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N2O4

Dinitrogen tetroxide
Full structural formula
Space-filling model
Nitrogen dioxide at different temperatures
Nitrogen dioxide at −196 °C, 0 °C, 23 °C, 35 °C, and 50 °C. (NO
2
) converts to the colorless dinitrogen tetroxide (N
2
O
4
) at low temperatures, and reverts to NO
2
at higher temperatures.
Names
IUPAC name
Dinitrogen tetroxide
Other names
Dinitrogen(II) oxide(-I)
Identifiers
10544-72-6 YesY
3D model (Jmol) Interactive image
ChEBI CHEBI:29803 YesY
ChemSpider 23681 YesY
ECHA InfoCard 100.031.012
EC Number 234-126-4
PubChem 25352
RTECS number QW9800000
UNII M9APC3P75A YesY
UN number 1067
Properties
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)
reacts
Vapor pressure 96 kPa (20 °C)
−23.0·10−6 cm3/mol
1.00112
Structure
planar, D2h
zero
Thermochemistry
304.29 J K−1 mol−1
+9.16 kJ/mol
Hazards
Safety data sheet External MSDS
Very Toxic T+ Corrosive C
R-phrases R26, R34
S-phrases (S1/2), S9, S26, S28, S36/37/39, S45
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazard OX: Oxidizer. E.g., potassium perchlorateNFPA 704 four-colored diamond
Flash point Non-flammable
Related compounds
Related nitrogen oxides
Nitrous oxide
Nitric oxide
Dinitrogen trioxide
Nitrogen dioxide
Dinitrogen pentoxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
YesY  (what is YesYN ?)
Infobox references

Dinitrogen tetroxide, commonly referred to as nitrogen tetroxide, is the chemical compound N2O4. It is a useful reagent in chemical synthesis. It forms an equilibrium mixture with nitrogen dioxide.

Dinitrogen tetroxide is a powerful oxidizer that is hypergolic (spontaneously reacts) upon contact with various forms of hydrazine, which makes the pair a popular bipropellant for rockets.

Dinitrogen tetroxide could be regarded as two nitro groups (-NO2) bonded together. It 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:

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

Nitrogen tetroxide is made by the catalytic oxidation of ammonia: steam is used as a diluent to reduce the combustion temperature. In the first step, the ammonia is oxidized into nitric 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:


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