Aromatic coupling diazonium salt

In basic solutions, of an activated aromatic substrate, diazonium salts decompose and eouple to give biaryls (Ar-Ar).2 9 Reaction of benzenediazonium ion (398) and toluene gave a 67% yield of 411, along with 19% of 412 and 14% of 413. This result suggests the coupling is rather unselective. Selectivity is observed only when the substituent is part of the aryl diazonium ion, as in the eonversion of 414 to 415. ... 

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. 

When an aqueous solution of a diazonium salt is added to an alkaline solution of a phenol, coupling occurs with formation of an azo-compound (p. 188). If ho vc cr the ntiueous solution of the diazonium salt, t. . ., />-bromohenzene diazonium chloride, is mixed with an excess of an aromatic hydrocarbon, and aqueous sodium hydroxide then added to the vigorously stirred mixture, the diazotate which is formed, e.g., BrC,H N OH, dissolves in the hydrocarbon and there undergoes decomposition with the formation of nitrogen and two free radicals. The aryl free radical then reacts with the hydrocarbon to give a... 

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... 

A reaction of aryl diazonium salts that does not involve loss of nitrogen takes place when they react with phenols and arylamines Aryl diazonium ions are relatively weak elec trophiles but have sufficient reactivity to attack strongly activated aromatic rings The reaction is known as azo coupling two aryl groups are joined together by an azo (—N=N—) function... 

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). 

Other typical electrophilic aromatic substitution reactions—nitration (second entr-y), sul-fonation (fourth entry), and Friedel-Crafts alkylation and acylation (fifth and sixth entries)—take place readily and are synthetically useful. Phenols also undergo electrophilic substitution reactions that are limited to only the most active aromatic compounds these include nitrosation (third entry) and coupling with diazonium salts (seventh entry). 

The coupling of enamines with aromatic diazonium salts has been used for the syntheses of monoarylhydrazones of a-diketones (370,488-492) and a-ketoaldehydes (488,493). Cleavage of the initial enamine double bond and formation of the phenylhydrazone of acetone and acetophenone has been reported with the enamines of isobutyraldehyde and 2-phenylpropionalde-hyde. Rearrangement of the initial coupling product to the hydrazone tautomer is not possible in these examples. 

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. 

There are apparent exceptions to the rule that aromatic compounds are azo coupling components only if they contain a hydroxy or an amino group. A long time ago Meyer and Tochtermann (1921) demonstrated that the 2,4,6-trinitrobenzenedi-azonium salt couples with mesitylene, isodurene (1,2,3,5-tetramethylbenzene), and pentamethylbenzene (see also Smith and Paden, 1934). That result was surprising at the time, but today it is, of course, understandable the diazonium salt used is pro-... 

When the normally acidic solution of a diazonium salt is made alkaline, the aryl portion of the diazonium salt can couple with another aromatic ring. Known as the... 

Coupling of diazonium salts with aromatic rings... 

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. 

Diazonium salts react with various nucleophiles in water (Eq. 11.62).106 In acidic aqueous solution, p-pheny I e ncbis di azo ni um ion reacts with alcohols more rapidly than it does with water.107 In the presence of nucelophiles such as halides, the substitution products are obtained. Furthermore, diazonium salts of aromatic compounds are excellent substrates for palladium-catalyzed coupling reactions such as the Heck-type reactions in water. 

N-Alkylation of primary aromatic amines increases their nucleophilic character, making them couple much more readily, the introduction of the azo group occurring in the 4-position. Thus, in contrast to aniline, N-methylaniline couples readily and N,N-dimethyl-aniline very readily with simple diazonium salts. Diphenylamine also couples in the 4-position, but less readily than N-methylaniline. 

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. 

The Gomberg coupling reaction of aryl diazonium salts with arenes is catalysed by quaternary ammonium salts [49], Although yields are variable, they are generally >50% [49, 50]. When aromatic solvents other than benzene are used, the appropriate biaryls are produced, e.g. 4-chlorobenzenediazonium tetrafluoroborate reacts with chlorobenzene to produce the 2,4 -, 3,4 - and 4,4 -dichlorobiphenyls in a ca. 67 18 15 ratio. 

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... 

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