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Chapter 2
2.5.6 Halogenation
[6, Ullmann, 2001, 15, Köppke, 2000, 16, Winnacker and Kuechler, 1982, 18, CEFIC, 2003]
See also Section 4.3.2.5 for environmental issues and treatment of waste streams from
halogenations.
Halogenation is one of the most important and versatile processes in chemistry. The industrial
application is dominated by chlorinations, due to the different reactivity and the higher price for
bromine, iodine and fluorine.
Side chain chlorinated alkyl aromatics, particularly those based on toluene and xylene, as well
as nucleus halogenated aromatics, have an exceptional place in organic fine chemistry, because
of their role as chemical intermediates in the manufacture of chemical products of almost all
kinds, including dyes, plastics, pharmaceuticals, flavours and fragrances, pesticides, catalysts
and inhibitors.
Bromination is a key process in anthraquinone chemistry and the manufacture of organic flameretardants.
Heavily halogenated aromatic hydrocarbons
Especially as a result of the environmental persistence of the heavily chlorinated benzenes,
toluenes and biphenyls, in recent years drastic measures have been applied to this range of
chemicals, such as prohibitions, and restrictions on their production and use, and legislation
regulating waste disposal. Possible side reactions of the chlorination process can result in the
formation of polychlorinated biphenyls or hexachlorobenzene. The combustion of aromatics
containing chlorine can lead to the formation of polychlorodibenzo dioxins/-furans
(PCDD/PCDF).
Chemical reaction
These chemicals are of major relevance on an industrial scale in substitutions of the aromatic
nucleus and in the substitution of aliphates. In both cases, hydrogen is replaced by halogen (X)
and the related hydrogen halide is created:
R – H + X2 � R – X + HX
Ar – H + X2 � Ar – X + HX
Both reactions are exothermic but the aliphate substitution follows a radical chain mechanism,
initialised by ultraviolet light (irradiation with mercury vapour lamps), while the halogenation
of the aromatic nucleus is based on an electrophilic mechanism supported by Friedel-Crafts
catalysts (i.e. Lewis acids such as FeCl3, AlCl3 …).
Generally, a mixture of isomers and/or compounds with a different degree of halogenation is
obtained and side reactions following alternative mechanisms cannot be completely suppressed.
The product mix depends on the aromatic/halogen ratio, the reaction conditions and the choice
of the catalyst.
A wide range of organic and aqueous solvents are currently in use, and especially
tetrachloromethane, tetrachloroethane, dichlorobenzene and trichlorobenzene are recommended
for halogenations [6, Ullmann, 2001].
48 Dezember 2005 OFC_BREF