Dinitrogen pentoxide is the chemical compound with the formula N2O5, also known as nitrogen pentoxide or nitric anhydride. It is one of the binary nitrogen oxides, a family of compounds that only contain nitrogen and oxygen. It exists as colourless crystals that melt at 41 °C. Its boiling point is 47 °C, and sublimes slightly above room temperature, yielding a colorless gas. Dinitrogen pentoxide 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dangerous oxidizer that once was used as a reagent when dissolved in chloroform for nitrations but has largely been superseded by NO2BF4. N2O5 is a rare example of a compound that adopts two structures depending on the conditions. The solid is a salt, nitronium nitrate, consisting of separate nitronium cations+ and nitrate anions−; but in the gas phase and under some other conditions it is a covalently bound molecule.
History
N2O5 was first reported by Deville in 1840, who prepared it by treating AgNO3 with Cl2.
Pure solid N2O5 is a salt, consisting of separated linear nitronium ions NO2+ and planar trigonal nitrate anions NO3−. Both nitrogen centers have oxidation state +5. It crystallizes in the space groupD46h with Z = 2, with the anions in the D3h sites and the cations in D3d sites. The vapor pressureP as a function of temperature T, in the range 211 to 305 K, is well approximated by the formula being about 48 torr at 0 °C, 424 torr at 25 °C, and 760 torr at 32 °C. In the gas phase, or when dissolved in a nonpolar solvents such as CCl4, the compound exists as covalently bound molecules O2N–O–NO2. In the gas phase, theoretical calculations for the minimum-energy configuration indicate that the O–N–O angle in each wing is about 134° and the N–O–N angle is about 112°. In that configuration, the two groups are rotated about 35° around the bonds to the central oxygen, away from the N–O–N plane. The molecule thus has a propeller shape, with one axis of 180° rotational symmetry When gaseous is cooled rapidly, one can obtain the metastablemolecular form, which exothermically converts to the ionic form above −70 °C. Gaseous absorbs ultraviolet light with dissociation into the radicals nitrogen dioxide and nitrogen trioxide . The absorption spectrum has a broad band with maximum at wavelength 160 nm.
Synthesis
A recommended laboratory synthesis entails dehydrating nitric acid with phosphorus oxide: Another laboratory process is the reaction of lithium nitrate and bromine pentafluoride, in the ratio exceeding 3:1. The reaction first forms nitryl flouride that reacts further with the lithium nitrate: The compound can also be created in the gas phase by reacting nitrogen dioxide or with ozone: However, the product catalyzes the rapid decomposition of ozone: Dinitrogen pentoxide is also formed when a mixture of oxygen and nitrogen is passed through an electric discharge. Another route is the reactions of or with
Reactions
Dinitrogen pentoxide reacts with water to produce nitric acid. Thus, dinitrogen pentoxide is the anhydride of nitric acid: Solutions of dinitrogen pentoxide in nitric acid can be seen as nitric acid with more than 100% concentration. The phase diagram of the system − shows the well-known negative azeotrope at 60% , a positive azeotrope at 85.7% , and another negative one at 87.5% . The reaction with hydrogen chloride also gives nitric acid and nitrosym chloride: Dinitrogen pentoxide eventually decomposes at room temperature into NO2 and O2. Decomposition is negligible if the solid is kept at 0 °C, in suitably inert containers. Dinitrogen pentoxide reacts with ammonia to give several products, including nitrous oxide, ammonium nitrate, nitramide and ammonium dinitramide, depending on reaction condiitons.
Dinitrogen pentoxide, for example as a solution in chloroform, has been used as a reagent to introduce the NO2 functionality in organic compounds. This nitration reaction is represented as follows: where Ar represents an arene moiety. The reactivity of the NO2+ can be further enhanced with strong acids that generate the "super-electrophile" HNO22+. In this use, has been largely replaced by nitronium tetrafluoroborate+−. This salt retains the high reactivity of NO2+, but it is thermally stable, decomposing at about 180 °C . Dinitrogen pentoxide is relevant to the preparation of explosives.
Atmospheric occurrence
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. Mixing ratios of several ppbv have been observed in polluted regions of the night-time troposphere. Dinitrogen pentoxide has also been observed in the stratosphere at similar levels, the reservoir formation having been postulated in considering the puzzling observations of a sudden drop in stratospheric NO2 levels above 50 °N, the so-called 'Noxon cliff'. Variations in N2O5 reactivity in aerosols can result in significant losses in tropospheric ozone, hydroxyl radicals, and NOx concentrations. Two important reactions of N2O5 in atmospheric aerosols are: 1) Hydrolysis to form nitric acid and 2) Reaction with halide ions, particularly Cl−, to form ClNO2 molecules which may serve as precursors to reactive chlorine atoms in the atmosphere.
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.
