Dyes, reactions synthetic

In the first chapter, devoted to thiazole itself, specific emphasis has been given to the structure and mechanistic aspects of the reactivity of the molecule most of the theoretical methods and physical techniques available to date have been applied in the study of thiazole and its derivatives, and the results are discussed in detail The chapter devoted to methods of synthesis is especially detailed and traces the way for the preparation of any monocyclic thiazole derivative. Three chapters concern the non-tautomeric functional derivatives, and two are devoted to amino-, hydroxy- and mercaptothiazoles these chapters constitute the core of the book. All discussion of chemical properties is complemented by tables in which all the known derivatives are inventoried and characterized by their usual physical properties. This information should be of particular value to organic chemists in identifying natural or Synthetic thiazoles. Two brief chapters concern mesoionic thiazoles and selenazoles. Finally, an important chapter is devoted to cyanine dyes derived from thiazolium salts, completing some classical reviews on the subject and discussing recent developments in the studies of the reaction mechanisms involved in their synthesis. 

The (A/-alkylated) lactam of 8-aminonaphthalenecarboxylic acid (47) also is a valuable dye iatemiediate, eg, for cyclometbine-type dyes used for dyeiag polyacrylonitrile fibers and other synthetics. 1,8-Naphtholactams are prepared in high yield and purity by the reaction of naphtholactones with RNH2 (R = H, Cl—4 alkyl, cycloalkyl, or optionally substituted aryl) in aqueous medium, usually in the presence of bisulfite at 150°C over a period of 15 h (143). 

The dianilinopolycarhocyanines, CgH5HN=CH-(-CH=CH NHCg Hg, are useful synthetic iatermediates for polymethine dyes, offering needed spectral and other properties (3,4,73,74). The asymmetrical polymethines appear to be ambivalent systems, and the number of possible reaction paths iacreases considerably as a result (75,76). 

Oxidation. Aromatic amines can undergo a variety of oxidation reactions, depending on the oxidizing agent and the reaction conditions. For example, oxidation of aniline can lead to formation of phenyUiydroxylamine, nitrosobenzene, nitrobenzene, azobenzene, azoxybenzene or -benzoquinone. Oxidation was of great importance in the early stages of the development of aniline and the manufacture of synthetic dyes, such as aniline black and Perkin s mauve. 

Stannic chloride is also used widely as a catalyst in Eriedel-Crafts acylation, alkylation and cycHzation reactions, esterifications, halogenations, and curing and other polymerization reactions. Minor uses are as a stabilizer for colors in soap (19), as a mordant in the dyeing of silks, in the manufacture of blueprint and other sensitized paper, and as an antistatic agent for synthetic fibers (see Dyes, application and evaluation Antistatic agents). 

Azo Coupling. The coupling reaction between an aromatic diazo compound and a coupling component is the single most important synthetic route to azo dyes. Of the total dyes manufactured, about 60% are produced by this reaction. Other methods iaclude oxidative coupling, reaction of aryUiydraziae with quiaones, and oxidation of aromatic amines. These methods, however, have limited iadustrial appHcations. 

Beryllium, calcium, boron, and aluminum act in a similar manner. Malonic acid is made from monochloroacetic acid by reaction with potassium cyanide followed by hydrolysis. The acid and the intermediate cyanoacetic acid are used for the synthesis of polymethine dyes, synthetic caffeine, and for the manufacture of diethyl malonate, which is used in the synthesis of barbiturates. Most metals dissolve in aqueous potassium cyanide solutions in the presence of oxygen to form complex cyanides (see Coordination compounds). 

Ring-Closure Reactions. Some interesting dyes are prepared by ring-closure reactions at or near the dye-forming step of a synthetic sequence. The stmctural identity of thiacyanine was originally estabUshed by the reaction of diethyl malonate and o-aminothiophenol. 

One principal use of cyclohexanol has been in the manufacture of esters for use as plasticizers (qv), ie, cyclohexyl and dicyclohexyl phthalates. In the finishes industry, cyclohexanol is used as a solvent for lacquers, shellacs, and varnishes. Its low volatiUty helps to improve secondary flow and to prevent blushing. It also improves the miscibility of cellulose nitrate and resin solutions and helps maintain homogeneity during drying of lacquers. Reaction of cyclohexanol with ammonia produces cyclohexylamine [108-91-8], a corrosion inhibitor. Cyclohexanol is used as a stabilizer and homogenizer for soaps and synthetic detergent emulsions. It is used also by the textile industry as a dye solvent and kier-boiling assistant (see Dye carriers). 

Although Lehn and his coworkers prepared a large number of cryptands and derived complexes over the years, the synthetic approach to these compounds remained essentially similar for most of them. Details are presented for a number of such compounds in ref. 26. The essential features of these syntheses were use of amide-forming reactions in the absence of templating ions with reliance on a high dilution step to form the second ring. An alternative approach for the synthesis of cryptands was developed by Dye and his coworkers. Their approach involved the use of a flow synthesis to replace the high dilution step. 

The procedure which had originally been used by Lehn et al. involved slow addition (over a period of ca. 8 h) of ca. 0.1 M solutions of diamine and diacyl halide in benzene. Dye et al. found that the reactions could be conducted more rapidly as long as stirring was kept efficient. This observation suggested the use of a mixing chamber of the type normally used for stopped-flow kinetic studies. Utilizing this type of set-up, the latter authors were able to obtain a 70% yield for 1, slightly inferior to the yield reported by Lehn, but a similar yield of 3 which is better than that previously ob-tained. Note that the chemical features of this synthetic method are essentially identical to the approach shown in Eq. (8.1) and differ primarily in the mechanics. 

In addition to the synthesis of industrially important azo dyes using heteroaromatic diazo components, intramolecular azo coupling reactions of heterocyclic diazonium ions also have interesting synthetic uses, because they lead to new fused-ring heterocycles, as shown by the diazotization of 2-amino-3-arylbenzimidazoles (12.7) in which, without isolation of the diazonium ion, a 1,2,4-triazine ring is formed (Kolodyazhnaya et al., 1973). Further examples of intramolecular azo couplings were reviewed by Tisler and Stanovnik (1980), and very extensively (403 references ) by Kishimoto et al. (1990). 

Humans have used dyes to create color since the dawn of history. Until the mid-nineteenth century, all dyes were of natural origin. Many came from plants, such as indigo, a dark blue dye that was extracted from the leaves of a native East Indian plant. In 1856, the young English chemist William Perkin stumbled upon the first synthetic dye. Perkin was trying to synthesize quinine, a valuable antimalaria dmg. None of his experiments met with success. As he was about to discard the residue from yet another failed reaction, Perkin noticed that it was colored with a purple tinge. He washed the residue with hot alcohol and obtained a purple solution from which strikingly beautiful purple crystals precipitated. Perkin had no idea what the substance was or what reactions had created it, but he immediately saw its potential as a new dye. 

The synthesis of carbonyl colorants uses a wide diversity of chemical methods, in which each individual product essentially has its own characteristic route. This is in complete contrast to the synthesis of azo dyes and pigments (Chapter 3) where a common reaction sequence is universally used. The subject is too vast to attempt to be comprehensive in a text of this type. The following section, therefore, presents an overview of some of the fundamental synthetic strategies which may be used to prepare some of the more important types of carbonyl colorants. 

As the chemical industry expanded, Perkin continued his own scientific research in the peace of his private laboratory. He had not lost his touch. Among the synthetic methods he discovered is one now called the Perkin reaction. He used it to make a synthetic substitute for a vegetable substance called coumarin, which has a pleasant, vanillalike odor. Coumarin spawned the synthetic perfume business and made luxurious scents available to all. Once again, a Perkin chemical started a new industry, albeit a modest one in comparison with dyes and pharmaceuticals. Despite the worldwide impact of Perkins discoveries, he was not knighted by the British monarchy until 1906, the fiftieth anniversary of his discovery of mauve. The world chemistry community feted him lavishly that year, and he traveled to the United States collecting further honors. A year later, at the age of 69, he died peacefully, at home. 

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