Heterocyclic Chemistry at a Glance, Second Edition. John A. Joule and Keith Mills.
© 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd.
Introduction
Heterocycles have great importance outside ‘pure’ chemistry and are of signifi cance, both as natural and synthetic com-
pounds, in many aspects of daily life and industry. The major areas of medicines and ‘natural products’/biomolecules
have been covered in their own chapters (18 and 17) but here we give an overview of some other important areas.
Classical aromatic chemistry – that is the chemistry of benzene derivatives – gives rise to many important industrial
and fi ne chemical applications. However there is only one neutral monocarbocyclic aromatic ring – benzene – but
there are 32 known monocyclic aromatic 5- and 6-membered heterocyclic rings (containing only N, O and S) and these
are great sources of diversity. As in medicine, the varied nature of these heterocyclic ring systems allows fi ne-tuning of
many chemical and physical properties for a variety of purposes.
Dyes and pigments (see also page 190)
The general term ‘colorants’ is used to describe pigments and dyes, the latter being soluble materials used for dying
cloth and so on. Many dyes are also used as stains for microscopy and analysis.
Natural dyes, mainly plant-based, have been used for centuries and many of these are heterocyclic, particularly anthocy-
anins and fl avonoids (see pages 165–166). Indigo is an important ancient plant-based dye, but is now produced syn-
thetically on a large scale to colour blue jeans (ca. 6–10 g per pair). Tyrian purple, an extremely expensive ancient dye
used in Roman times, is the 6,6-dibromo derivative of indigo, and was extracted from a sea snail. Indigo carmine, a
food colorant, is the disodium salt of the 5,5-disulfonic acid and also has a number of medical uses, such as the ability
to selectively identify tissue during endoscopy.
The fi rst (1857) mass-produced synthetic dye – W.H. Perkin’s mauve (mauveine) (a bright purple) – was an acciden-
tally-prepared heterocycle, from the oxidation of a mixture of substituted anilines. This is generally held to be the start
of the Fine Chemicals industry. This process was developed before the structural theory of aromatic and heterocyclic
chemistry was established, but later mauveine was shown to be a mixture of several closely related compounds, domi-
nantly mauveines A and B (phenazinium salts), the fi nal detail being established only in 1994.