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 Nitrogen dioxide at −196 °C, 0 °C, 23 °C, 35 °C, and 50 °C. (NO 2) converts to the colorless dinitrogen tetroxide (N 2O 4) at low temperatures, and reverts toNO 2 at higher temperatures. | |||
| Names | |||
|---|---|---|---|
| IUPAC name Dinitrogen tetroxide | |||
| Identifiers | |||
3D model (JSmol) | |||
| ChEBI | |||
| ChemSpider |
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| ECHA InfoCard | 100.031.012 | ||
| EC Number |
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| 2249 | |||
| RTECS number |
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| UNII | |||
| UN number | 1067 | ||
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| Properties | |||
| N2O4 | |||
| Molar mass | 92.010 g·mol−1 | ||
| Appearance | White solid, colorless liquid, orange gas | ||
| Density | 1.44246 g/cm3 (liquid, 21 °C) | ||
| Melting point | −11.2 °C (11.8 °F; 261.9 K) and decomposes to NO2 | ||
| Boiling point | 21.69 °C (71.04 °F; 294.84 K) | ||
| Reacts to form nitrous and nitric acids | |||
| Vapor pressure | 96 kPa (20 °C)[1] | ||
| −23.0·10−6 cm3/mol | |||
Refractive index (nD) | 1.00112 | ||
| Structure | |||
| Planar,D2h | |||
| small, non-zero | |||
| Thermochemistry | |||
Std molar entropy(S⦵298) | 304.29 J/K⋅mol[2] | ||
Std enthalpy of formation(ΔfH⦵298) | +9.16 kJ/mol[2] | ||
| Hazards | |||
| GHS labelling: | |||
    | |||
| Danger | |||
| H270,H314,H330,H335,H336 | |||
| P220,P244,P260,P261,P264,P271,P280,P284,P301+P330+P331,P303+P361+P353,P304+P340,P305+P351+P338,P310,P312,P320,P321,P363,P370+P376,P403,P403+P233,P405,P410+P403,P501 | |||
| NFPA 704 (fire diamond) | |||
| Flash point | Non-flammable | ||
| Safety data sheet (SDS) | External SDS | ||
| Related compounds | |||
Related compounds | |||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Dinitrogen tetroxide, commonly referred to asnitrogen tetroxide (NTO), and occasionally (usually among ex-USSR/Russian rocket engineers) asamyl, is thechemical compound N2O4. It is a usefulreagent in chemical synthesis. It forms anequilibrium mixture withnitrogen dioxide. Its molar mass is 92.011 g/mol.
Dinitrogen tetroxide is a powerfuloxidizer that ishypergolic (spontaneously reacts) upon contact with various forms ofhydrazine, which has made the pair a commonbipropellant for rockets.
Dinitrogen tetroxide could be regarded as twonitro groups (-NO2) bonded together. It forms anequilibrium mixture withnitrogen dioxide.[5] The molecule is planar with an N-N bond distance of 1.78 Å and N-O distances of 1.19 Å. The N-N distance corresponds to a weak bond, since it is significantly longer than the average N-N single bond length of 1.45 Å.[6] This exceptionally weak σ bond (amounting to overlapping of thesp2 hybrid orbitals of the two NO2 units[7]) results from the simultaneous delocalization of the bonding electron pair across the whole N2O4 molecule, and the considerable electrostatic repulsion of the doubly occupied molecular orbitals of each NO2 unit.[8]
Unlike NO2, N2O4 isdiamagnetic since it has no unpaired electrons.[9] The liquid is also colorless but can appear as a brownish yellow liquid due to the presence of NO2 according to the following equilibrium:[9]
Higher temperatures push the equilibrium towards nitrogen dioxide. Inevitably, some dinitrogen tetroxide is a component ofsmog containing nitrogen dioxide.
SolidN2O4 is white, and melts at −11.2 °C.[9]
Nitrogen tetroxide is made by thecatalyticoxidation ofammonia (theOstwald process): steam is used as adiluent to reduce the combustion temperature. In the first step, the ammonia is oxidized intonitric oxide:
Most of the water is condensed out, and the gases are further cooled; the nitric oxide that was produced is oxidized to nitrogen dioxide, which is then dimerized into nitrogen tetroxide:
and the remainder of the water is removed asnitric acid. The gas is essentially pure nitrogen dioxide, which is condensed into dinitrogen tetroxide in a brine-cooled liquefier.[10]
Dinitrogen tetroxide can also be made through the reaction of concentrated nitric acid and metallic copper. This synthesis is practical in a laboratory setting. Dinitrogen tetroxide can also be produced by heating metal nitrates.[11] The oxidation of copper by nitric acid is a complex reaction forming various nitrogen oxides of varying stability which depends on the concentration of the nitric acid, presence of oxygen, and other factors. The unstable species further react to form nitrogen dioxide which is then purified and condensed to form dinitrogen tetroxide.
Nitrogen tetroxide is used as an oxidizing agent in one of the most important rocket propellant systems because it can be stored as a liquid at room temperature.Pedro Paulet, aPeruvianpolymath, reported in 1927 that he had experimented in the 1890s with a rocket engine that used spring-loaded nozzles that periodically introduced vaporized nitrogen tetroxide and apetroleum benzine to aspark plug for ignition, with the engine putting out 300 pulsating explosions per minute.[12][13] Paulet would go on to visit the German rocket associationVerein für Raumschiffahrt (VfR) and on March 15, 1928, Valier applauded Paulet's liquid-propelled rocket design in the VfR publicationDie Rakete, saying the engine had "amazing power".[14] Paulet would soon be approached byNazi Germany to help develop rocket technology, though he refused to assist and never shared the formula for his propellant.[15]
In early 1944, research on the usability of dinitrogen tetroxide as an oxidizing agent for rocket fuel was conducted by German scientists, although the Germans only used it to a very limited extent as an additive forS-Stoff (fuming nitric acid). It became the storable oxidizer of choice for many rockets in both theUnited States andUSSR by the late 1950s. It is ahypergolic propellant in combination with ahydrazine-basedrocket fuel. One of the earliest uses of this combination was on theTitan family of rockets used originally asICBMs and then aslaunch vehicles for many spacecraft. Used on the U.S.Gemini andApollo spacecraft and also on theSpace Shuttle, it continues to be used as station-keeping propellant on most geo-stationary satellites, and many deep-space probes. It is also the primary oxidizer for Russia'sProton rocket.
When used as a propellant, dinitrogen tetroxide is usually referred to simply asnitrogen tetroxide and the abbreviationNTO is extensively used. Additionally, NTO is often used with the addition of a small percentage ofnitric oxide that reacts to formdinitrogen trioxide, which inhibitsstress-corrosion cracking of titanium alloys, and in this form, propellant-grade NTO is referred to asmixed oxides of nitrogen (MON) and can be distinguished by its green-blue color. Larger additions of nitric oxide, up to 25-30%, also lower the freezing point of NTO, improving storability in space conditions.[16] Most spacecraft now use MON instead of NTO; for example, the Space Shuttle reaction control system used MON3 (NTO containing 3% NO by weight).[17]
On 24 July 1975, NTO poisoning affected three U.S.astronauts on the final descent to Earth after theApollo-Soyuz Test Project flight. This was due to a switch accidentally left in the wrong position, which allowed theattitude control thrusters to fire after the cabin fresh air intake was opened, allowing NTO fumes to enter the cabin. One crew member lost consciousness during descent. Upon landing, the crew was hospitalized for five days for chemical-inducedpneumonia andpulmonary edema.[18][19]
The tendency of N2O4 to reversibly break into NO2 has led to research into its use in advanced power generation systems as a so-called dissociating gas.[20] "Cool" dinitrogen tetroxide is compressed and heated, causing it to dissociate intonitrogen dioxide at half the molecular weight. This hot nitrogen dioxide is expanded through a turbine, cooling it and lowering the pressure, and then cooled further in a heat sink, causing it to recombine into nitrogen tetroxide at the original molecular weight. It is then much easier to compress to start the entire cycle again. Such dissociative gasBrayton cycles have the potential to considerably increase efficiencies of power conversion equipment.[21]
The high molecular weight and smaller volumetric expansion ratio of nitrogen dioxide compared to steam allows the turbines to be more compact.[22]
N2O4 was the main component of the "nitrin" working fluid in the decommissionedPamir-630D portable nuclear reactor which operated from 1985 to 1987.[23]
Nitric acid is manufactured on a large scale via N2O4. This species reacts with water to give bothnitrous acid andnitric acid:
The coproduct HNO2 upon heatingdisproportionates toNO and more nitric acid. When exposed to oxygen, NO is converted back into nitrogen dioxide:
The resulting NO2 and N2O4 can be returned to the cycle to give the mixture of nitrous and nitric acids again.
N2O4 undergoesmolecular autoionization to give [NO+] [NO3−], with the formernitrosonium ion being a strong oxidant. Various anhydroustransition metal nitrate complexes can be prepared from N2O4 and base metal.[24]
If metal nitrates are prepared from N2O4 in completely anhydrous conditions, a range of covalent metal nitrates can be formed with many transition metals. This is because there is a thermodynamic preference for the nitrate ion to bond covalently with such metals rather than form an ionic structure. Such compounds must be prepared in anhydrous conditions, since the nitrate ion is a much weaker ligand than water, and if water is present the simple nitrate of thehydrated metal ion will form. The anhydrous nitrates concerned are themselves covalent, and many, e.g. anhydrouscopper nitrate, are volatile at room temperature. Anhydrous titanium nitrate sublimes in vacuum at only 40 °C. Many of the anhydrous transition metal nitrates have striking colours. This branch of chemistry was developed byCliff Addison and Norman Logan at theUniversity of Nottingham in the UK during the 1960s and 1970s when highly efficient desiccants anddry boxes started to become available.
In even slightly basic solvents, N2O4 adds toalkenes radically, giving mixtures ofnitro compounds andnitrite esters. Pure or in entirely nonbasic solvents, the compounds autoionizes as above, to givenitroso compounds andnitrate esters.[25]
