Phenols coupling with diazonium

Naphthols undergo the usual reactions of phenols. Coupling with diazonium salts is particularly important in dye manufacture (see Sec. 23.17) the orientation of this substitution is discussed in the following section. 

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 phenolic group of tyrosine undergoes iodina-tion (Eq. 3-44), acylation, coupling with diazonium compounds, and other reactions. 

As already mentioned, hydroxypyrazines exist in tautomeric equilibria with the corresponding pyrazinones which are normally the predominant species in the equilibria. Some of the reactions of hydroxypyrazines are reminiscent of those of phenols they can, for example, be coupled with diazonium salts and brominated and nitrated in either the ortho or para position to the hydroxyl group. Coupling with diazonium salts occurs in neutral or weakly alkaline solution, but if the reaction is carried out in 1 M sodium hydroxide solution, arylation of the pyrazine ring takes place. From hydroxy-pyrazine and benzenediazonium chloride 47% 2-hydroxy-3-phenyl-and 4% 2-hydroxy-3,6-diphenylpyrazine are obtained. 

Nitrogen s extra pair of electrons, which is responsible for the usual basicity of nitrogen compounds, is involved in the tt cloud, and is not available for sharing with acids. In contrast to most amines, therefore, pyrrole is an extremely weak base (ATj, -- 2.5 X 10 14). By the same token, there is a high electron density in the ring, which causes pyrrole to be extremely reactive toward electrophilic substitution it undergoes reactions like nitrosation and coupling with diazonium salts which are characteristic of only the most reactive benzene derivatives, phenols and amines. 

Like other aromatic compounds, these five-membered heterocycles undergJ nitration, halogenation, sulfonation, and Friedel-Crafts acylation. They are mucji more reactive than benzene, and resemble the most reactive benzene derivatives (amines and phenols) in undergoing such reactions as the Reimer-Tiemann reaction, nitrosation, and coupling with diazonium salts. 

The early work on this compound was confused by Pschorr s failure to recognize its phenolic nature. He noted the alkali-soluble nature of the methiodide and the production of a mono-acetyl derivative, but concluded, on account of the apparent insolubility of the parent base in alkali, that fission of the cyclic ether fink occurred during the reactions of the base with acetic anhydride and methyl iodide [7]. That /3-ethylthiocodide is indeed a phenol was demonstrated by Morris and Small [3] it gives coloured solutions with alcoholic ferric chloride and readily couples with diazonium salts in alkaline solution to give red dyes moreover it is soluble in alkali, though not readily so, being a very weakly acidic phenol. 

Phenol readily couples with diazonium salts to yield coloured compounds. The latter can be nsed for the photometric detection of phenol as in the case of diazotized 4-nitroaniline. Sahcylic acid (2-hydroxybenzoic acid) can be prodnced by the Kolbe-Schmitt reaction (stndied by the density functional method ) from sodinm phenolate and carbon dioxide, whereas potassium phenolate gives the para compound. Alkylation and acylation of phenol can be carried out with aluminium chloride as catalyst methyl groups can also be introduced by the Mannich reaction. Diaryl ethers can only be produced under extreme conditions. 

The fact that the one-pot synthesis of caUxarenes works best with p-f-butylphenoP was beneficial for the whole area, since the f-butyl groups are easUy removed by trans-butylation with AICI3 in toluene (as solvent and acceptor), converting calixarenes 2a-e into the p-unsubstituted calixarenes 2na-e. Thus, the p-positions at the wide rim are available for virtually all kinds of electrophilic substitution reactions which are possible with phenols . Sulphonation, nitration, bromination (or iodination), bromomethyla-tion, aminomethylation, formylation, acylation and coupling with diazonium salts are examples. 

Examples of the selective substitution of phenol units in partially O-alkylated (or 0-acylated) calixarenes (reaction c)) are the bromination , iodination, nitration , formylation , chloromethylation , alkylation and coupling with diazonium salts. ... 

Pyridones and pyrazolones, tautomers of the hydroxypyridines and hydroxypyrazoles, respectively, are phenol-like in behaviour and are sufficiently activated to couple with diazonium salts, giving commercially valuable azo dyes. 

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... 

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). ... 

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. 

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