Diazonium salts reduction with sodium sulfite

The most feasible method consists in the reduction of diazonium salts with sodium sulfite. Although this method is given in several laboratory manuals, the results were not found entirely satisfactory. The present directions provide for a lengthy but essential heating of the diazonium-sulfite mixture, omit the useless zinc dust reduction, and supply exact details for preparation on a fairly large laboratory scale. 

Aryl hydrazines are formed by the reduction of diazonium salts with tin(II) chloride in hydrochloric acid (Scheme 8.24). The reduction can also be achieved by treatment with sodium sulfite solution. 

Copper(I) chloride is made by reduction of copper(II) sulfate with sodium sulfite (which is produced as required from the cheaper sodium bisulfite). The white solid is left covered with the reducing solution for protection against air oxidation until it is to be used and then dissolved in hydrochloric acid. On addition of the diazonium salt solution, a complex forms and rapidly decomposes to give p-chlorotoluene and nitrogen. The mixture is very discolored, but steam distillation leaves most of the impurities and all salts behind and gives material substantially pure except for the presence of a trace of yellow pigment, which can be eliminated by distillation of the dried oil. 

It should be pointed out that in an attempt to prepare 3-(2-hydrazinophenyl) quinoxalin-2(l//)-one 157 by reduction of corresponding diazonium salt from 3-(2-aminophenyl)quinoxalin-2(lfl)-ones 53 with sodium sulfite, instead of the intended product 157 a compound with the molecular formula C14H10N4O has been isolated, which the authors (Wiedermannova et al. 2003) considered to be 6H-quinoxalino[l,6-c][l,2,3]benzotriazin-13(12fl)-one 158. However, later (LuCka et al. 2007) it was proved that the structure proposed by the authors (Wiedermannova et al. 2003) was incorrect. By NMR spectroscopy (Lucka et al. 2007) and by X-ray analysis (Frysova et al. 2011) it was shown that the correct stmcture is l-(l//-imidazol-3-yl)-l//-benzimidazol-2(3fT)-one 155 (Scheme 6.67). 

Phenylhydrazine has been prepared by the reduction of benzene diazonium salts with sulfites 1 by the reduction of benzene diazonium chloride with stannous chloride 2 by the reduction of benzene diazonium hydrate with zinc or sulfur dioxide 3 by the reduction of sodium benzene diazotate with sodium stannite 4 by the reduction of diazoamino benzene 5 by the reduction of nitrosophenyl hydroxylamine or its methyl ether 6 and by the action of hydrazine hydrate on phenol.7... 

Conversion of diazonium salts to hydrazines was achieved by reduction with zinc in acidic medium (yields 85-90%) [59<5], with stannous chloride (yield 70%) [184, and sodium sulfite (yields 70-80%) [253,597]. Nitro groups present in the diazonium salts survive the reduction unharmed [184,253],... 

Isocyanates and isothiocyanates are reduced to methylamines on treatment with LiAlH4. LiAIH4 does not usually reduce azo compounds622 (indeed these are the products from LiAIH4 reduction of nitro compounds, 9-67), but these can be reduced to hydrazo compounds by catalytic hydrogenation or with diimide623 (see 5-9). Diazonium salts are reduced to hydrazines by sodium sulfite. This reaction probably has a nucleophilic mechanism.624... 

The reduction of diazonium salts by sodium sulfite forms monosub-stituted arylhydrazines. An improved procedure for the synthesis of phenylhydrazine in 84% yield is typical. Arylhydrazine salts substituted in the nucleus with halo," ether, carboxyl, or nitro groups have been prepared. The free bases are liberated from the salts by the action of aqueous sodium hydroxide or sodium acetate. 

This reaction was initially reported by Fischer in 1875. It is a synthesis of phenylhydrazine involving the diazotization of aniline, the reduction of the resulting diazonium salt with an excess amount of sodium sulfite, and the hydrolysis of phenylhydrazine sulfonic acid salt with hydrochloric acid. In addition, the diazonium salt can also be reduced by zinc dust in acetic acid. This process has become a standard industrial method for the production of arylhydrazines. 

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