# Dinitrogen pentoxide

Dinitrogen pentoxide (also known as nitrogen pentoxide or nitric anhydride) is the chemical compound with the formula N$_2$O$_5$. 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 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dangerous oxidizer that once was used as a reagent when dissolved in chloroform for nitrations but has largely been superseded by nitronium tetrafluoroborate (NO$_2$BF$_4$).
N$_2$O$_5$ 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 [NO$_2$]$^+$ and nitrate anions [NO$_3$]$^−$; but in the gas phase and under some other conditions it is a covalently-bound molecule.

## History

N$_2$O$_5$ was first reported by Deville in 1840, who prepared it by treating silver nitrate (AgNO$_3$) with chlorine.

## Structure and physical properties

Pure solid N$_2$O$_5$ is a salt, consisting of separated linear nitronium ions NO+2 and planar trigonal nitrate anions NO−3. Both nitrogen centers have oxidation state +5. It crystallizes in the space group D$^4$ $_{6h}$ (C6/mmc) with Z = 2, with the NO−3 anions in the D$_{3h}$ sites and the NO+2 cations in D$_{3d}$ sites.
The vapor pressure P (in atm) as a function of temperature T (in kelvin), in the range 211 to 305 K (−62 to 32 °C), 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 O$_2$N−O−NO$_2$. In the gas phase, theoretical calculations for the minimum-energy configuration indicate that the O−N−O angle in each −NO$_2$ wing is about 134° and the N−O−N angle is about 112°. In that configuration, the two −NO$_2$ 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 (C$_2$)
When gaseous N$_2$O$_5$ is cooled rapidly ("quenched"), one can obtain the metastable molecular form, which exothermically converts to the ionic form above −70 °C.
Gaseous N$_2$O$_5$ absorbs ultraviolet light with dissociation into the free radicals nitrogen dioxide NO$_2$$^•$ and nitrogen trioxide NO$_3$$^•$ (uncharged nitrate). The absorption spectrum has a broad band with maximum at wavelength 160 nm.

## Preparation

A recommended laboratory synthesis entails dehydrating nitric acid (HNO$_3$) with phosphorus(V) oxide:
    P$_4$O$_{10}$ + 12 HNO$_3$ → 4 H$_3$PO$_4$ + 6 N$_2$O$_5$
Another laboratory process is the reaction of lithium nitrate LiNO$_3$ and bromine pentafluoride BrF$_5$, in the ratio exceeding 3:1. The reaction first forms nitryl fluoride FNO$_2$ that reacts further with the lithium nitrate:
    BrF$_5$ + 3 LiNO$_3$ → 3 LiF + BrONO$_2$ + O$_2$ + 2 FNO$_2$
    FNO$_2$ + LiNO$_3$ → LiF + N$_2$O$_5$
The compound can also be created in the gas phase by reacting nitrogen dioxide NO$_2$ or N$_2$O$_4$ with ozone:
    2 NO$_2$ + O$_3$ → N$_2$O$_5$ + O$_2$
However, the product catalyzes the rapid decomposition of ozone:
    2 O$_3$ + N$_2$O$_5$ → 3 O$_2$ + N$_2$O$_5$
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 POCl$_3$ or nitryl chloride NO$_2$Cl with silver nitrate AgNO$_3$

## Reactions

Dinitrogen pentoxide reacts with water (hydrolyses) to produce nitric acid HNO$_3$. Thus, dinitrogen pentoxide is the anhydride of nitric acid:
    N$_2$O$_5$ + H$_2$O → 2 HNO$_3$
Solutions of dinitrogen pentoxide in nitric acid can be seen as nitric acid with more than 100% concentration. The phase diagram of the system H$_2$O−N$_2$O$_5$ shows the well-known negative azeotrope at 60% N$_2$O$_5$ (that is, 70% HNO$_3$), a positive azeotrope at 85.7% N$_2$O$_5$ (100% HNO$_3$), and another negative one at 87.5% N$_2$O$_5$ ("102% HNO$_3$").
The reaction with hydrogen chloride HCl also gives nitric acid and nitryl chloride NO$_2$Cl:
    N$_2$O$_5$ + HCl → HNO$_3$ + NO$_2$Cl
Dinitrogen pentoxide eventually decomposes at room temperature into NO$_2$ and O$_2$. Decomposition is negligible if the solid is kept at 0 °C, in suitably inert containers.
Dinitrogen pentoxide reacts with ammonia NH$_3$ to give several products, including nitrous oxide N$_2$O, ammonium nitrate NH$_4$NO$_3$, nitramide NH$_2$NO$_2$ and ammonium dinitramide NH$_4$N(NO$_2$)$_2$, depending on reaction conditions.

### Decomposition of dinitrogen pentoxide at high temperatures

Dinitrogen pentoxide between high temperatures of 600 and 1,100 K (327–827 °C), is decomposed in two successive stoichiometric steps:
    N$_2$O$_5$ → NO$_2$ + NO$_3$
    2 NO$_3$ → 2 NO$_2$ + O$_2$
In the shock wave, N$_2$O$_5$ has decomposed stoichiometrically into nitrogen dioxide and oxygen. At temperatures of 600 K and higher, nitrogen dioxide is unstable with respect to nitrogen oxide NO 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 N$_2$O$_5$ at high temperatures, it can also be decomposed in carbon tetrachloride CCl$_4$ at 30 °C (303 K). Both N$_2$O$_5$ and NO$_2$ are soluble in CCl$_4$ 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 O$_2$ that is produced over time because the only form to obtain O$_2$ is with the N$_2$O$_5$ decomposition. The equation below refers to the decomposition of N$_2$O$_5$ in CCl$_4$:
    2 N$_2$O$_5$ → 4 NO$_2$ + O$_2$(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

N$_2$O$_5$ can also be decomposed in the presence of nitric oxide NO:
    N$_2$O$_5$ + NO → 3 NO$_2$
The rate of the initial reaction between dinitrogen pentoxide and nitric oxide of the elementary unimolecular decomposition.

## Applications
### Nitration of organic compounds

Dinitrogen pentoxide, for example as a solution in chloroform, has been used as a reagent to introduce the −NO$_2$ functionality in organic compounds. This nitration reaction is represented as follows:
    N$_2$O$_5$ + Ar−H → HNO$_3$ + Ar−NO$_2$
where Ar represents an arene moiety. The reactivity of the NO+2 can be further enhanced with strong acids that generate the "super-electrophile" HNO2+2.
In this use, N$_2$O$_5$ has been largely replaced by nitronium tetrafluoroborate [NO$_2$]$^+$[BF$_4$]$^−$. This salt retains the high reactivity of NO+2, but it is thermally stable, decomposing at about 180 °C (into NO$_2$F and BF$_3$).
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 NO$_2$ 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 NO$_2$ levels above 50 °N, the so-called 'Noxon cliff'.
Variations in N$_2$O$_5$ reactivity in aerosols can result in significant losses in tropospheric ozone, hydroxyl radicals, and NO$_x$ concentrations. Two important reactions of N$_2$O$_5$ in atmospheric aerosols are hydrolysis to form nitric acid and reaction with halide ions, particularly Cl$^−$, to form ClNO$_2$ molecules which may serve as precursors to reactive chlorine atoms in the atmosphere.

## Hazards

N$_2$O$_5$ 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.