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is an unstable and potentially dangerous oxidizer that once was used as a reagent for nitrations but has largely been superseded by NO₂BF₄ (nitronium tetrafluoroborate).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

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 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.

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⁻ 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:

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

N₂O₅ is of interest for the preparation of explosives.

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⁺.

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.

References