Dinitrogen pentoxide (also known as nitrogen pentoxide or nitric anhydride) is the chemical compound with the formula . It is one of the binary nitrogen oxides, a family of compounds that contain only nitrogen and oxygen. It exists as colourless crystals that sublime slightly above room temperature, yielding a colorless gas.

Dinitrogen pentoxide has been used for nitrations but has largely been superseded by nitronium tetrafluoroborate ().

Structure

Solid is a salt, nitronium nitrate, consisting of linear nitronium cations () and planar trigonal nitrate anions (). The N-O distances in the cation are and for the anion. Gaseous is a covalently-bound molecule. It crystallizes in the space group D (C6/mmc) with .

In the gas phase or when dissolved in nonpolar solvents such as carbon tetrachloride, the compound exists as covalently-bonded molecules . In the gas phase, theoretical calculations for the minimum-energy configuration indicate that the angle in each wing is about 134° and the angle is about 112°. In that configuration, the two groups are rotated about 35° around the bonds to the central oxygen, away from the plane. The molecule thus has a propeller shape, with one axis of 180° rotational symmetry (C).

Physical properties

The vapor pressure (in atm) as a function of temperature (in kelvin), in the range , is well approximated by the formula

being about at , at , and at , ( below the melting point).

When gaseous is cooled rapidly ("quenched"), one can obtain the metastable molecular form, which exothermically converts to the ionic form above .

Gaseous absorbs ultraviolet light with dissociation into the free radicals nitrogen dioxide and nitrogen trioxide (uncharged nitrate). The absorption spectrum has a broad band with maximum at wavelength .

Preparation

was first reported by the French chemist Henri Deville in 1840, who prepared it by treating silver nitrate () with chlorine.

A recommended laboratory synthesis entails dehydrating nitric acid () with phosphorus(V) oxide ():

Another laboratory process is the reaction of lithium nitrate () and bromine pentafluoride (), in the ratio exceeding 3:1. The reaction first forms nitryl fluoride () 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 phosphoryl chloride () or nitryl chloride () with silver nitrate ().

Reactions

Dinitrogen pentoxide reacts with water (hydrolyses) to produce nitric acid (). Thus, dinitrogen pentoxide is an acidic oxide, 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% (that is, 70% ), a positive azeotrope at 85.7% (100% ), and another negative one at 87.5% ("102% ").

The reaction with hydrogen chloride () also gives nitric acid and nitryl chloride ():

Dinitrogen pentoxide reacts with ammonia () to give several products, including nitrous oxide (), ammonium nitrate (), nitramide (), and ammonium dinitramide (), depending on reaction conditions.

Thermal decomposition

Decomposition is negligible if the solid is kept at . It decomposes at room temperature into nitrogen dioxide () and O2:

Its solutions in carbon tetrachloride () decompose at . Both and are soluble in and remain in solution while oxygen is insoluble and escapes. The volume of the oxygen allows determination of the reaction kinetics:

In the presence of nitric oxide (NO), the decomposition proceeds as follows:

Applications

Nitration of organic compounds

Dinitrogen pentoxide, for example as a solution in chloroform (), has been used as a reagent to introduce the \sNO2 functionality in organic compounds. This nitration reaction is represented as follows:

Where Ar represents an arene moiety. The reactivity of the can be further enhanced with strong acids that generate the superelectrophile .

In this use, has been largely replaced by nitronium tetrafluoroborate (). This salt retains the high reactivity of , but it is thermally stable, decomposing at about (into nitryl fluoride () and boron trifluoride ()).

Dinitrogen pentoxide is relevant to the preparation of explosives.

Atmospheric occurrence

In Earth's atmosphere, dinitrogen pentoxide is an important reservoir of the species that are responsible for ozone depletion: its formation provides a null cycle with which and are temporarily held in an unreactive state. Mixing ratios of several parts per billion by volume have been observed in polluted regions of the nighttime 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 levels above 50° N, the so-called 'Noxon cliff'.

Variations in reactivity in aerosols can result in significant losses in tropospheric ozone, hydroxyl radicals, and concentrations. Two important reactions of in atmospheric aerosols are hydrolysis to form nitric acid and reaction with halide ions, particularly Cl-, to form nitryl chloride () molecules which may serve as precursors to reactive chlorine atoms in the atmosphere.

Hazards

is a strong oxidizer that forms explosive mixtures with organic compounds and ammonium salts. Dinitrogen pentoxide can degrade to the highly toxic nitrogen dioxide gas.

References