Diazonium ions phenols from

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... 

Phenols from Diazonium Ion Intermediates. Aryl diazonium ions can be converted to phenols by heating in water. Under these conditions, there is probably formation of a phenyl cation. 

Various diazonium salts have been prepared using this approach55 and thermoanalytically characterized.56 Both the structure of the diazonium moiety and the counterion clearly influence the stability of the diazonium moiety. The thermally stable diazonium ion 71 (Z = Cl, Y = CH20) [ty2 (25°) > 100 days] is also capable of scavenging various nucleophiles (amines, phenols, and anilines).57 This resin with a tetrafluoroborate counterion (resin 66) is called T2 diazonium resin and it is now commercially available from Novabiochem. 

Radical arylations of phenols differ in some respects from those of phenolates (Scheme 37). First, the decreased nucleophilicity of the phenol, such as 100, allows the use of unmasked aryl diazonium chlorides 101 as radical sources. Given that an efficient reductant is present in the reaction mixture and that the diazonium salt is added slowly, biphenyl alcohols 102 can be prepared in moderate to good yields [153,154]. In this way, the concentration of the salt 101 is kept low at any time and homocoupling reactions (addition of the aryl radical to diazonium ions) as well as azo coupling to the phenol 100 can be successfully overcome. 

The formation of this product of a consecutive disubstitution is apparently not easy to understand. It is known that an arylazo residue diecreases the reactivity of a phenol or a naphthol for substitution by a second diazonium ion by 3 to 5 orders of ten. In phenols and naphthol the favorable effect of promoting the dissociation of a proton from the substrate does not work it was found only for C-acids (see discussion in the preceding Sect. 4.1). 

There have been a number of studies of the reaction of diazoacetic ester in aprotic solvents, mainly with carboxylic acids (Bronsted and Bell, 1931 Hartman et al., 1946 and references cited). However, the information available hardly justifies conclusions about the mechanism. Addition of relatively basic phenols causes an acceleration in rate which can be interpreted in terms of nucleophilic catalysis of a rate-determining displacement of nitrogen, but the kinetic order in acid varies between one and two. Formally, a mixed order would result if proton loss from the diazonium ion was effected by carboxylate ions alone, while the less discriminating displacement of nitrogen involved competition between anions and unionized molecules. However, there are examples of high or mixed orders in other acid-catalysed reactions (Bronsted and Bell, 1931 Bell, 1941 1959) and in all probability large medium effects play a role. 

Cohen, T., Dietz, A. G., Jr., Miser, J. R. A simple preparation of phenols from diazonium ions via the generation and oxidation of aryl radicals by copper salts. J. Org. Chem. 1977,42, 2053-2058. 

The determination of phenols according to VDI3485, Part 1 (1988) is a sum method. The phenolic compounds react on diazotized p-nitroaniline to produce an azo dye (see Eq. 7). In the first step of the reaction, a diazonium ion is formed from p-nitroaniline and sodium nitrite in acidic solution. This diazonium ion is a strong electrophilic reagent and attacks phenol in the para position with formation of an azo coupling by substitution. 

SP (Vis) AA accelerates dediazoniation of diazonium ions. The derivatization product is monitoring from coupling unreacted diazonium ion with phenol to give an azo dye... 

Photodecomposition of some ortfeo-substituted aryldiazonium ions leads to unusual reaction products due to hydrogen atom transfer between the ortho-substituent and the radical center derived by loss of nitrogen from the diazonium ion. Thus, 2-diethylaminobenzenediazonium salts yield N-ethylaniline via the intermediate 3. Deuterium is not incorporated into the benzene ring from either DjO or CD3OD.2 Decomposition of the diazonium salt 4 leads to a benzocyclobutene as the principal product. The influence of the nitro substituent is important, because the un-nitrated diazonium salt is decomposed to give a phenol in the normal manner. ... 

The phenanthrene 1,2- and 3,4-diones are synthetically accessible from the related 8 phenols. Oxidation of 2-phenanthrol with either Fremy s salt ((KS0 )2N0) or phenylseleninic anhydride gave phenanthrene 1,2-dione directly (55). Unexpectedly, oxidation of 3-phenanthrol with (KSOg NO yielded 2,2-dihydroxybenz(e)indan-l,3-d-ione (Figure 10). However, phenanthrene 3,4-dione was readily obtained from 3-phenanthrol by Fieser s method entailing diazonium coupling, reduction, and oxidation of the resulting 4-amino-3-phen-anthrol with chromic acid (56). 

Apart from complex formation involving metal ions (as discussed in Chapter 4), crown ethers have been shown to associate with a variety of both charged and uncharged guest molecules. Typical guests include ammonium salts, the guanidinium ion, diazonium salts, water, alcohols, amines, molecular halogens, substituted hydrazines, p-toluene sulfonic acid, phenols, thiols and nitriles. 

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]. 

Treatment of an aromatic amine with nitrous acid gives dia-zonium compounds. These couple with phenols to give dyes. Displacement of nitrogen from a diazonium compound by a halide or cyanide ion takes place with copper catalysis and is a useful synthetic method. 

Various aromatic amines, phenols, and compounds containing active methylene groups can be titrated with arenediazonium salts, from which 4-bromo-l-naphthale-nediazonium chloride seems to be the most widely applicable titrant. Compounds that react slowly with arenediazonium salts can be determined by back-titration when the excess of arenediazonium salt is back-titrated with either sodium tetraphenylborate or 2,4-diaminotoluene. Indirect determination is useful for secondary amines, which react with arenediazonium ions to form triazenes. The determination of diazonium salts of ampholytic character is based on the reaction of these salts with l-phenyl-3-methyl-5-pyrazolone, the excess of which is titrated with 4-bromo-l-naphthalenediazonium chloride solution. 

The preparation of phenols by hydrolysis of benzediazonium ions is well known in the literature (ref. 10). It involves the preparation of a diazonium salt, e.g. the diazonium hydrogenosulfate by treatment of the aniline hydrogen sulphate with sodium nitrite in dilute aqueous sulphuric acid, followed by hydrolysis in more concentrated aqueous sulphuric acid. The temperature of the hydrolysis is maintained at the boiling point of the aqueous acid by proper adjustment of the concentration of the sulphuric acid and the phenol formed removed from the reaction medium by means of steam distillation in order to minimise the coupling of the formed phenol with the diazonium salt introduced. 

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