoxide]]

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  &rarr;  HNO3  +  Ar-NO2

N2O5 is of interest for the preparation of explosives.

NO2BF4
Replacement of the NO3&minus; portion of N2O5 with BF4&minus; 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+.

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.