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Dinitrogen pentoxide is the binary nitrogen oxide N2O5, also known as nitrogen pentoxide. This unstable and potentially dangerous oxidizer was once of interest as a reagent for nitrations but it has largely been superseded by NO2BF4, which is more stable and effects the same reactions. N2O5 exists as colorless crystals that sublime at 32.4 °C. The salt decomposes at room temperature into NO2 and O2.
Syntheses N2O5 was first reported by Deville in 1840, who prepared this species by treating AgNO3 with Cl2. The recommended laboratory synthesis entails the dehydration of nitric acid (HNO3) with phosphorus(V) oxide: P4O10 + 12 HNO3 4 H3PO4 + 6 N2O5 Structure The structure of N2O5 in the solid state reveals separated, linear NO2+ and planar NO3− ions. Thus, the solid could be called nitronium nitrate. Both nitrogen centers have oxidation states V. The intact molecule O2N-O-NO2 does exist in the gas phase (obtained by subliming N2O5) and when the solid is extracted into the nonpolar solvent CCl4. In the gas phase, the O-N-O and N-O-N angles are 133° and 114°, respectively. 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 reacts with water to produce nitric acid (HNO3), as such nitrogen pentoxide is the anhydride of nitric acid. Dinitrogen pentoxide sometimes used as a source of the NO2 functionality, often using it as a solution in chloroform: N2O5 + R-H HNO3 + R-NO2 F− has been reported as a proton acceptor to neutralize the HNO3 coproduct. N2O5 is of interest for the preparation of explosives. Chemistry of NO The reactive portion of N2O5 is the cation NO2+. NO3− is unreactive kinetically. Thus, when the NO3− portion of N2O5 is replaced with BF4−, the high reactivity of NO2+ is retained. The great advantage of NO2BF4 is that it is stable thermally (up to 180 °C where it decomposes to 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 and could form explosive mixtures with organic compounds and ammonium salts. The decomposition of dinitrogen pentoxide produces the highly toxic nitrogen dioxide, signaled by the appearance of its characteristic reddish-brown color. | |||||||||||||||||||||
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