Dinitrogen tetroxide

Dinitrogen tetraoxide

Full structural formula Full structural formula Space-filling model
Names
IUPAC name Dinitrogen tetraoxide
Other names Dinitrogen(II) oxide(-I)
Identifiers
CAS Number	10544-72-6 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:29803 Y
ChemSpider	23681 Y
ECHA InfoCard	100.031.012
EC Number	234-126-4
PubChem CID	25352
RTECS number	QW9800000
UN number	1067
CompTox Dashboard (EPA)	DTXSID00893116
InChI InChI=1S/N2O4/c3-1(4)2(5)6 Y Key: WFPZPJSADLPSON-UHFFFAOYSA-N Y InChI=1/N2O4/c3-1(4)2(5)6 Key: WFPZPJSADLPSON-UHFFFAOYAS
SMILES [O-] [N+](=O)[N+]([O-])=O
Properties
Chemical formula	N2O4
Molar mass	92.011 g/mol
Appearance	colourless liquid / orange gas
Density	1.44246 g/cm3 (liquid, 21 °C)
Melting point	−11.2 °C (11.8 °F; 261.9 K)
Boiling point	21.69 °C (71.04 °F; 294.84 K)
Solubility in water	reacts
Vapor pressure	96 kPa (20 °C)
Refractive index (nD)	1.00112
Structure
Molecular shape	planar, D2h
Dipole moment	zero
Thermochemistry
Std molar entropy (S⦵298)	304.29 J K−1 mol−1
Std enthalpy of formation (ΔfH⦵298)	+9.16 kJ/mol
Hazards
NFPA 704 (fire diamond)	3 0 0 OX
Flash point	Non-flammable
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Dinitrogen tetroxide (nitrogen tetroxide) is the chemical compound N₂O₄. It is a useful reagent in chemical synthesis. It forms an equilibrium mixture with nitrogen dioxide; some call this mixture dinitrogen tetroxide, while some call it nitrogen dioxide.

Dinitrogen tetroxide is a powerful oxidizer. N₂O₄ is hypergolic with various forms of hydrazine, i.e., they burn on contact without a separate ignition source, making them popular bipropellant rocket fuels.

Structure and properties

Dinitrogen tetroxide forms an equilibrium mixture with nitrogen dioxide. 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.

Unlike NO₂, N₂O₄ is diamagnetic since it has no unpaired electrons. The liquid is also colorless but can appear as a brownish yellow liquid due to the presence of NO₂ according to the following equilibrium:

N₂O₄ ⇌ 2 NO₂

Higher temperatures push the equilibrium towards nitrogen dioxide. Inevitably, some dinitrogen tetroxide is a component of smog containing nitrogen dioxide.

Production

Nitrogen dioxide is made by the catalytic oxidation of ammonia: steam is used as a diluent to reduce the combustion temperature. Most of the water is condensed out, and the gases are further cooled; the nitric oxide that was produced is oxidized to nitrogen dioxide, and the remainder of the water is removed as nitric acid. The gas is essentially pure nitrogen tetroxide, which is condensed in a brine-cooled liquefier.

Use as a rocket propellant

Dinitrogen tetroxide is one of the most important rocket propellants ever developed, much like the German-developed hydrogen peroxide–based T-Stoff oxidizer used in their World War II rocket-propelled combat aircraft designs such as the Messerschmitt Me 163 Komet, and by the late 1950s it became the storable oxidizer of choice for rockets in both the USA and USSR. It is a hypergolic propellant often used in combination with a hydrazine-based rocket fuel. One of the earliest uses of this combination was on the Titan rockets used originally as ICBMs and then as launch vehicles for many spacecraft. Used on the U.S. Gemini, Apollo spacecraft and the Space Shuttle, it continues to be used on most geo-stationary satellites, and many deep-space probes. It now seems likely that NASA will continue to use this oxidizer in the next-generation 'crew-vehicles' which will replace the shuttle. It is also the primary oxidizer for Russia's Proton rocket and China's Long March rockets.

When used as a propellant, dinitrogen tetroxide is usually referred to simply as 'Nitrogen Tetroxide' and the abbreviation 'NTO' is extensively used. Additionally, NTO is often used with the addition of a small percentage of nitric oxide, which inhibits stress-corrosion cracking of titanium alloys, and in this form, propellant-grade NTO is referred to as "Mixed Oxides of Nitrogen" or "MON". Most spacecraft now use MON instead of NTO; for example, the Space Shuttle reaction control system uses MON3 (NTO containing 3wt%NO). [1]

On 24 July 1975, NTO poisoning nearly killed the three U.S. astronauts on board the Apollo-Soyuz Test Project during its final descent. This was due to a switch left in the wrong position, which allowed NTO fumes to vent out of the Apollo spacecraft then back in through the cabin air intake from the outside air after the external vents were opened. One crewmember lost consciousness during descent. Upon landing, the crew was hospitalized 14 days for chemical-induced pneumonia and edema.

Power generation using N₂O₄

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. "Cool" nitrogen tetroxide is compressed and heated, causing it to dissociate into nitrogen 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 gas Brayton cycles have the potential to considerably increase efficiencies of power conversion equipment.

Chemical reactions

Intermediate in the manufacture of nitric acid

Nitric acid is manufactured on a large scale via N₂O₄. This species reacts with water to give both nitrous acid and nitric acid:

N₂O₄ + H₂O → HNO₂ + HNO₃

The coproduct HNO₂ upon heating disproportionates to NO and more nitric acid. When exposed to oxygen, NO is converted back into nitrogen dioxide:

2 NO + O₂ → 2 NO₂

The resulting NO₂ (and N₂O₄, obviously) can be returned to the cycle to give the mixture of nitrous and nitric acids again.

Synthesis of metal nitrates

N₂O₄ behaves as the salt [NO⁺] [NO₃⁻], the former being a strong oxidant:

2 N₂O₄ + M → 2 NO + M(NO₃)₂

where M = Cu, Zn, or Sn.

If metal nitrates are prepared from NTO 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 hydrated nitrate will form. The anhydrous nitrates concerned are themselves covalent, and many, e.g. anhydrous copper nitrate, are volatile at room temperature. Anhydrous titanium nitrate sublimes in vacuum at only 40 degrees C. Many of the anhydrous transition metal nitrates have striking colours. This branch of chemistry was developed by Clifford Addisson and Noramn Logan at Nottingham University in the UK during the 1960s and 1970s when highly efficient desiccants and dry boxes started to become available.

See also: Nitrosonium tetrafluoroborate

See also

• T-Stoff

External links

• International Chemical Safety Card 0930
• National Pollutant Inventory – Oxides of nitrogen fact sheet
• NIOSH Pocket Guide to Chemical Hazards: Nitrogen tetroxide
• Air Liquide Gas Encyclopedia: NO₂ / N₂O₄
• Poliakoff, Martyn (2009). "The Chemistry of Lunar Lift-Off: Our Apollo 11 40th Anniversary Special". The Periodic Table of Videos. University of Nottingham.