Gases - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 30

At relatively low temperatures (200 ∘ C), nitrogen is the only product.
At higher temperatures, nitrous oxide formation begins passing through a
maximum at 400 ∘ C [28]. The desired nitric oxide (NO) begins to be formed
at 300 ∘ C and the yield of nitric oxide continues to increase with temperature.
Most plants operate at about 900 ∘ C. However, even at this temperature, both
nitrogen and nitrous oxide are byproducts.
At the stoichiometric ratio of ammonia to oxygen, the ammonia/air
mixture is explosive. Therefore lower levels of ammonia are used. The lower
explosion level (LEL) declines with pressure so low pressure systems can
use a higher level of ammonia than high pressure systems [29]. There are
other potential explosive hazards. For example, formation of ammonium
nitrite or ammonium nitrate is a concern and precautions must be taken in
order to avoid explosions.
Nitric acid is used to make nitrate salts for fertilizer application. Ammo-
nium nitrate and potassium nitrate are common. Ammonium nitrate is also
used for explosives. Nitric acid is used in the production of adipic acid which
in turn is used to make nylon 6,6 and also some polyesters. In the synthesis
of adipic acid, cyclohexane is oxidized by a radical process to a mixture of
cyclohexanol and cyclohexanone. This mixture is then oxidized further with
nitric acid and a catalyst to adipic acid.

OH O

O 2
Catalyst

+

Cyclohexanol Cyclohexanone

OH O

O

HNO 3
Catalyst

O H

H O

+

O

Adipic acid

Nitric acid is also used in the nitration of aromatic rings. For example, if
toluene is nitrated twice to form dinitrotoluene, the intermediate mixture of
predominantly 2,4-dinitrotoluene and 2,6-dinitrotoluene can be reduced to
the corresponding diamines which are useful for polyurethane manufacture.
If three nitro groups are added, the explosive trinitrotoluene (TNT) is formed.
Recognize that because each nitro group introduced withdraws electron
density from the aromatic ring, the reaction rate slows by several orders of
magnitude for each nitro group introduced. Because of this, it is possible
to selectively add one, two, or three nitro groups to synthesize different
target molecules. The introduction of the second nitro group requires higher
temperatures and the third, higher still.