Preparation of azo-dyes

Alternatively, dissolve 12 g. of dimethylaniline in a suspension of 20 g. of sulphanilic acid in 100 c.c. of water, cool in ice, and add the nitrite solution slowly. The sodium salt of the dye then separates directly. 

As a variation diazotised anthranilic acid may be converted into methyl red by coupling with dimethylaniline. 

The azo-dye obtained is the indicator methyl orange which is much used in alkalimetry. The yellow dilute solution of helianthine is turned red by acids. 

The yellow sodium salt is derived from this azo- form, whilst a red quinonoid salt is formed by the action of acids. 

Probably the free acid, which is also red, has the constitution of an intramolecular quinonoid salt. 

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It is used as a first component in the preparation of azo-dyes, amlnoazo-dyes See azo-dyes, o-aminobenzoic acid See anthranilic acid. 

The mixture of xylidines has been used as a first component of azo-dyes. The chief constituent of the mixture is m-xylidine (4-amino-1,3-xylene). It can be separated by crystallization from glacial ethanoic acid. It is also used for the preparation of azo-dyes. 

The reaction just described is carried out technically on a large scale in iron vessels provided with stirrers, for /9-naphthol, as well as the numerous sulphonic acids which can be obtained from it by the action of sulphuric acid, is widely used for the preparation of azo-dyes. Moreover, by the action of ammonia under pressure, /3-naphthylamine is produced from /S-naphthol ... 

Uses. In the preparation of azo dyes corrosion inhibitor chemical intermediate... 

Treatment of the 2-oxopyrido[l,2-a]pyrimidine (22 R = H) with tri-ethyloxonium fluoroborate and subsequently with m-nitrobenzaldehyde gave the pyrido[l,2-a]pyrimidines(200). Hydrolysis of 200 with hydrochloric acid yielded 2-hydrazono-2/f-pyrido[l,2-a]pyrimidine following oxidative coupling with 1-hydroxy-2-napthanilide or 4-acetylaminodi-phenylamine, this was utilized for the preparation of azo dyes.252... 

The direct diazotization and preparation of azo-dyes in one electrochemical process was discovered by Lob.1 His method— an anodic process—is based on the following principle ... 

The fraction obtained by predpitation with potassium chloride is G salt which contains 5 per cent, at the most, of R salt as an impurity. In the laboratory, this crude G salt can be purified by recrystalhzation from water. The method used in the plant is based on the fact that G salt couples with diazonium compounds much less readily that either R salt or Schaeffer salt. A test is made to determine the content of easily coupling sulfonic acids by titrating a sample with diazobenzene or diazoxylene in dilute solution. The crude G salt is then treated with the correct amount, as indicated by the titration, of a slowly reacting diazo compound, usually diazoxylene. Under these conditions, only the impurities co le, leaving a solution of pure R salt after the dye which is formed is removed. The G salt solution can be used directly in the preparation of azo dyes. The dye from the impurities is sold under the name Ponceau. 

Remarks. Nitro-p-dichlorobenzene can be reduced to p-dicbloroaniline in tbe usual way with iron and a small amount of acid. Tbe chlorine ortbo to tbe nitro group is labile and can be replaced by various substituents, less easily, however, than the chlorine in 2,4-dinitrochlorobenzene, but more easity than that in o-nitrochloro-benzene. The replacement of the chlorine by hydrojyl by the action of aqueous sodium hydroxide is described in the following preparation. Reaction wiUi sodium methylate yields 4-chloro-2-nitroanisole, which is reduced to 4-chloro-2-am sidine (chloramsidine P, or fast red R base). With ammonia under pressure, 4-chloro-2-nitroaniline (fast red 3CL base) is formed. These bases are important starting materials for the preparation of azo dyes, especial pigment and lake coIots, as well as ice colors. 

Salicylic and cresotinic acids are used in the preparation of azo dyes and of tri-phenylmethane dyes of the aurine type (eriochrome azurol B). These acids confer upon the dyes the property of going onto metal mordants, particularly chromium mordants, and also of forming complex compounds, in substance, with suitable metal salts. Salicylic acid is also widely used in the pharmaceutical industry. 

The reduction takes about 2 hours, after which 5 grams of soda ash is added and the solution is filtered. The filtrate, after evaporation to 200 cc., can be used directly for the preparation of azo dyes without isolating the chloroaminobenzoic acid. The yield, determined by titration, is about 92 per cent of the theoretical amount. 

The coupling reaction is important in the industrial preparation of azo dyes as well as in the analytical determination of diazonium compounds. The reaction has been reviewed, and experimental procedures have been given,... 

Technical Xylidine.—The para-xylene is also present in the technical product which of course yields only one xylidine, viz., 1-4-dimethyl 2-amino benzene. From the or//fo-xylene present the vicinal or I-2-di-methyl 3-amino benzene is obtained. The technical xylidine contains these three isomeric compounds and is used in the preparation of azo-dyes. Of the amino derivatives of the higher homologues only one will be mentioned. 

Rijkuris A., Biseniece S. Sergeeva, V.N. (1978) Composition and Properties of Tar Formed During Thermolysis of Lignocellulose. Preparation of Azo Dyes from Thermolysis Tar. Khim. Drev, 1 68, CA. 90 153480. 

OTHER COMMENTS used as an intermediate in the preparation of polyurethane foams, elastomeric fibers, and polyamides used in industry as a curing agent for epoxy resins and urethane elastomers, and as a corrosion inhibitor for iron useful in the determination of tungsten and sulfates has also been used in the preparation of azo dyes, as a rubber processing chemical, and as an epoxy resin hardening agent. 

Another group of compounds which can be synthesized by a MCR under MW conditions are pyridones. Pyridone derivatives 15 have found many applications in the preparation of azo-dyes, especially as disperse dyes for polymeric materials [42]. It is possible to prepare these compounds by one-pot three-component condensation of a jS-cyanoester, a /i-keto ester, and a primary amine (Scheme 17.11) [43]. 

Hoshino and Saji [7] first reported the preparation of azo-dye thin films using FTMA. These films were prepared by controlled-potential electrolysis of the FTMA micelle aqueous solution solubilizing the azo dye at 4-0.3 V versus SCE. The azo-dye molecules exist as sol-ubilizate in the micelles, and are then released from the micelles and deposited... 

The novel use of phase-transfer catalysis in electrophilic reactions has allowed the preparation of azo-dyes of type (25), where the amino component is a particularly unreactive coupling agent. Although the generality of the reaction is unfortunately limited, in certain cases it is the only means of preparing the required C-coupled product (Scheme 38),... 

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