Dinitrogen pentoxide

Template:Chembox Dinitrogen pentoxide (also known as nitrogen pentoxide or nitric anhydride) is the chemical compound with the formula Template:Chem2. 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dangerous oxidizer that once was used as a reagent when dissolved in chloroform for nitrations but has largely been superseded by nitronium tetrafluoroborate (Template:Chem2).

Template:Chem2 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 Template:Chem2 and nitrate anions Template:Chem2; but in the gas phase and under some other conditions it is a covalently-bound molecule.

History

Template:Chem2 was first reported by Deville in 1840, who prepared it by treating silver nitrate (Template:Chem2) with chlorine.

Structure and physical properties

Pure solid Template:Chem2 is a salt, consisting of separated linear nitronium ions Template:Chem2 and planar trigonal nitrate anions Template:Chem2. Both nitrogen centers have oxidation state +5. It crystallizes in the space group DTemplate:Su (C6/mmc) with Z = 2, with the Template:Chem2 anions in the D_3h sites and the Template:Chem2 cations in D_3d sites.

The vapor pressure P (in atm) as a function of temperature T (in kelvin), in the range Template:Cvt, 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 Template:Chem2. In the gas phase, theoretical calculations for the minimum-energy configuration indicate that the Template:Chem2 angle in each Template:Chem2 wing is about 134° and the Template:Chem2 angle is about 112°. In that configuration, the two Template:Chem2 groups are rotated about 35° around the bonds to the central oxygen, away from the Template:Chem2 plane. The molecule thus has a propeller shape, with one axis of 180° rotational symmetry (C₂)

When gaseous Template:Chem2 is cooled rapidly ("quenched"), one can obtain the metastable molecular form, which exothermically converts to the ionic form above −70 °C.

Gaseous Template:Chem2 absorbs ultraviolet light with dissociation into the free radicals nitrogen dioxide Template:Chem2 and nitrogen trioxide Template:Chem2 (uncharged nitrate). The absorption spectrum has a broad band with maximum at wavelength 160 nm.

Preparation

A recommended laboratory synthesis entails dehydrating nitric acid (Template:Chem2) with phosphorus(V) oxide:

Template:Chem2

Another laboratory process is the reaction of lithium nitrate Template:Chem2 and bromine pentafluoride Template:Chem2, in the ratio exceeding 3:1. The reaction first forms nitryl fluoride Template:Chem2 that reacts further with the lithium nitrate:

Template:Chem2

Template:Chem2

The compound can also be created in the gas phase by reacting nitrogen dioxide Template:Chem2 or Template:Chem2 with ozone:

Template:Chem2

However, the product catalyzes the rapid decomposition of ozone:

Template:Chem2

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 Template:Chem2 or nitryl chloride Template:Chem2 with silver nitrate Template:Chem2

Reactions

Dinitrogen pentoxide reacts with water (hydrolyses) to produce nitric acid Template:Chem2. Thus, dinitrogen pentoxide is the anhydride of nitric acid:

Template:Chem2

Solutions of dinitrogen pentoxide in nitric acid can be seen as nitric acid with more than 100% concentration. The phase diagram of the system Template:Chem2−Template:Chem2 shows the well-known negative azeotrope at 60% Template:Chem2 (that is, 70% Template:Chem2), a positive azeotrope at 85.7% Template:Chem2 (100% Template:Chem2), and another negative one at 87.5% Template:Chem2 ("102% Template:Chem2").

The reaction with hydrogen chloride Template:Chem2 also gives nitric acid and nitryl chloride Template:Chem2:

Template:Chem2

Dinitrogen pentoxide eventually decomposes at room temperature into [[nitrogen dioxide|Template:Chem2]] and [[oxygen|Template:Chem2]]. Decomposition is negligible if the solid is kept at 0 °C, in suitably inert containers.

Dinitrogen pentoxide reacts with ammonia Template:Chem2 to give several products, including nitrous oxide Template:Chem2, ammonium nitrate Template:Chem2, nitramide Template:Chem2 and ammonium dinitramide Template:Chem2, depending on reaction conditions.

Decomposition of dinitrogen pentoxide at high temperatures

Dinitrogen pentoxide between high temperatures of Template:Cvt, is decomposed in two successive stoichiometric steps:

Template:Chem2

Template:Chem2

In the shock wave, Template:Chem2 has decomposed stoichiometrically into nitrogen dioxide and oxygen. At temperatures of 600 K and higher, nitrogen dioxide is unstable with respect to nitrogen oxide Template:Chem 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 Template:Chem2 at high temperatures, it can also be decomposed in carbon tetrachloride Template:Chem2 at Template:Cvt. Both Template:Chem2 and Template:Chem2 are soluble in Template:Chem2 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 Template:Chem2 that is produced over time because the only form to obtain Template:Chem2 is with the Template:Chem2 decomposition. The equation below refers to the decomposition of Template:Chem2 in Template:Chem2:

Template:Chem2

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

Template:Chem2 can also be decomposed in the presence of nitric oxide Template:Chem2:

Template:Chem2

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 [[Nitro compound|Template:Chem2]] functionality in organic compounds. This nitration reaction is represented as follows:

Template:Chem2

where Ar represents an arene moiety. The reactivity of the Template:Chem2 can be further enhanced with strong acids that generate the "super-electrophile" Template:Chem2.

In this use, Template:Chem2 has been largely replaced by nitronium tetrafluoroborate Template:Chem2. This salt retains the high reactivity of Template:Chem2, but it is thermally stable, decomposing at about 180 °C (into [[nitryl fluoride|Template:Chem2]] and [[boron trifluoride|Template:Chem2]]).

Dinitrogen pentoxide is relevant to the preparation of explosives.

Atmospheric occurrence

In the atmosphere, dinitrogen pentoxide is an important reservoir of the Template:Chem2 species that are responsible for ozone depletion: its formation provides a null cycle with which Template:Chem2 and Template:Chem2 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 Template:Chem2 levels above 50 °N, the so-called 'Noxon cliff'.

Variations in Template:Chem2 reactivity in aerosols can result in significant losses in tropospheric ozone, hydroxyl radicals, and Template:Chem2 concentrations. Two important reactions of Template:Chem2 in atmospheric aerosols are hydrolysis to form nitric acid and reaction with halide ions, particularly [[chloride|Template:Chem2]], to form [[ClNO2|Template:Chem2]] molecules which may serve as precursors to reactive chlorine atoms in the atmosphere.

Hazards

Template:Chem2 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.

Cited sources

• Template:Cite book