colored compounds and dyes

Aqueous solutions of dyes are used to color paper, leather, textiles, and other materials. Pigments are finely ground solids dispersed in a liquid for application as coatings such as paints and inks or for blending with other materials. Most dyes are compounds of carbon (i.e., organic), while pigments are both inorganic (e.g. titanium and ferric oxides) and organic compounds. The colors of dyes and pigments are due to the absorption of visible light by the compounds. Dyes used in ancient times include alizarin, Tyrian purple, and indigo; the last remains common madder plant, Rubia tinctorum. Other red dyes of this class include kermes, derived from the insect Coccus ilicis, and cochinela, derived from the insect coccus cacti. Tyrian purple, a purple dye of the indigoid class, is derived from a sea snail, murex brandaris. Indigo, or woad, a blue dye of the indigoid class, comes from the leaves of the indigo plant, Indigofera tinctoria L. Yellow dyes of the flavenoid class include weld, from the seeds, stems, and leaves of Reseda luteols; quercetin, from the bark of the North American oak, Quercus tinctoria nigra; and safflower, from the dried petals of Carthamus tinctorius. The synthetic dye industry arose directly from studies of coal tar, a by-product of the production of coke from soft coal (with a yield of roughly 8 gallons per ton, or 30 liters per metric ton, of coal). Virtually all dyes were obtained from natural sources—mainly vegetable—until 1856, when Englishman William H. Perkin serendipitously discovered the first commercially successful synthetic dye, mauve, as a product of dichromate oxidation of impure aniline. Its introduction triggered a decline in the dominance of natural dyes in world markets. Although mauve had a short commercial lifetime, its early success led to experimentation that produced several better dyes within a relatively few years. In 1858, aniline’s reaction with stannic chloride was found to give aniline red. Later named magenta, it was the first of the triphenylmethane dyes, and it triggered the second phase of the synthetic dye industry. Other reagents were found to give better yields, leading to vigorous patent activity and several legal disputes. Inadvertent addition of excess aniline in a magenta preparation resulted in the discovery of aniline blue. From the molecular formulas of these dyes, Hofmann showed that aniline blue was magenta with three more phenyl groups (-C6H5), but the chemical structures were still unknown. In a careful study, the British chemist Edward Chambers Nicholson showed that pure aniline produced no dye, a fact also discovered at a CIBA plant in Basel, Switzerland, that was forced to close because the aniline imported from France no longer gave satisfactory yields. Hofmann demonstrated that toluidine (CH3C6H4NH2) must be present to produce these dyes. All the dyes were found to be mixtures of two major components. Emil Fischer, who showed that the methyl carbon of p-toluidine becomes the central carbon bonded to three aryl groups, established the rosaniline structures in 1878. Magenta was found to be a mixture of pararosaniline and a homolog having a methyl group (-CH3) ortho to one of the amino groups (-NH2). All of the early synthetic dyes, including mauve, were prepared from aniline containing unknown amounts of toluidine. Recognition of the tetravalency of carbon and the nature of the benzene ring were key factors required to deduce the molecular structures of the well-known natural dyes and the new synthetics (mauve, magenta, and the azo dyes). Only one natural dye, logwood, is now used commercially, to a small degree, to dye silk, leather, and nylon black. Synthetic indigo rather than the natural product is used mainly for cotton denim (as in blue jeans). The Badische Anilin- & Soda-Fabrik (BASF) of Germany placed synthetic indigo on the market in 1897; development of its process was financed by profits from synthetic alizarin, marketed first in 1869. In 1868, Carl Graebe and Carl Leibermann had recognized that dyes contain sequences of conjugated double bonds: X=C-C=C-C=C-…, where X is carbon, oxygen, or nitrogen. In 1876, Otto Witt proposed that dyes consist of conjugated systems, called chromophores, and salt-forming groups, or auxochromes, polar substituents that modify their colors. Although these ideas remain valid, they have been broadened by better recognition of the role of specific structural features. Dye chromophores are conjugated systems involving two or more benzene rings linked with other unsaturated groups. Early commercial synthetic chromophore dyes include chrysoidine (an azobenzene), alizarin (an anthraquinone), fluorescein (a xanthene), and pararosaniline (a triphenylmethane). Chrysoidine is one of the azo dyes, named thus because the products have aryl rings linked through a -N=N- unit, called an azo group. Pthalocyanines, introduced in 1934, are analogs of two natural porphyrins, chlorophyll and hemoglobin. Phthalocyanines became commercially available in the 1930s with the parent and its copper complex marketed as Monastral Fast Blue B and G, respectively. A second new group of pigments was also developed in the 20th century. Quinacridone was introduced in 958. Its seven crystalline forms range in color from yellowish-red to violet; the violet b and red g forms are used as pigments, both classified as CI Pigment Violet 19. Reactive dyes, introduced in 1956, are anchored to the textile fiber by covalent bonds. Their discovery ended the search for a process to overcome the inadequacies of the common methods for dyeing cotton. With the introduction of reactive dyes, cotton could finally be dyed in bright shades with monazo dyes for yellows to reds, anthraquinones for blues, and copper phthalocyanines for bright turquoise colors.