18 ChemMatters | FEBRUARY/MARCH 2018 www.acs.org/chemmatters
and to the partial synthesis of indigotin. In
1882, von Baeyer published the “Baeyer–
Drewson indigo syntheses.” It turned out
to be an easy route for producing indigo at
laboratory scale. In 1905, Von Baeyer was
awarded the Nobel Prize, recognizing his work
in organic dyes and other advances in organic
The synthesis of indigo remained imprac-
tical, so the search was on for alternative
starting materials. In 1901, Johannes Pfleger
and Karl Heumann came up with an industrial
mass-production synthesis of indigo from
aniline (Fig. 3a). Aniline was readily avail-
able in large quantities from coal, so it was
an economically feasible route to synthesiz-
ing indigo. At the time synthetic indigo was
launched, natural indigo production was
19,000 metric tons, and soon declined to
1,000 metric tons by 1914.
Variations using coal-based aniline are still
in use today, but modern production of indigo
starts with petroleum-based naphthalene
(Fig. 3b) through a different synthetic route
that is even more economically viable.
Aniline was first isolated in 1826 by Otto
Unverdorben by destructive heating and
distillation of indigo. Naph-
thalene was traditionally
used in moth balls, which
accounted for their charac-
We have certainly come
a long way from the ancient
methods of extracting
naturally occurring indigo
from the woad plant to the
techniques of synthetic indigo. If the syn-
thesis of this dye weren’t so successful,
who knows what would be in fashion
for casual wear these days!
Steingruber, E. Indigo and
Indigo Colorants. Ullmann’s
Encyclopedia of Industrial
Weinheim, Germany, 2004.
Cannon, J; Cannon, M. Dye Plants
and Dyeing. A & C Black: London, 2003.
Kumar Samanta, A.; Konar, A. Dyeing of
Textiles with Natural Dyes. Perrin Akcakoca
Kumbasar, E. (Ed.); In Tech Open.com, 2011,
pp 29–56: https://www.intechopen.com/books/
Ebert, E. Sense of Fashion: Tie-Dye Gets Modern.
Savannah Now.com/blogs/fashion (last updated
Jan 22, 2016): http://savannahnow.com/column-
Cooper, R.; Deakin, J. J. Botanical Miracles:
Chemistry of Plants that Changed the World. CRC
Press, Taylor & Francis Group: Boca Raton, Fla.,
Jeffrey Deakin is a former teacher of chemistry and physics in secondary schools in the
United Kingdom. Raymond Cooper is currently a
Visiting Professor and lecturer at the Hong Kong
Polytechnic University, Hong Kong. Both are Fellows
of the Royal Society of Chemistry.
remains inside the
Over time, though,
bonding is an intermolecular force, which is
weaker than a chemical
bond, some indigo molecules will separate from the cellulose
molecules in the fabric, and the blue
color will slowly fade. This is triggered by frequent washing of the
fabric. Faded denim apparel is
considered fashionable today,
and some people prefer
jeans with a faded blue
color over dark blue.
During the 1800s, natural
indigo production could no longer meet the
demands of the clothing industry, and a
search for synthetic indigo started.
Fascinated by the blue dye, the German
chemist Adolf von Baeyer started work in
1865 that soon led to the discovery of indole
This strip of cloth shows the saturation of
indigo dye from 0– 10.
FadED deNIm aPpARel
anD soME peOPle
pRefER jeANs WitH
a fADed BluE coLOr
ovER daRk BluE.
Traditional boro (patchwork) denim kimono
Figure 3. Molecular structures of (3a) aniline and