Chemistry Reference
In-Depth Information
4.2 Synthetic access
The numerous methods for the synthesis of nitric oxide and nitrogen dioxide will not be exhaustively
surveyed here, but both are important commercially available industrial gases. Because of their reactivity,
however, in particular the nitric oxide disproportionation reaction (Equation 4.4),
11
cylinders of nitric oxide
are invariably contaminated with nitrous oxide and nitrogen dioxide. Considerable thought in the use of
these cylinders is required, especially for any exacting analytical, physical, or biological applications. At
a minimum, passing the gas through a strong base is required to remove the bulk of the nitrogen dioxide,
but it is difficult to remove the final traces of nitrogen dioxide. Extra fractionation steps are required to
remove the nitrous oxide. Failure to heed these precautions has led to some monumental misunderstandings
in nitric oxide's biology.
12
Indeed, several chemistry texts mistakenly describe solid nitric oxide as blue
in color when in fact it is colorless
13
; the blue color is due to contamination by dinitrogen trioxide.
14
As a
consequence, the prudent approach is to prepare these gases directly as needed. The direct stoichiometric
reduction of nitrite by acidic aqueous solution of ferrous ions (Equation 4.5),
15
or the acid-promoted
disproportionation of nitrite,
16
are useful for atmospheric pressure experiments where water is not an
issue,
17
and if these conditions are problematic then the dry chromium trioxide reduction of nitrite/nitrate
(Equation 4.6) is a useful alternative.
18
For nitrogen dioxide the thermal decomposition of plumbous nitrate
(Equation 4.7)
19
or dinitrogen pentoxide
20
are facile.
(4.5)
Fe
+
2
+
NO
2
−
+
2H
+
Fe
+
3
+
NO
+
H
2
O
~600 K
(4.6)
3KNO
2
+
KNO
3
+
Cr
2
O
3
2K
2
CrO
4
+
4NO
~700 K
(4.7)
2Pb(NO
3
)
2
2PbO
+
4NO
2
+
O
2
For mechanistic studies, isotopic labelling with
18
Oor
15
N are powerful tools,
21,22
which are readily
adapted to a host of applications using any of these methods. As an example of the power in the use of
isotopes, note that it is possible to specifically label either of the nitrogen atoms with
15
N in nitrous oxide.
23
4.3 Physical properties
Being open shell stable room temperature gases, both nitric oxide and nitrogen dioxide have received
sustained considerable attention and are physically well characterized species. Table 4.2 shows some of
this data for the two monomeric gaseous species. In the course of the pioneering physical chemistry
in this area it was suggested that nitric oxide, as was known for nitrogen dioxide, dimerizes at low
temperatures and in the solid state (Equation 4.1). For example, although paramagnetic as a gas, nitric
oxide is diamagnetic as a solid or liquid.
24,25
Also, the observed and calculated values for
S
◦
differ by
3.1 J/deg, which is within experimental error of
2
R ln2, and this is explained in terms of a disorder of
[NO]
2
dimers in the crystal.
26
In contrast, the nitrogen dioxide dimer, dinitrogen tetroxide (Equation 4.2),
is well characterized. The van der Waals adduct of nitric oxide and nitrogen dioxide, dinitrogen trioxide
(N
2
O
3
, Equation 4.3), is also a well characterized adduct.
Self and cross dimerization reactions of nitric oxide and nitrogen dioxide are both of considerable impor-
tance. The resulting species, dinitrogen dioxide (N
2
O
2
) from nitric oxide, dinitrogen tetroxide (N
2
O
4
) from
nitrogen dioxide, and their cross dimerization product, dinitrogen trioxide are all relatively weakly bound
1
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