Reactions Catalyzed by Micelles

Therefore Rufer studied an azo coupling reaction with uncharged coupling components, namely that of p-methoxybenzenediazonium ions with 1-naphthylamine and l-amino-2-methylnaphthalene, buffered to pH 7 at 25 °C. Both reactions are strongly catalyzed by sodium dodecylsulfate (up to 1100 times with l-amino-2-methyl-naphthalene) in cases where the surfactant concentration was higher than the critical micelle concentration (CMC). For the coupling reaction with 1-naphthylamine, both the o/p-ratio and the amount of the 2,4-bisphenylazo product are dependent on the sodium dodecylsulfate concentration. The formation of the 2,4-bisphenylazo com- 

2 For a detailed discussion of general base catalysis and kinetic hydrogen isotope effects in azo coupling reactions, see Section 4.3. The problem of competitive N-coupling of diazonium ions with aromatic amines and its implications on the mechanism shown in (45) is treated in Section 4.4. 

The catalytic effect can be explained by the increased concentrations of both reagents in the neighborhood of the micelles The diazonium ion is attracted by the opposite charge of the micelles and the naphthylamine by solubilization in the micelles. Rufer succeeded in treating these effects not only qualitatively, but also on a quantitative basis. 

Hashida and coworkers have studied the effects of cationic, anionic and nonionic surfactants on azo coupling reactions. By using diazonium ions 108-110) as well as coupUng components 111, 112) bearing a different overall charge, they could confirm the statement that micellar effects are governed by the formal charge of both reactants. 

If the two reacting species have opposite formal charge, the reaction rate is usually depressed by cationic as well as by anionic micellar reagents, as was shown previously This is due to the fact that either of the reagents will, for electrostatic reasons, be excluded from the micelle. This electrostatic impasse was successfully circumvented by functionalizing a cationic surfactant with an arenediazonium ion moiety The azo coupling with the micellar diazonium ion surfactant 113 was faster than the azo coupling with the model diazonium ion 114 by factors of 122 and 244, in reactions with 2-naphthol and with 2-naphthol-6-sulfonate, respectively. 

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