Diazonium compounds, coupling

The nitrous acid liberated at the anode reacts with the amine to form a diazonium compound, and this immediately condenses with the phenolic substance which is present to produce the azo-dye. Cooling is desirable though not so important as in the ordinary chemical method, since the diazonium compound couples with the phenol immediately it is formed but a diaphragm must be used to separate anode from cathode. Orange II, Congo red, dianisidine blue, and many other colours have been formed in this way. 

In producing the developed colours, the direct dyestuff is diazotized on tile fibre, and the diazonium compound couples with a suitable developer. With the coupled dyes the process is reversed. The cotton is dyed with a direct dye containing an amino or hydroxyl group and the dyed fibre is then treated with a solution of a diazotized base. An example is Benzo Para Deep Brown G (C.I. direct brown 152), (24),... 

Diazonium salts couple readily with aromatic primary amines, giving diazoamino compounds. If for instance an aqueous solution of aniline sulphate is diazotised with a deficiency of nitrous acid, only part of it is converted into benzenediazonium sulphate and the latter then couples with the unchanged aniline to give diazoaminobenzene. The reaction is carried out at the opti-CeHsNHj.HjSO + HONO = CbHsNjHSO, + zHaO... 

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

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). ... 

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). 

The reaction is very common in pyrazolone chemistry. Since alkoxypyrazoles and tautomerizable pyrazolones undergo this reaction and 3-pyrazolin-5-ones, like antipyrine, do not, it is assumed that the reaction takes place at C-4 of the OH tautomer. Pyrazolone diazo coupling is an important industrial reaction since the resulting azo derivatives are used as dyestuffs. For instance, tartrazine (Section 4.04.4.1.3) has been prepared this way. 3,5-Pyrazolidinediones react with aryldiazonium salts resulting in the introduction of a 4-arylazo group. As has been described in Section 4.04.2.1.4(v), diazonium salts couple in the 3-position with indazole to give azo compounds. 

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). 

The aromatic amino group is diazotized in the flrst reaction step. The diazonium compound so formed is then coupled with N-(l-naphthyl)-ethylenediamine to yield an azo dye. 

Titanium(III) chloride (particularly in slightly alkaline medium) reduces the p-nitro groups of the thiophosphate insecticides to amino groups, which are then reacted with nitrite in acid mediiun in a second step to yield a diazonium compound as intermediate. This is then coupled to N-(l-naphthyl)-ethylenediamine dihydrochloride to yield an azo dye [3]. In the case of benzodiazepines the first reaction step includes an additional acid hydrolysis to the corresponding benzophenone derivative [2]. 

In the presence of acids, sulfanilic acid — like other primary aromatic amines — reactt with nitrite to yield a diazonium compound that can couple with a suitable aromatic amine to yield an azo dye. 

Diazonium salts couple to hydroxy-substituted vicinal triazoles (101) with subsequent rearrangement of the hydroxy arylazo compounds (102) to the carbamoyl tetrazole (104).170 An open-chain intermediate (103) has been proposed.169 This rearrangement is similar to that of the benzoyl... 

The triazole derivative, in turn, produces an intensely colored substance, III, with a diazonium compound under the experimental conditions devised for this analysis. The reactions leading to the formation of the colored substance have not yet been fully elucidated, but it appears that opening of the triazole ring as well as coupling is involved. The chemistry of the color formation will be discussed in another publication. 

Diazo A family of reprographic processes (including Diazotype), based on the coupling of diazonium compounds with dye couplers to form colored compounds. Exposure of the dia-zonium compounds to near-ultraviolet radiation destroys them, so illuminated areas do not develop color. 

The production of azo pigments relies almost exclusively on the azo coupling reaction [1,2] to afford the azo group. Diazotization of an aromatic amine yields a diazonium compound, which subsequently reacts with a coupling component ( coupling ). 

In most cases, diazonium salts are unstable in the dry state and are sensitive to heat and impact. Since isolation is not necessary for azo pigment production, the diazonium compound is coupled with the coupling component as it is formed in solution or suspension. 

A coupling reaction is an electrophilic substitution of the diazonium compound with a nucleophilic partner (coupling component RH) ... 

This undesirable effect largely compromises the advantage that increased temperature has in accelerating a coupling reaction. Rather than risking decomposition of the diazonium compound by elevating the reaction temperature, it is therefore much more useful to increase the pH or the concentrations of the reactants in order to enhance the rate of a coupling reaction. 

The solution is then transferred into the coupling vessel equipped with a mechanical stirrer and, possibly in the presence of a surfactant, precipitated with acetic acid, hydrochloric acid, or phosphoric acid. The coupling component may also be precipitated indirectly i.e., the appropriate mixture of acid and emulsifier is filled into the kettle first and the alkaline solution of the coupling component is then added gradually to the clear solution by gravity flow. The clarified solution of the diazonium compound is then introduced into or onto this coupling suspension. 

Coupling reactions in organic solvents are occasionally carried out with masked diazonium compounds e.g., with special diazonium moieties which are incorporated into a larger organic structure [4], for instance in a diazoamino compound (15) or a benzotriazinone (16) ... 

The diazonium compound itself is liberated by adding a strong organic acid, such as halogenated acetic acid, after the coupling component has been introduced into the reaction mixture. 

Another variety of the continuous-coupling technique operates by transporting the coupling component suspension as a laminar flow upwards inside a vertical reaction tube. Portions of the diazonium compound, dissolved in an acidic aqueous medium, are added through appropriately located inlets in the walls of the reaction tube. The concentration of the added solution decreases as the reaction mixture flows upward and is designed to synchronize the uppermost inlet for the diazonium salt solution with the stoichiometric end point of the coupling reaction. 

Structurally, there are two basic types of disazo pigments, depending on whether the bifunctional element is introduced through the diazonium compound or through the coupling component. A bifunctional diazo component results in products of the type... 

In order to manufacture a diarylide yellow pigment, the diamine, dissolved in hydrochloric acid or sulfuric acid, is bisdiazotized with an aqueous sodium nitrite solution. The resulting diazonium compound is subsequently coupled onto two equivalents of acetoacetarylide. Since a material with a very fine particle size is needed for a fast and complete coupling reaction, the coupling component is prepared by dissolution in an alkaline medium and reprecipitation with an acid, such as acetic acid or hydrochloric acid. Dichlorobenzidine cannot be diazotized stepwise. A lack of sodium nitrite does not produce the monodiazonium compound, but an excess of 3,3 -dichlorodiaminodiphenyl remains. 

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