Diazonium dyes

Amador, E. Salvatore, A. C. Serum gluatmic-oxalacetic transaminase activity. Revised manual and automated methods using diazonium dyes. Am. J. Clin. Path. (1971), 55, 686-697. 

Figure 27.7 The azo dye (also called a diazonium dye) forms in a coupling reaction between the diazonium ion and an alkaline solution of phenol.

Diazonium salts react with other aromatic compounds (such as phenol) to form dyes this is known as a coupling reaction. Diazonium dyes are commercially important. 

The stability of diazonium dyes arises from the extensively delocalised n bonding electron system. 

It is extensively used in the preparation of dyestuffs. Combines with diazonium salts to form oxyazo-colouring matters. Gives rise to fluorescein dyes on fusion with phthalic anhydride. Used for production of plasticizers, resins, adhesives. 

Azo-dye formation. Dissolve 2-3 drops of aniline in 1 ml. of cone. HCl and add 3 ml. of water. Shaike to dissolve any hydrochloride which may have separated and cool in ice. Add a few drops of 20% sodium nitrite solution. Add this cold diazonium solution to a cold solution of the phenol in an excess of aqueous NaOH solution. Solutions or precipitates of azo-dyes ranging in colour from orange through scarlet to dark red, according to the phenol used, are obtained. Note in particular that i-naphthol gives a brownish-red, 2-naphthol a scarlet precipitate. Catechol decomposes. 

Form diazonium compounds which couple with alkaline 2-naphthol to give red dyes (p. 339). 

Forms diazonium derivative which can be coupled with 2-naphthol or with dimethylaniline to form azo-dyes. 

Diazotisation. Dissolve 0 2 g. of the substance in about 5 ml. of dil. HCl, warming if necessary. Cool in ice-water and add sodium nitrite solution drop by drop the end of the diazotisation is marked by the complete decolorisation of the solution. Pour the diazonium solution into a cold solution of 2-naphthol in a considerable excess of NaOH solution a brilliant red dye is produced. 

The solution must be strongly acid in order to avoid the coupbng reaction between the undecomposed diazonium salt and the phenol (see under Azo Dyes). For the preparation of phenol and the cresols, the aqueous solution of the diazonium compound is warmed to about 50° at higher temperatures the reaction may become unduly vigorous and lead to appreciable quantities of tarry compounds... 

Some reference to the use of nitrous acid merits mention here. Primary aromatic amines yield diazonium compounds, which may be coupled with phenols to yield highly-coloured azo dyes (see Section IV,100,(iii)). Secondary aromatic amines afford nitroso compounds, which give Liebermann a nitroso reaction Section IV,100,(v). Tertiary aromatic amines, of the type of dimethylaniline, yield p-nitroso derivatives see Section IV,100,(vii). ... 

The diazonium salt of 2-aminothiazole couples with 2-dimethylamino-4-phenylthiazole, giving the corresponding azo dye (194) (Scheme 123) used for dyeing synthetic fibers (404). 

Coupling of the diazonium salts with phenols and amines forms the basis for manufacture of a number of commercial dyes. 

Dihydroxynaphthalene [83-56-7] behaves similarly to 1-naphthol coupling takes place mainly in the 4-position by simple diazonium compounds, and in the 2-position with diazophenols. Diazotized 2-arninophenol-4-sulfonic acid [98-37-3] couples with 1,5-dihydroxynaphthalene to produce the important mordant dye Diamond Black PV [2052-25-7] (see stmcture 53) (Cl Mordant Black 9 Cl 16500). 

Synthesis. Almost without exception, azo dyes ate made by diazotization of a primary aromatic amine followed by coupling of the resultant diazonium salt with an electron-rich nucleophile. The diazotization reaction is carried out by treating the primary aromatic amine with nitrous acid, normally generated in situ with hydrochloric acid and sodium nitrite. The nitrous acid nitrosates the amine to generate the N-nitroso compound, which tautomerizes to the diazo hydroxide. 

The alternative approach is to pad the fabric with the alkaline naphthol and dry, foUowed by printing directly onto this prepared fabric diazonium salts or stabilized diazonium salts. Coupling is instant and the only further treatment needed is to remove aU the uncoupled naphthol and surface azo pigment in a subsequent washing treatment. Because the choice of colors is limited from one naphthol component, other shades are obtained by using other classes of dye alongside the azoic colors, eg, reactives. This approach is widely used in the production of African prints. 

Some weak electrophilic reagents, which are usually inert toward azoles, also react with quaternized azoles. Diazonium salts yield phenylhydrazones (Scheme 48) in a reaction analogous to the Japp-Klingemann transformation of /S-keto esters into phenylhydrazones in the dithiolylium series illustrated the product has bicyclic character. Cyanine dye preparations fall under this heading (see also Section 4.02.1.6.5). Monomethine cyanines are formed by reaction with an iodo quaternary salt, e.g. Scheme 49. Tri- and penta-methinecar-bocyanines (384 n = 1 and 2, respectively) are obtained by the reaction of two molecules of a quaternary salt with one molecule of ethyl orthoformate (384 n = 1) or/S-ethoxyacrolein acetal (384 n =2), respectively. 

When treated with 4-dimethylaminobenzaldehyde, diazonium salts, or phthalic anhydride, these salts produced the corresponding styryl or azamethine dyes derived from l,2,3-thiadiazolo[4,5-/] or [5,4-/]quinolines. The Xmax of azamethine... 

Almost all dyes are quinones or azaquinones of compound 2. The majority are used for thermal transfer processes a recent example is compound 306 (98USP5792587). The second major class are azo dyes, prepared either from aromatic diazonium salts as in compound 307 (95GEP4319296) or from a triazolopyridine 3-diazonium salt as in 308 (81BRP2054630). 

Diazonium salts are important intermediates in organic synthesis, e.g. for the Sandmeyer reaction. The most important use is the coupling reaction with phenols or aromatic amines to yield azo dyes (see Diazo coupling). 

Arylamines are converted by diazotization with nitrous acid into arenediazonium salts, ArN2+ X-. The diazonio group can then be replaced by many other substituents in the Sandmeyer reaction to give a wide variety of substituted aromatic compounds. Aryl chlorides, bromides, iodides, and nitriles can be prepared from arenediazonium salts, as can arenes and phenols. In addition to their reactivity toward substitution reactions, diazonium salts undergo coupling with phenols and arylamines to give brightly colored azo dyes. 

Modified PAN fibres containing aldehyde groups can be used to obtain chemically stained fibres. The chemical addition of dyes can be conducted following two schemes A) The fibres of the above composition are treated with aromatic amines, e.g., l-amino-8-hydroxynaphthalene-3,6-disulfonic acid ( H-acid ), and then azocoupling is carried out with a diazonium salt ... 

Aromatic diazonium compounds became industrially very important after Griess (1866a) discovered in 1861/62 the azo coupling reaction, by which the first azo dye was made by C. A. Martius in 1865 (see review by Smith, 1907). This is still the most important industrial reaction of diazo compounds. Hantzsch and Traumann (1888) discovered that a heteroaromatic amine, namely 2-aminothiazole, can also be diazotized. Heteroaromatic diazonium compounds were, however, only used for azo dyes much later, to a small extent in the 1930 s, but intensively since the 1950 s (see Zollinger, 1991, Ch. 7). 

C-coupling is of outstanding importance in the azo coupling reaction for the synthesis of azo dyes and pigments. An aromatic or heteroaromatic diazonium ion reacts with the so-called coupling component, which can be an aromatic primary, secondary, or tertiary amine, a phenol, an enol of an open-chain, aromatic, or heteroaromatic carbonyl compound, or an activated methylene compound. These reactions at an sp2-hybridized carbon atom will be discussed in Chapter 12. In the... 

In another investigation (Loewenschuss et al., 1976) dediazoniation was studied in TFE and in acetonitrile in the presence of pyridine. There is UV and NMR evidence for the formation of a diazopyridinium cation in addition, -CIDNP absorption and emission signals were observed. Systems containing diazonium salts and pyridine are important in industrial chemistry, as pyridine is used as a proton acceptor in the diazo coupling reaction (see Sec. 12.8) in a considerable number of syntheses of azo dyes. At the same time pyridine has an unfavorable effect on the yield because of the competing homolytic dediazoniation. 

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