Page 46 - Chemical and process design handbook

Speight_Part 1_N&O 11/7/01 3:02 PM Page 1.32

NITRATION

Nitration is the insertion of a nitro group (–NO ) into an organic com-
2
pound, usually through the agency of the reaction of a hydrocarbon with
nitric acid. Concentrated sulfuric acid may be used as a catalyst.

ArH + HNO → ArNO + H O
3 2 2
More than one hydrogen atom may be replaced, but replacement of each
succeeding hydrogen atom represents a more difficult substitution.
The nitrogen-bearing reactant may be:

1. Strong nitric acid
2. Mixed nitric and sulfuric acid
3. A nitrate plus sulfuric acid
4. Nitrogen pentoxide (N O )
2 5
5. A nitrate plus acetic acid

Both straight chain and ring-type carbon compounds can be nitrated;
alkanes yield nitroparaffins.
The process for the production of nitrobenzene from benzene involves the
use of mixed acid (Fig. 1), but there are other useful nitrating agents, e.g.,
inorganic nitrates, oxides of nitrogen, nitric acid plus acetic anhydride, and
nitric acid plus phosphoric acid. In fact, the presence of sulfuric acid in quan-
tity is vital to the success of the nitration because it increases the solubility
of the hydrocarbon in the reaction mix, thus speeding up the reaction, and
+
promotes the ionization of the nitric acid to give the nitronium ion (NO ),
2
which is the nitrating species. Absorption of water by sulfuric acid favors the
nitration reaction and shifts the reaction equilibrium to the product.
Nitration offers a method of making unreactive paraffins into reactive
substances without cracking. Because nitric acid and nitrogen oxides are
strong oxidizing agents, oxidation always accompanies nitration. Aromatic
nitration reactions have been important particularly for the manufacture of
explosives. Nitrobenzene is probably the most important nitration product.

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