Diazonium salts, amine reactions coupling

Salts of primary aromatic amines react with nitrous acid to produce diazonium salts. The reaction is usually performed by adding a cold solution of sodium nitrite to a cold solution of the arylamine in aqueous mineral acid. The end point of the reaction is conveniently determined by the detection of excess nitrous acid with potassium iodide-starch paper. Sulfamic acid has long been used both in industry and in the laboratory to remove excess nitrous acid. It has been found to react with the more active diazo compounds. In most cases, high temperatures are avoided to prevent the formation of phenols and the decomposition of the unstable nitrous acid. An excess of mineral acid is necessary to prevent coupling between the diazonium salt and unreacted amine (cf. method 494). If the amine salt is somewhat insoluble, a fine crystalline form, which is produced by rapid crystallization from a warm aqueous solution, may be employed. ... 

There are many variations, and a wide range of aromatic diazonium salts can be used. One method for generating the diazonium salt is treatment of an aromatic amine with nitrous acid (HONO via reaction of HNO3/HCI or NaN02/2 HCl).i62 These diazonium salts can be coupled to other aromatic compounds in the presence of base in what is commonly called the Gomberg-Bachmann reaction. 63 Treatment of 4-bromo-aniline with nitrous acid, for example, gave the diazonium salt (231) which reacted with benzene to give the... 

The most representative example of this type of reaction is the so-called Griess reaction, originally developed for the kinetic determination of nitrite. Nitrite in a weakly acidic solution reacts with an amine (usually sulfanilic acid) to form the corresponding diazonium salt, which then couples with a naphthylamine or naphthol. This reaction can also be used for the kinetic determination of nitrate after reduction in a column, as well as for measuring other species such as sulfonamides (Bratton-Marshall reaction), and benzodiazepines and N-methylcarba-mate pesticides, the hydrolysis of which yields the corresponding benzophenones (which can be diazoti-zed) and naphthols (useful for the coupling reaction), respectively. 

The intermediate azo carbonic acid can be prepared under typical aqueous dia-zotisation and coupling conditions and can be readily isolated. However, because of the absence of solubilizing groups on the amine and the coupling component, preparation of the diazonium salt and subsequent coupling can be difficult Therefore these reactions are preferably carried out in organic solvents such as... 

The most important color reaction of diazonium salts is their coupling with a suitable passive component, i.e., with a phenol in alkaline medium, or with an aromatic amine in acid medium (see p. 341). Among phenols, resorcinol and 2-naphthol or R-salt are suitable passive components among amines, 1-naphthylamine or N-l-naphthylethylenediamine are suitable. 

The most important reaction of the diazonium salts is the condensation with phenols or aromatic amines to form the intensely coloured azo compounds. The phenol or amine is called the secondary component, and the process of coupling with a diazonium salt is the basis of manufacture of all the azo dyestuffs. The entering azo group goes into the p-position of the benzene ring if this is free, otherwise it takes up the o-position, e.g. diazotized aniline coupled with phenol gives benzeneazophenol. When only half a molecular proportion of nitrous acid is used in the diazotization of an aromatic amine a diazo-amino compound is formed. 

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... 

The most noteworthy reaction of azo-compounds is their behaviour on reduction. Prolonged reduction first saturates the azo group, giving the hydrazo derivative (C NH-NH C), and then breaks the NH NH linkage, with the formation of two primary amine molecules. If method (1) has been employed to prepare the azo-compound, these two primary amines will therefore be respectively (a) the original amine from which the diazonium salt was prepared, and (6) the amino derivative of the amine or phenol with which the diazonium salt was coupled. For example, amino-azobenzene on complete reduction gives one equivalent of aniline, and one of p-phenylene diamine, NHaCeH NH benzene-azo-2-naphthoI similarly gives one equivalent of aniline and one of... 

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. 

A diazonium salt is a weak electrophile, and thus reacts only with highly electron-rich species such as amino and hydroxy compounds. Even hydroxy compounds must be ionized for reaction to occur. Consequendy, hydroxy compounds such as phenols and naphthols are coupled in an alkaline medium (pH > of phenol or naphthol typically pH 7—11), whereas aromatic amines such as N,N diaLkylamines are coupled in a slightly acid medium, typically pH 1—5. This provides optimum stabiUty for the dia2onium salt (stable in acid) without deactivating the nucleophile (protonation of the amine). 

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). 

The reactions of arenediazonium ions with 7V-alkyl- or 7V-arylhydroxylamines were investigated by Bamberger (1920b, and earlier papers). Mitsuhashi et al. (1965) showed that the l,3-diaryl-3-hydroxytriazenes are tautomeric with 1,3-diaryltriazene-3-oxides (Scheme 6-16). Oxidation of 1,3-diaryltriazenes with peroxybenzoic acid in ether yields the same product as that from diazonium salts and TV-arylhydroxyl-amine. The infrared spectrum of the product obtained by coupling diazotized relabeled aniline with A/-phenylhydroxylamine indicates that the diaryltriazene-oxide is the preponderant tautomer. 

Coupling reactions with diazonium salts to yield intensely colored azo derivatives have often been used for the detection of phenols, primary aromatic amines and electron-rich heterocyclics. 

A sample of the reaction mixture should not become turbid when sodium acetate solution is added. But if now a few drops of a solution of an aniline salt are poured in, there is precipitated yellow diazoaminobenzene, which can be re-dissolved with concentrated hydrochloric acid after a few small pieces of ice have been added. Further, when a few particles of j8-naphthol or of R-acid are dissolved in a small excess of 2 A-sodium hydroxide solution and a sample of the diazo-solution is added, an intense red colour is produced. The development of this colour, which results from coupling , is an infallible test for a diazonium salt, and hence also for the corresponding primary aromatic amine. 

Accordingly, many reactions can be performed on the sidewalls of the CNTs, such as halogenation, hydrogenation, radical, electrophilic and nucleophilic additions, and so on [25, 37, 39, 42-44]. Exhaustively explored examples are the nitrene cycloaddition, the 1,3-dipolar cycloaddition reaction (with azomethinylides), radical additions using diazonium salts or radical addition of aromatic/phenyl primary amines. The aryl diazonium reduction can be performed by electrochemical means by forming a phenyl radical (by the extrusion of N2) that couples to a double bond [44]. Similarly, electrochemical oxidation of aromatic or aliphatic primary amines yields an amine radical that can be added to the double bond on the carbon surface. The direct covalent attachment of functional moieties to the sidewalls strongly enhances the solubility of the nanotubes in solvents and can also be tailored for different... 

Perhaps the best-known method of preparing aromatic azo compounds involves the coupling of diazonium salts with sufficiently reactive aromatic compounds such as phenols, aromatic amines, phenyl ethers, the related naphthalene compounds, and even sufficiently reactive aromatic hydrocarbons. Generally, the coupling must be carried out in media which are neutral or slightly basic or which are buffered in the appropriate pH range. The reaction may also be carried out in nonaqueous media. While some primary and secondary aromatic amines initially form an A-azoamine, which may rearrange to the more usual amino-C-azo compound, tertiary amines couple in a normal manner. 

Thiadiazoles amination, 6, 562 diazonium salts coupling reactions, 6, 484 electrophilic attack at carbon, 5, 56... 

The diazonium sulphate is used in preference to the diazonium chloride, since the presence of chloride ions gives rise to small quantities of the aryl chloride as a by-product. The solution must be acidic in order to avoid the coupling reaction between unreacted diazonium salt and the phenol (see Section 6.7.2, p. 946). For the preparation of phenols and cresols, the aqueous solution of the diazonium compound is warmed to about 50 °C at higher temperatures the reaction may become unduly vigorous and lead to appreciable quantities of tarry compounds. For certain substituted amines, a higher temperature (e.g. boiling 40 60% sulphuric acid) is necessary to decompose the diazonium salt... 

Azo compounds are prepared by the interaction of a diazonium salt with a phenol in the presence of sodium hydroxide or with an amine in the presence of sodium acetate. The coupling reaction is an electrophilic substitution involving the diazonium ion which reacts at the position of greatest electron availability, i.e. the position ortho or para to the electron releasing phenoxy or amino groups. 

Coupling Components With Condensed Cyclic Ammo nium Residues. Heterocyclic compounds in which the condensed benzene ring is substituted by a hydroxyl or an amino group can be coupled with diazonium compounds and may also be quatemized, either prior or subsequent to the coupling reaction, to yield cationic azo dyes. l,2-Dialkyl-6-nitroindazolium salts are reduced to the 6-amino compounds and then coupled with diazonium salts of aromatic amines. These dyes (e g., 21) color polyacrylonitrile in bright yellow to orange shades [64],... 

Aryldiazonium salts are weak electrophiles. Consequently, they undergo Ar-SE reactions via sigma complexes (azo couplings) only with the most strongly activated aromatic compounds. Only phenolates and secondary and tertiary aromatic amines react with them. Primary aromatic amines react with diazonium salts, too, but via their N atom. Thus, triazenes, that is, compounds with the structure Ar—N=N—NH—Ar are produced. Phenol ethers or nonde-protonated phenols can react with aryldiazonium salts only when the latter are especially good... 

Another common laboratory reaction of amines is diazotization to provide unstable and highly reactive diazonium salts. Plimmer et al (14) have isolated an aromatic triazene (XV) from soil containing 3,4-dichloroaniline (XIV) and presented evidence that it is formed by "natural diazotization of the aniline followed by coupling with a second amine molecule (Fig. 5). If this is true— that the natural nitrite commonly found in soil and water can bring about diazotization—a new dimension must be added to both the natural mechanisms of herbicide degradation and the generation of new series of potentially dangerous transformation products. 

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