Obtaining anthraquinone by oxidizing anthracene with bichromate and sulfuric acid later became the first commercial method for the manufacture of anthraquinone from anthracene. For many years, anthraquinone provided the dyestuff industry with one of the greatest and most prolific building blocks for the manufacture of valuable dyestuffs noted for their outstanding fastness properties. However, in recent times production has fallen off considerably due to the high cost of manufacturing and the discovery of other classes of dyestuffs with good fastness properties. When sublimed, anthraquinone forms a pale yellow, crystalline material, needlelike in shape. It melts at 286°C and boils at 379–381°C. At much higher temperatures, decomposition occurs. Anthraquinone has only a slight solubility in alcohol or benzene and is best recrystallized from glacial acetic acid or high boiling solvents such as nitrobenzene or dichlorobenzene. It is very soluble in concentrated sulfuric acid. In general, anthraquinone is a relatively inert compound exhibiting stability toward oxidation. Only the reduction products involving the keto groups are of any academic or industrial importance. In the dyestuff industry, anthraquinone still ranks high as an intermediate for the production of dyes and pigments having properties unattainable by any other class of dyes or pigments. At this writing (1993) the only two processes for its production are by the Friedel‐Crafts reaction utilizing benzene, phthalic anhydride, and anhydrous aluminum chloride, and by the vapor‐phase catalytic oxidation of anthracene, the latter being preferred. Anthraquinone is a comparatively safe compound. It is a mild allergen and may cause skin irritation. It presents only a slight fire hazard on exposure to heat. The use of benzene in the Friedel‐Crafts process presents serious safety and health problems. Besides its major use in the manufacture of intermediates for anthraquinone dyes and pigments, anthraquinone is finding increasing interest as a catalyst in wood pulping in polymerization of various materials for plastics, and in the isomerization of vegetable oils.

From the earliest times humans admired the beautiful natural colors of plants and minerals and sought to enhance human appearance by color. Ancient peoples used these dyes in cosmetics,food, and as medicine. The greatest use of natural dyes occurred when the art of weaving was developed. A few synthetic dyes were manufactured before 1856, but the big break through in synthetic dye manufacture came from Perkin who developed the synthesis for the dye Mauve. Synthetic dyes replaced natural dyes, but natural dyes are making a comeback. Hand dyeing is popular and there is an ever increasing demand for natural dyes as food colorings. This article discusses those natural dyes formerly manufactured. Basic categores include anthraquinones; napthoquinone dyes; indigoid dyes, and the natural food colors.The use of biogenic sources for color is a common practice, and for the most part, these do not produce adverse reactions. However, a few biogenic substances have been linked to allergic type reactions. Literature makes reference to carmine and annatto. Regulations of natural dyes are not numerous, but are discussed here.

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Anthraquinones Naphthoquinone Dyes Indigoid Dyes Natural Food Colors Health, Safety, and Environmental Factors of Natural Dyes

With Perkin's syntheses of Mauve in 1856, the age‐old usage of natural dyes for textiles and foods greatly diminished. However, scientific interest in natural dyes continued. In 1868, the first laboratory synthesis of a natural dye, alizarin, was accomplished, followed by the manufacture of the dye. Later, indigotin was synthesized and ultimately manufactured. The structure of many other natural dyes was determined and verified by synthesis: anthraquinones, naphthoquinones, flavones, anthocyaniddins, and others. In the field of anthraquinones, some remarkable discoveries were reported: laccaic acid was shown to be a mixture of acids, and the wrong structure had been assigned to carminic and kermisic acid. The correct structure of the latter was established using the newly developed regiospecific method of synthesis. The structure of Tyrian Purple was determined and the dye synthesized. Much later, the precursors of the dye were isolated and identified. Since the early 1970s there has been a renewal of interest in natural dyes for use as food colorants. This was prompted mainly by FDA's deletion of certain synthetic dyes used for coloring foods. Some natural dyes are on FDA's list of approved dyes for coloring foods. However, their range of color is limited, so that other sources of natural dyes have been investigated. Among these was the red beet whose color is due primarily to a red dye and a small amount of a yellow dye. Both were isolated and synthesized and found to have unique chemical structures. By a suitable modification of the chlorophyll molecule, a useful food dye has been obtained.