Azo Coupling Reactions in Organic Solvents

Comjmred with the abundance of data dealing with azo coupling reactions in aqueous solutions, relatively few investigations deal with reactions in organic solvents. The first reports were simply stating that specific diazonium salts, which are soluble in benzene and in dipolar aprotic solvents can react with coupling components in these media. 

The solvent effects on azo coupling reactions with different coupling components were studied by Hashida and coworkers No large solvent effect was found 

The same conclusion was made in a kinetic study of solvent effects in reactions of benzenediazonium tetrafluoroborate with substituted phenols As expected due to the difference in solvation, the effect of the p-sul ituents is smaller in protic than in dipolar aprotic solvents. Ortho-alkyl substitution of phenol was found to increase the coupling rate, again as would be expected for electron releasing substituents. However, this rate acceleration was larger in protic than in dipolar aprotic solvents, since in the former case the anion solvation is much stronger to begin with, and therefore steric hindrance to solvation will have a larger effect  

Fischer and Zollinger have studied the reaction of p-chlorobenzenediazonium chloride with 2-naphthol-l-sulfonate. A very interesting dependence of the reaction products and reaction rate on the solvent was observed The initiaUy formed complex 102 decomposes under ring fragmentation in aqueous base (see (44) in Sect. 2.2.3) it does not react in water, ethanol or dimethylsulfoxide, reacts very slowly in acetone or glacial acetic acid to give a small amount of azo compound 7dJ, while in dichloromethane, chloroform, benzene or pyridine quantitative, though slow, formation of the azo compoimd is observed. Ba on kinetic measurements an Sisil-type mechanism was postulated for the formation of the azo dye 103 from the CT-complex 102 

In several investigations of the azo coupling of arenediazonium ions with aromatic amines in organic solvents general base catalysis and consequently a primary deuterium isotope effect were observed 

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