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2.5.7 Nitration
Chapter 2
[6, Ullmann, 2001, 15, Köppke, 2000, 16, Winnacker and Kuechler, 1982, 18, CEFIC, 2003, 46,
Ministerio de Medio Ambiente, 2003]
For environmental issues and treatment of waste streams, see Section 4.3.2.6.
Liquid phase nitration is a dominant step in the manufacture of common high explosives and
important for the production of a wide range of aromatic intermediates for dyes, agrochemicals,
pharmaceuticals or other fine chemicals. A typical nitration reaction is higly exothermic,
therefore, for a safe mode of reaction, a dosage controlled process with precautions securing no
accumulation of reactants is necessary. Typical nitroaromatic production is based on high yield
processes, with more than 80 % of the total cost being the cost of the raw materials. Integral
requirements of all efficient nitration processes are sulphuric acid regeneration and isomer
control and separation. Nitration of the important naphthalene mono- and disulphonic acids is
usually performed with the formed sulphonated mass. Among the typical raw materials are
halogenated aromatics, which can contribute to the AOX load of waste water streams.
Chemical reaction
Nitration is the irreversible introduction of one or more nitro groups into an aromatic system by
electrophilic substitution of a hydrogen atom. O-nitration to give nitrates and N-nitration to give
nitramines are far less important for aromatic compounds but relevant for the manufacture of
explosives.
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HNO 3 / H 2SO 4
H NO 2
- H 2O
Figure 2.20: Nitration of an aromatic compound
Nitration is normally carried out in a liquid phase reaction with a mixture of nitric and sulphuric
acids (mixed acid) and occasionally with nitric acid. A typical mixed acid, for example for
mononitration, consists of 20 % nitric acid, 60 % sulphuric acid and 20 % water (this is referred
to as 20/60/20 mixed acid). The strength of the mixed acid and the temperature can be varied to
maximise the formation of the required isomer. Stronger mixed acid and higher temperature
lead to oxidative side reactions. An important side reaction leads to phenolic by-products.
Operations
Figure 2.21 shows a typical sequence of operations for the nitration of aromatic compounds,
possible input materials and associated waste streams. The reaction is carried out in cast iron,
stainless steel or enamel-lined mild steel reactors. Temperatures vary normally
between 25 and 100 °C. The substrate is dissolved in the sulphuric acid phase and the mixed
acid is subsequently added. On completion of the reaction, the batch is transferred into water to
give a two phase mixture of diluted acid and an organic product phase.
After phase separation, liquid products are purified by distillation. The remaining acid phase can
be extracted with the feed material in order to recover organic compounds. Solid products are
crystallised (where necessary, by the addition of cold water). The crude nitroaromatic is washed
with water and diluted NaOH to remove the acids and phenolic by-products. Depending on the
quality requirements, a recrystallisation from water or organic solvent may be necessary. Isomer
separation is carried out within the crystallisation, washing or distillation steps.
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