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.<ref name=conn1979>Connell, Peter Steele. (1979) The Photochemistry of Dinitrogen Pentoxide. Ph. D. thesis, Lawrence Berkeley National Laboratory.</ref>
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dangerous oxidizer that once was used as a reagent when dissolved in chloroform for nitrations but has largely been superseded by nitronium tetrafluoroborate (Template:Chem2).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.<ref name=angus1949>Template:Cite journal</ref>
HistoryEdit
Template:Chem2 was first reported by the French chemist Henri Deville in 1840, who prepared it by treating silver nitrate (Template:Chem2) with chlorine.<ref>Template:Cite journal</ref><ref name=b1>Template:Cite book</ref>
Structure and physical propertiesEdit
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 D3h sites and the Template:Chem2 cations in D3d sites.<ref name=wils1982/>
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
- <math> \ln P = 23.2348 - \frac{7098.2}{T}</math>
being about 48 torr at 0 °C, 424 torr at 25 °C, and 760 torr at 32 °C (9 °C below the melting point).<ref name=mcda1988>Template:Cite journal</ref>
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) <ref name=parth1996>Template:Cite journal</ref>
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.<ref name=Holl/>
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.<ref name=osbo2000>Template:Cite journal</ref>
PreparationEdit
A recommended laboratory synthesis entails dehydrating nitric acid (Template:Chem2) with phosphorus(V) oxide:<ref name=Holl>Template:Holleman&Wiberg</ref>
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:<ref name=wils1982/>
The compound can also be created in the gas phase by reacting nitrogen dioxide Template:Chem2 or Template:Chem2 with ozone:<ref name=yao1982>Template:Cite journal</ref>
However, the product catalyzes the rapid decomposition of ozone:<ref name=yao1982/>
Dinitrogen pentoxide is also formed when a mixture of oxygen and nitrogen is passed through an electric discharge.<ref name=wils1982>Template:Cite journal</ref> Another route is the reactions of Phosphoryl chloride Template:Chem2 or nitryl chloride Template:Chem2 with silver nitrate Template:Chem2<ref name=wils1982/><ref>Template:Cite journal</ref>
ReactionsEdit
Dinitrogen pentoxide reacts with water (hydrolyses) to produce nitric acid Template:Chem2. Thus, dinitrogen pentoxide is the anhydride of nitric acid:<ref name=Holl/>
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").<ref name=lloyd1955>Template:Cite journal</ref>
The reaction with hydrogen chloride Template:Chem2 also gives nitric acid and nitryl chloride Template:Chem2:<ref name=wilk1976>Template:Cite journal</ref>
Dinitrogen pentoxide eventually decomposes at room temperature into [[nitrogen dioxide|Template:Chem2]] and [[oxygen|Template:Chem2]].<ref>Template:Cite book</ref><ref name=yao1982/> Decomposition is negligible if the solid is kept at 0 °C, in suitably inert containers.<ref name=wils1982/>
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.<ref name=fren2002>Template:Cite journal</ref>
Decomposition of dinitrogen pentoxide at high temperaturesEdit
Dinitrogen pentoxide between high temperatures of Template:Cvt, is decomposed in two successive stoichiometric steps:
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.<ref name=garry>Template:Cite journal</ref>
Decomposition of dinitrogen pentoxide in carbon tetrachloride at 30 °CEdit
Apart from the decomposition of Template:Chem2 at high temperatures, it can also be decomposed in carbon tetrachloride Template:Chem2 at Template:Cvt.<ref name=Herrera>Jaime, R. (2008). Determinación de orden de reacción haciendo uso de integrales definidas. Universidad Nacional Autónoma de Nicaragua, Managua.</ref> 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:
And this reaction follows the first order rate law that says:
- <math>-\frac{d[\mathrm{A}]}{dt} = k [\mathrm{A}]</math>
Decomposition of nitrogen pentoxide in the presence of nitric oxideEdit
Template:Chem2 can also be decomposed in the presence of nitric oxide Template:Chem2:
The rate of the initial reaction between dinitrogen pentoxide and nitric oxide of the elementary unimolecular decomposition.<ref>Template:Cite journal</ref>
ApplicationsEdit
Nitration of organic compoundsEdit
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:
where Ar represents an arene moiety.<ref>Template:Cite journal</ref> 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.<ref name=b1/><ref>Template:Cite journal</ref>
Atmospheric occurrenceEdit
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.<ref>Template:Cite book</ref> Mixing ratios of several parts per billion by volume have been observed in polluted regions of the nighttime troposphere.<ref>Template:Cite journal</ref> Dinitrogen pentoxide has also been observed in the stratosphere<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> Two important reactions of Template:Chem2 in atmospheric aerosols are hydrolysis to form nitric acid<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
HazardsEdit
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.
ReferencesEdit
Cited sourcesEdit
Template:Oxides Template:Nitrogen compounds Template:Oxygen compounds