Aniline and azo dyes are important classes of synthetic dyes manufactured from substances derived from coal tar and products of oil refining. Most aniline dyes range from pink through purple to black; a few are green or blue. Most azo dyes are red, orange, or yellow. Dyes are used to modify the colors of such substances as polymers and textiles; they are also used in inks and coatings.
Until the mid-19th century, the only available dyes were of animal or vegetable origin. These dyes include madder, a moderately strong red dye taken from the root of a herb of the same name and known to the Egyptians as early as 1500 B.C.E.; indigo, a dark blue dye with a reddish cast derived from indigo plants; cochineal, a scarlet dye and food colorant taken from the dried bodies of cochineal beetles; and saffron, an orange dye and spice extracted from dried stigmas of the purple-flowered Crocus sativus.
The rarest and most highly prized dye was purpura, a purple dye extracted from the Murex mollusk. Purpura was used to dye the robes of the Persian nobility as early as the sixth century B.C.E. and then the clothing of Roman nobility. In the late fourth century, Emperor Theodosium of Byzantium—now the city of Istanbul, Turkey— prohibited the use of purple-dyed garments by persons other than members of his family; the penalty for disobedience was death.
In the mid-19th century, purple clothing would again catch the interest of a ruler—this time the British Queen Victoria. The difference was that Victoria’s preferred shade was that of the first commercial synthetic dye, mauveine.
Aniline dyes from coal tar
Following the development of coal carbonization by the British inventor William Murdock in the 1790s, an industry developed for the production of coal gas for lighting and heating. The industry produced large amounts of coal tar—a by-product that was at first thought to be of little use.
In 1832, Friedlieb Runge, the new technical director of a municipal gasworks in Oranienburg near Berlin, Germany, started a series of experiments with coal tar. In one of his experiments, he isolated an oil by standard distillation of coal tar and then steam distillation of the first distillate. When Runge treated this oil with a solution of calcium chloride in water, chlorine gas (Cl2) was evolved and the liquid turned blue.
Runge found that this liquid, which he called Kyanöl (German for "blue oil"), left a yellow stain on pine. Furthermore, he found that moderately strong oxidizing agents, such as chlorine and nitric acid (HNO3), could convert the blue oil into blue, red, violet, and even black dyes.
Runge foresaw the commercial prospects for these dyes, which were in fact the first synthetic aniline dyes. His plans to develop a dye manufacturing plant were stifled, however: the financial director of the gasworks saw Runge as a rival and refused financial support for his project.
A number of other German chemists were interested in the components of coal tar. Among them was August Hofmann, who purified the aromatic compounds benzene (C6H6) and methylbenzene (C6H5CH3, also called toluene) and developed methods for converting those compounds into their nitrated derivatives, such as nitrobenzene (C6H5NO2), and subsequently into amines, such as phenylamine (aniline, C6H5NH2), which he had also isolated directly from coal tar. Hofmann was invited to Britain to be the first director of the Royal College of Chemistry in London, which opened in 1845.
