N2O4

Dinitrogen tetroxide

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 to NO 2 at higher temperatures.
Names
IUPAC name Dinitrogen tetroxide
Other names Dinitrogen(II) oxide(-I)
Identifiers
CAS Number	10544-72-6 Y
3D model (Jmol)	Interactive image
ChEBI	CHEBI:29803 Y
ChemSpider	23681 Y
ECHA InfoCard	100.031.012
EC Number	234-126-4
PubChem	25352
RTECS number	QW9800000
UNII	M9APC3P75A Y
UN number	1067
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)
Magnetic susceptibility (χ)	−23.0·10−6 cm3/mol
Refractive index (nD)	1.00112
Structure
Molecular shape	planar, D2h
Dipole moment	zero
Thermochemistry
Std molar entropy (So298)	304.29 J K−1 mol−1
Std enthalpy of formation (ΔfHo298)	+9.16 kJ/mol
Hazards
Safety data sheet	External MSDS
EU classification (DSD)	T+ C
R-phrases	R26, R34
S-phrases	(S1/2), S9, S26, S28, S36/37/39, S45
NFPA 704	0 3 2 OX
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).
Y  (what is YN ?)
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: