Dinitrogen pentoxide

Dinitrogen pentoxide (also known as nitrogen pentoxide or nitric anhydride) is the chemical compound with the formula N2O5. It is one of the binary nitrogen oxides, a family of compounds that only contain nitrogen and oxygen. It exists as colourless crystals that sublime 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dangerous oxidizer that once was used as a reagent when dissolved in chloroform for nitrations but has largely been superseded by nitronium tetrafluoroborate (NO2BF4).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 [NO2]+ and nitrate anions [NO3]-; 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 silver nitrate (AgNO3) with chlorine.

Structure and physical properties
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 group D$-0 cm^{3}&thinsp;mol^{−1}$ (C6/mmc) with Z = 2, with the NO3- anions in the D3h sites and the NO2+ cations in D3d sites.

The vapor pressure P (in atm) as a function of temperature T (in kelvin), in the range 211 to 305 K, is well approximated by the formula
 * $$ \ln P = 23.2348 - \frac{7098.2}{T}$$

being about 48 torr at 0 °C, 424 torr at 25 °C, and 760 torr at 32 °C (9 °C below the melting point).

In the gas phase, or when dissolved in nonpolar solvents such as carbon tetrachloride, the compound exists as covalently-bonded molecules O2N\sO\sNO2. In the gas phase, theoretical calculations for the minimum-energy configuration indicate that the O\sN\sO angle in each \sNO2 wing is about 134° and the N\sO\sN angle is about 112°. In that configuration, the two \sNO2 groups are rotated about 35° around the bonds to the central oxygen, away from the N\sO\sN plane. The molecule thus has a propeller shape, with one axis of 180° rotational symmetry (C2)

When gaseous N2O5 is cooled rapidly ("quenched"), one can obtain the metastable molecular form, which exothermically converts to the ionic form above −70 °C.

Gaseous N2O5 absorbs ultraviolet light with dissociation into the free radicals nitrogen dioxide NO2^{•} and nitrogen trioxide NO3^{•} (uncharged nitrate). The absorption spectrum has a broad band with maximum at wavelength 160 nm.

Preparation
A recommended laboratory synthesis entails dehydrating nitric acid (HNO3) with phosphorus(V) oxide:
 * P4O10 + 12 HNO3 → 4 H3PO4 + 6 N2O5

Another laboratory process is the reaction of lithium nitrate LiNO3 and bromine pentafluoride BrF5, in the ratio exceeding 3:1. The reaction first forms nitryl fluoride FNO2 that reacts further with the lithium nitrate:
 * BrF5 + 3 LiNO3 → 3 LiF + BrONO2 + O2 + 2 FNO2
 * FNO2 + LiNO3 → LiF + N2O5

The compound can also be created in the gas phase by reacting nitrogen dioxide NO2 or N2O4 with ozone:
 * 2 NO2 + O3 → N2O5 + O2

However, the product catalyzes the rapid decomposition of ozone:
 * 2 O3 + N2O5 → 3 O2 + N2O5

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 POCl3 or nitryl chloride NO2Cl with silver nitrate AgNO3

Reactions
Dinitrogen pentoxide reacts with water (hydrolyses) to produce nitric acid HNO3. Thus, dinitrogen pentoxide is the anhydride of nitric acid:
 * N2O5 + H2O → 2 HNO3

Solutions of dinitrogen pentoxide in nitric acid can be seen as nitric acid with more than 100% concentration. The phase diagram of the system H2O−N2O5 shows the well-known negative azeotrope at 60% N2O5 (that is, 70% HNO3), a positive azeotrope at 85.7% N2O5 (100% HNO3), and another negative one at 87.5% N2O5 ("102% HNO3").

The reaction with hydrogen chloride HCl also gives nitric acid and nitryl chloride NO2Cl:
 * N2O5 + HCl → HNO3 + NO2Cl

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 NH3 to give several products, including nitrous oxide N2O, ammonium nitrate NH4NO3, nitramide NH2NO2 and ammonium dinitramide NH4N(NO2)2, depending on reaction conditions.

Decomposition of dinitrogen pentoxide at high temperatures
Dinitrogen pentoxide between high temperatures of 600 and(-), is decomposed in two successive stoichiometric steps:
 * N2O5 → NO2 + NO3
 * 2 NO3 → 2 NO2 + O2

In the shock wave, N2O5 has decomposed stoichiometrically into nitrogen dioxide and oxygen. At temperatures of 600 K and higher, nitrogen dioxide is unstable with respect to nitrogen oxide and oxygen. The thermal decomposition of 0.1 mM nitrogen dioxide at 1000 K is known to require about two seconds.

Decomposition of dinitrogen pentoxide in carbon tetrachloride at 30 °C
Apart from the decomposition of N2O5 at high temperatures, it can also be decomposed in carbon tetrachloride CCl4 at 30 C. Both N2O5 and NO2 are soluble in CCl4 and remain in solution while oxygen is insoluble and escapes. The volume of the oxygen formed in the reaction can be measured in a gas burette. After this step we can proceed with the decomposition, measuring the quantity of O2 that is produced over time because the only form to obtain O2 is with the N2O5 decomposition. The equation below refers to the decomposition of N2O5 in CCl4:


 * 2 N2O5 → 4 NO2 + O2(g)

And this reaction follows the first order rate law that says:


 * $$-\frac{d[\mathrm{A}]}{dt} = k [\mathrm{A}]$$

Decomposition of nitrogen pentoxide in the presence of nitric oxide
N2O5 can also be decomposed in the presence of nitric oxide NO:


 * N2O5 + NO → 3 NO2

The rate of the initial reaction between dinitrogen pentoxide and nitric oxide of the elementary unimolecular decomposition.

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:


 * N2O5 + Ar\sH → HNO3 + Ar\sNO2

where Ar represents an arene moiety. The reactivity of the NO2+ can be further enhanced with strong acids that generate the "super-electrophile" HNO2(2+).

In this use, N2O5 has been largely replaced by nitronium tetrafluoroborate [NO2]+[BF4]-. This salt retains the high reactivity of NO2+, but it is thermally stable, decomposing at about 180 °C (into NO2F and BF3).

Dinitrogen pentoxide is relevant to the preparation of explosives.

Atmospheric occurrence
In the atmosphere, dinitrogen pentoxide is an important reservoir of the NO_{x} 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 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 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 NO_{x} concentrations. Two important reactions of N2O5 in atmospheric aerosols are hydrolysis to form nitric acid and 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.