Phenolic compounds reaction with diazonium

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. 

Benzene and some of its derivatives react with solutions of mercuric nitrate in concentrated nitric acid to give nitrophenols. These reactions, known as oxynitrations may proceed by mercuration followed by nitroso-demercuration the resulting nitroso compound becomes a diazonium compound and then a phenol, which is nitrated. ... 

Reaction with arenediazonium salts Adding a phe nol to a solution of a diazonium salt formed from a primary aromatic amine leads to formation of an azo compound The reaction is carried out at a pH such that a significant portion of the phenol is pres ent as its phenoxide ion The diazonium ion acts as an electrophile toward the strongly activated ring of the phenoxide ion... 

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). 

Toluene from Toluidine.—It is often desirable to obtain tbe hydiocarbon from the base. The process of diazotisntion offers the only convenient method. The diazonium salt may be reduced by alcohol (Reaction 1, p. 162) or, as in the piesent instance, by sodium stannite. Less direct methods are the con-veision of the diazonium compound into (i) the hydrazine (see p. 174), (2) the acid and distillation with lime (p. 200), (3) the halogen derivative and reduction with sodium amalgam, 01, finally (4) the phenol and distillation with zinc dust. 

Methyl Orange.--The first point to notice in this reaction is that the diazonium salt forms no diazoamino-compotind with the dimethylanilinc, but at once pioduces an azo-compound. This is always the case with tertiary amines, some secondary amines like diphenyl.amine and the phenols. The reaction may be regarded as typical of the formation of all azo-colounng m.atters. At least two substances are requisite in this process on the one hand. an aromatic compound containing an amino-groujD in the nucleus, and, on the other, a base or phenol... 

The synthesized CPMV-alkyne 42 was subjected to the CuAAC reaction with 38. Due to the strong fluorescence of the cycloaddition product 43 as low as 0.5 nM, it could be detected without the interference of starting materials. TMV was initially subjected to an electrophilic substitution reaction at the ortho-position of the phenol ring of tyrosine-139 residues with diazonium salts to insert the alkyne functionality, giving derivative 44 [100]. The sequential CuAAC reaction was achieved with greatest efficiency yielding compound 45, and it was found that the TMV remained intact and stable throughout the reaction. 

At low acid concentrations, nitric oxide tends to form. This evidently may attack nitrosophenol to form diazonium compounds directly. The diazonium salts, in turn, may couple with unreacted phenol to give colored products. Nitrous acid may also produce nitrophenols from phenols. The mechanism of this reaction may involve oxidation of initially formed nitrosophenols, homolytic attack by nitrogen dioxide, or nucleophilic attack by nitrite ions [1]. 

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

Reaction CXXII. Action of Phenols and Aromatic Amines on Diazonium Compounds.—Diazonium compounds combine with phenols and aromatic bases to form azo dyestuffs. 

The reaction of diazo cation with phenolate yielding the azo dye may proceed through the formation of the diazo ether. Kekule came to this conclusion in 1870. Zollinger (1958), considering this conclusion, proposed and explained the mechanism by which the diazo ethers convert into the C-diazo compounds, that is, into the hydroxyazo dyes. The diazo ether preliminarily dissociates into the phenolate ion and the diazonium ion i.e., a two-stage intermolecular reaction takes place. The CIDNP effect suggests that the diazo ether may reversibly convert into the radical pair ... 

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... 

Reaction with Water Yields Phenols.—With water at ordinary or at raised temperatures diazonium salts readily decompose, the nitrogen is set free and the hydroxyl compound of the corresponding hydrocarbon is formed, i.e. the diazo group is entirely replaced by the hydroxyl group. Benzene diazonium chloride thus yields hydroxy benzene or phenol. 

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. 

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