Benzenediazonium salt, reduction

A comparison of the products from the four benzenediazonium salts makes it clear that an increase in the electrophilicity of the diazonium ion favors homolytic dediazoniation in borderline solvents. As discussed in Section 8.6, increased electrophilicity is accompanied by an increase in the reduction potential (Ei/2), which is a measure of the tendency to add an electron and form an arenediazenyl radical (Ar-N ). 

The quite negative reduction potentials of spin traps (Table 2) make them less amenable to participation in the radical anion mechanism, as first established in the cathodic reduction of benzenediazonium salts at a controlled potential in the presence of PBN (Bard et al., 1974). In fact, the lower cathodic limit of the spin trapping method is set not by the nitrone but by the spin adduct formed. 

Occasionally, the successful application of the Marcus expressions (5.35) and (5.37) to a reaction can support its designation as outer-sphere. The reduction of a series of substituted benzenediazonium salts by Fe(CN)5 and (Me5cp)2pe conforms to the simple Marcus expression and represents supporting evidence for the formulation of these reactions as outer sphere (or non-bonded electron transfer in organic systems)... 

The initial product, nitrosobenzene, is so easily reduced to N-phenylhydroxyl-amine that it has not been isolated from the reduction medium, but its presence has been established by reaction in solution with hydroxylamine to yield a benzenediazonium salt, which couples readily with 1-naphthylamine to form the dyestuff 2-phenylazo-1 -naphthylamine. 

It is reported that alkaline sodium hydrotulfite (Na2S204) is effective -in converting certain diazo compounds to the hydrocarbons,119 a statement supported solely by reference to the work of Grandmougin.130 Actually he obtained only a small yield of benzene from benzenediazonium salts but suggested that on further study the process might prove to be useful. Upon reduction with hydrosulfite, the diazo oxide XXIV is converted to the phenol XXV in 95% yield (crude product.)131... 

The electrochemical reduction of benzenediazonium salts has been shown to be a convenient source of free aryl radicals, which were used in several chemical reactions. 

Daasbjerg, K. and K. J. Sehested. Reduction of substituted benzenediazonium salts by solvated electrons in aqueous neutral solution studied by pulse radiolysis. J. Phys. 

In 1858 Peter Griess (in time taken from his duties in an English brewery) discovered diazonium salts. In 1875 Emil Fischer (at the University of Munich) found that reduction of benzenediazonium chloride by sulfur dioxide yields phenylhydrazine. Nine years later, in 1884, Fischer reported that the phenylhydrazine he had discovered could be used as a powerful tool in the study of carbohydrates. 

Numerous studies have now focussed on this technique of using diazonium salts for modifying electrode surfaces for a whole host of applications[9,57,58,63-65]. For example, Hong and Porter[66] have reported the electrochemical reduction of benzenediazonium tetrafluoroborate in acetonitrile containing tetrabutylammonium tetrafluoroborate to... 

Tin(n) chloride has often been used for reduction of arenediazonium salts in acid solution, a procedure that usually gives the tin double salt of the arylhydrazine hydrochloride, these salts being readily decomposed by alkali. This process is simpler than the sulfite method and is very suitable for laboratory purposes the reduction occurs at a low temperature (about 0°), but it fails with diazonium salts of the anthraquinone series, with / -nitrobenzene-diazonium chloride, and with o-benzenetetrazonium dichloride. Victor Meyer exemplifies the use of tin(n) chloride by the reduction of benzenediazonium chloride to phenylhydrazine.12... 

The following methods of reduction have also been recommended for conversion of arenediazonium salts into arylhydrazines Electrolytic reduction of benzenediazonium chloride has been carried out,15 giving a 100% yield of phenylhydrazinium chloride. Reduction with triphenylphosphine has been recommended16 as a general method of preparing arylhydrazines from arenediazonium salts. Good results were also obtained by using sulfur dioxide as reductant,17 and sodium dithionite in hydrochloric acid solution has also proved useful.18... 

Hydrazines.—The diazonium salts are converted into amines by active reduction if, however, the process is regulated, hydrazines are formed. When one mole of benzenediazonium chloride is treated with the amount of stannous chloride and hydrochloric acid equivalent in reducing power to four grams of hydrogen, phenylhydrazine hydrochloride is formed —... 

Substituted benzenediazonium tetrafluoroborates were reduced on Cu, Fe, Ti, Mo, Al, Zn and Ni electrodes in aqueous solution in the presence of acrylamide, in order to use the free radicals formed by the 1-electron mechanism as a polymerization initiator Only when Cu cathodes were used was a high degree of conversion of acrylamide achieved. With Fe cathodes, the polymerization occurred to a lesser extent and with the other metals it did not exceed 5-7%. No explanation was offered for this fact and a catalytic effect of Cu or Cu ions on the reduction of the diazonium salts cannot be excluded. The 4-methoxy and 4-methyl derivatives proved to be the most effective initiators with almost no polymerization being observed with the 4-nitro compound. These results were explained by a competitive reaction of the intermediate free radical with H2O. The occurrence of the polymerization by a radical mechanism was proved by using hydroquinone as an inhibitor, and this fact confirms the le mechanism for the electrochemical reduction of the diazonium salts. 

The electrochemical reduction of benzenediazonium chloride was also studied in the presence of unsaturated compounds like styrene, using Pt as cathode and Cu, Fe or Ti as anode. The main processes observed were addition of a phenyl and a chloro group to the double bond [PhCH(Cl)CH2Ph (I)] and additive dimerization [PhCH2CH(Ph)CH(Ph)-CH2Ph (II)]. These results are due to the catalytic effect of the cations formed by the anodic dissolution of the metal on the reaction between the diazonium salts and the unsaturated compound. The absence of products (I) and (II) when both electrodes were Pt confirms this redox catalysis. 

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