Decomposition, diazonium salt to phenol

Phenols. Cohen and co-workers have described complete details for conversion of diazonium salts to phenols by a redox procedure first published in 1967. The classical method involves thermal decomposition in a strongly acidic medium and involves a cationic intermediate. In the newer method the diazonium... 

Modifications of the classical method are mainly concerned with the decomposition of the diazonium salt. Thermal [74], photochemical [75], and electrochemical [76] processes were found to be effective for the generation of the radical intermediate. Ti(III)-catalyzed Pschorr reaction of aryl diazonium salts and phenols gave symmetrical biaryls in excellent yield [77]. 

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... 

Because of the small concentration of the 2 1 complex the last term can be ignored. From the extreme rate values in the absence of zinc and with an excess of zinc, 2i and 22 are determined as 2.4 X 104 min.-1 and 1.57 min.-1 respectively. These values can be combined with the trend in the rate constants to give the stability constant of the reactive complex, presumably Zn(OR)(OAc), as 3 X 107. For the simple zinc complex in water the literature values of the stability constant for the 1 1 complex vary from 2.5 X 108 to 6.3 X 108. The diazo coupling reaction of the complex indicates the smaller effect of coordination vis a vis protonation since this reaction is very sensitive to such effects and does not proceed with phenols. Unfortunately the choice of cations for such a reaction is restricted since the cation should not interfere with the analytical methods used to obtain the kinetic data nor should it introduce additional reactions such as occur with transition metal cations which can catalyze the decomposition of the diazonium salt via a redox process. 

The decomposition of arenediazonium tetrafluoroborates RC6H4N2 + BF4 (R = Me, OMe, Cl, Br, N02, C02R, OPh) in aqueous solutions (pH 1-7) which gives phenols and phenyl-diazenylphenols is first order with respect to the concentration of the diazonium salt.246 At lower pH, the rates (k) are constant but above a certain pH value, k increases with pH linearly. Correlations with the Hammett constants er have been proposed. 

The published research on the photochemical decomposition of di-azonium salts suggests that the two processes, a heterocyclic and a homolytic process, analogous to those of the thermal decomposition may occur. Various workers 36 187 have reported that phenols are formed when diazonium salts are photolyzed in water and aryl ethers result when an alcohol replaces water as the solvent. Homer and Stohr122 report that a process analogous to reductive deamination occurs in preference to ether formation results in alcohols. The importance of free radical intermediates in the photodecomposition, based on magnetic susceptibility measurements, has been stressed.25 Lee and his co-workers171 have recently suggested that in ethanol the photodecomposition of a diazonium salt occurs via a radical intermediate while in water an ionic process predominates. Thus, photodecomposition of a nitrobenzene diazonium chloride in water yielded both a nitrophenol and a chloronitrobenzene in ethanol, on the other hand, the major product of photolysis was the reduction product, nitrobenzene. 

It appears, therefore, that in this case the primary decomposition product is phenol and that by coupling with undecomposed diazonium salt it is converted into the azophenol. Addition of zinc oxide to the reaction mixture raises the yield of phenol to 60% while that of the azo compound falls to approximately 30%. 

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. ... 

Primary aromatic amines, such as aniline (phenylamine) forms a more stable diazonium ion at 0-5°C. Above 5°C, it will decompose to give phenol and N2. Diazonium salts can be isolated in the crystalline form but are usually used in solution and immediately after preparation, due to rapid decomposition on standing even with little ambient heat. Solid diazonium salts can be explosive on shock or on mild warming. 

A famous example of a reaction which must be kept cold is the diazotization of anilines to make diazonium salts. The reaction involves treating the amine with nitrous acid (HONO) made from NaN02 and HCl. You need not think about the mechanism at this stage— you will meet it in Chapter 22—but the key point is that the product is a rather unstable but very useful diazonium salt. The diazotization takes place readily at room temperature, but unfortunately so does the decomposition of the product to give a phenol. By lowering the temperature, we supply insufficient energy for the phenol formation, but the diazotization still works just fine. 

Coupling to phenols, naphthols, and related hydroxyl compounds is carried out in alkaline solution. Under alkaline conditions the hydroxy compound is soluble, and coupling usually is rapid. Ordinarily the coupling must be carried out in the cold to prevent decomposition of the diazonium salt. 

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. ... 

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