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Chapter 2
2.5.11 Reduction of aromatic nitro compounds
[6, Ullmann, 2001, 16, Winnacker and Kuechler, 1982, 19, Booth, 1988]
For environmental issues and treatment of waste streams, see Section 4.3.2.7.
One of the most industrially important reduction processes in industrial use is the conversion of
an aromatic nitro or dinitro compound into an arylamine or arylene diamine. Aromatic amines
are widely used as dye intermediates, especially for azo dyes, pigments, and optical brighteners;
as intermediates for photographic chemicals, pharmaceuticals, and agricultural chemicals; in
polymers via isocyanates for polyurethanes; and as antioxidants. Among reduction methods,
there are three of major relevance in organic fine chemistry:
• catalytic hydrogenation, which is extremely important industrially because of its universal
applicability; most processes can be carried out successfully by catalytic hydrogenation
• Béchamp and Brinmeyr reduction with iron, which is the classical method
• alkali sulphide reduction, which is the selective method in specific cases, such as in the
manufacture of nitroamines from dinitro compounds, the reduction of nitrophenols, the
reduction of nitroanthraquinones and the manufacture of aminoazo compounds from the
corresponding nitroazo derivative.
All three methods are also applied to halogenated nitro compounds, and can thus contribute to
AOX loads in waste water streams.
2.5.11.1 Catalytic reduction with hydrogen
Chemical reaction
The catalytic reduction of the nitro compounds is very exothermic. To reduce these hazards, the
concentration of nitro compound, the amount and partial pressure of the hydrogen, the
temperature, and the activity of the catalyst, are controlled.
R
NO 2
+ 3 H 2
Catalyst
Figure 2.23: Catalytic reduction of aromatic nitro compounds
Most aromatic nitro compounds are hydrogenated in the liquid phase. In this case, the pressure
and temperature can be changed independently. The temperature is limited by the hydrogenation
reaction of the aromatic ring which occurs above 170 – 200 °C.
Normally, the reduction is carried out at 100 – 170 °C. Sensitive compounds are hydrogenated
at lower temperatures (20 – 70 °C) or at lower pressures (1 – 50 bar). 1 – 50 bar are used
normally.
56 Dezember 2005 OFC_BREF
R
NH 2