CH226.22
•Compounds of Group V elements: Nitrogen oxides
Dinitrogen monoxide, N2O (O.S. = +1)
At first sight, you may find the large range of nitrogen oxides confusing, but by
keeping track of the nitrogen oxidation numbers, the structure and chemistry of
nitrogen oxides should become clearer. All nitrogen oxides are linear or planar, with 
orbitals delocalized over the whole molecule.
Dinitrogen monoxide (nitrous oxide), N2O (nitrogen O.N. = +1), is a non-toxic gas; it
is a linear molecule which is isoelectronic with CO2. N2O is made by careful
decomposition of ammonium nitrate at 250 °C: NH4NO3 (s) = N2O (g) + 2H2O
 –0.33
 +0.88
1.13 Å
Note the N≡N bond distance in N2: 1.10 Å
 –0.55
1.19 Å
N2O is used as a general anesthetic (‘laughing gas’). The gas is well-soluble in fatty
compounds. In whipped cream, it is dissolved in the fatty cream until it leaves the
can, when it becomes gaseous and thus creates foam. Used in this way, it produces
whipped cream four times the volume of the liquid, whereas whipping air into cream
only produces twice the volume (from Wikipedia).
2
Nitrogen monoxide, NO (O.S. = +2)
Nitrogen monoxide (nitric oxide), NO, is a colorless gas and one of the most reactive
of the nitrogen oxides. NO reacts immediately with atmospheric O2 to give the
characteristic brown color of gaseous NO2; it is this reaction that is largely
responsible for NO2 in polluted air (nitrogen oxides are emitted by car exhausts).
NO is made industrially from ammonia:
1.15 Å
4NH3 + 5O2 → 4NO + 6H2O (at 1300 K, on a Pt catalyst)
NO is an odd-electron paramagnetic molecule; it has a bond order of 2.5.
When NO is cooled to a liquid, some N2O2 dimers are formed; these mainly have the
cis structure:
2
102°
3
Nitrogen dioxide, NO2 (O.S. = +4)
Nitrogen dioxide, NO2 is an odd-electron paramagnetic molecule. The unpaired
electron resides mainly on the nitrogen, which enables two molecules of NO2 to form
an N—N bond. When NO2 is liquefied, it forms dinitrogen tetroxide, N2O4. At the
boiling temperature of N2O4 (21.5 °C) the liquid contains about 16% NO2:
2NO2 (g)  N2O4 (l)
1.75 Å
2
•
1.20 Å
1.18 Å
•
•
−196
brown
colorless
0
23
35 °C
(from Wikipedia)
“The US Air Force buys, stores and transports liquid dinitrogen tetroxide (N2O4) in
support of the nation's space program. N2O4 is a liquid oxidizer which is combined
with rocket fuel for spontaneous ignition. It is used primarily in Titan and Delta
rockets and the Space Shuttle.” (from: www.fas.org/spp/military/program/launch/fact01.htm) 4
Nitrogen dioxide, NO2
Nitrogen dioxide can be prepared by the reaction of copper metal with concentrated
nitric acid :
Cu (s) + 4HNO3 (l) → Cu(NO3)2 (aq) + 2H2O + 2NO2 (g)
NO2 is produced, together with oxygen, when nitrates of heavy metals are heated.
Lead nitrate is most commonly used in this reaction:
2Pb(NO3)2 (s) → 2PbO (s) + 4NO2 (g) + O2
NO2 and N2O4 are not only toxic but also corrosive, because they react with water to
form nitric and nitrous acids:
N2O4 (g) + H2O → HNO3 (aq) + HNO2 (aq)
5
Dinitrogen trioxide, N2O3 (O.S. = +3)
Dinitrogen trioxide, N2O3, can be isolated only at low temperatures as a blue solid
(Tm = –101 °C) and deep blue liquid, when stoichiometric amounts of NO and NO2
are combined. The molecule is planar. As the temperature is raised, the liquid
becomes greenish as it disproportionates to NO and the brown NO2:
N2O3  ON + NO2
6
Dinitrogen pentoxide, N2O5 (O.S. = +5)
Dinitrogen pentoxide, N2O5 (nitrogen oxidation number +5), is the true anhydride of
nitric acid; it can be made by dehydrating concentrated nitric acid with P2O5 at low
temperature:
6HNO3 (aq) + P2O5 (s) → 3N2O5 (s) + 2H3PO4 (aq)
N2O5 is a highly reactive, colorless solid, which has been shown to have an ionic
structure containing NO2+ and NO3– ions (nitronium nitrate).
Dinitrogen pentoxide reacts violently with water, yielding HNO3, and is a powerful
oxidizing agent.
7