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2.1.2 Isomers and by-products
Chapter 2
The stepwise introduction of substituents and their subsequent modification (see Figure 2.1), in
combination with the unavoidable drastic reaction conditions (e.g. reactivity of halogens,
oxidative effects of concentrated sulphuric/nitric acids, higher reaction temperatures for
deactivated aromatics), can lead to an increasing number of side reactions and unwanted byproducts,
e.g:
• position isomers
• higher and lower substituted compounds
• modification of substituents
• oxidation products
• by-products derived from the by-products if work-up is omitted (“one pot synthesis”).
Cl Cl
OFC_BREF Dezember 2005 21
NO 2
Chlorination Nitration Exchange
OCH 3
OCH3 OCH3
OCH3 OCH3 NH NH
Reductive
coupling Rearrangement
Figure 2.1: Illustrative example of a synthesis using several unit processes
Often recovery and re-use of isomers or by-products is technically possible (e.g. as starting
material in other plants or sectors). But in many cases a recovery appears difficult due to
economical, ecological or legal reqirements. If a recovery is not possible, unwanted
isomers/by-products have to be separated from the product and contribute to waste or waste
water streams.
The nitration of toluene given in Table 2.3 can be used as an illustrative example. In this case,
the isomers are separated and purified by distillation.
Starting material Process Isomers By-products
Nitrophenols
Toluene
Nitration with
HNO3
o-Nitrotoluene (59.5 %)
m-Nitrotoluene (4 %)
p-Nitrotoluene (36 %)
Nitrocresols
Nitrohydroxy benzoic acids
Phenylnitromethane
Tetranitromethane
Table 2.3: Example for the formation of isomers and by-products
NH 2
NO 2
NH 2