One of the main tasks of organic chemistry is to investigate and reproduce artificially the carbon-containing chemicals that help drive the vital processes in animals and plants. This is important not only for broadening knowledge of chemical and biological phenomena, but also for seeking ways of applying chemistry to everyday life. Uncovering and replicating Nature’s colourful secrets through the chemical analysis and synthesis of its natural dyes was a central theme of Adolf von Baeyer’s work.

Inspired by his mentor August Kekulé’s pioneering theory, which proposed that you can assign certain atoms to their correct positions within a molecule by virtue of their behaviour in chemical reactions, Baeyer meticulously carried out such reactions to formulate the structures of many compounds. The most important of these was his derivation of the approximate structure of indigo, a beautiful and distinctive blue dye extracted from the plant of the same name for use in textiles and printing. In addition, Baeyer discovered a prominent collection of dyes, the phthaleins, and worked out their chemical composition. By studying the chemical makeup of phthalein dyes, Baeyer showed how they are all derivatives of the same carbon compound, triphenylmethane, and he revealed how minute differences in their structures could explain their colour differences. Both indigo and phthalein dyes belong to the same group of chemicals, named aromatic on the basis that their carbon atoms form a ring, and Baeyer developed theories to propose how the number of carbon atoms present in rings belonging to a subgroup of these compounds, called hydroaromatic, could explain why certain compounds are more stable than others.

It took almost two decades for Baeyer to progress from the approximate structure of indigo to calculating its precise chemical formula, but within that time he already had enough information to produce the dye synthetically in three ways from different sets of raw materials rich in carbon atoms. These achievements set the scene for developing ways of producing artificial versions of dyes on an industrial scale, which could be cheaper and perform better than naturally occurring dyes. Starting from easily available products from the petrochemical industry and even waste products, chemists developed variations of Baeyer’s breakthrough experiments to produce synthetic indigo, and eventually almost all the colours of the rainbow.