Micellar effects on nucleophilic substitutions in nonpolar solvents

Abstract

The rate constant for the decomposition of sodium 1,1-dimethoxy2,4,6-trinitrocyclohexadienylide (1), k[subscript obs], in benzene in the presence of dodecylammonium carboxyl ate aggregates is greater by a factor of 10� than that in pure benzene. Phosphatidyl ethanolamine, phosphatidyl choline, anionic micellar Aerosol-OT (Sodium bis( 2-ethylhexyl) sulfosuccinate) and nonionic micellar Igepal CO-530 (nonylphenoxy polyoxyethen (5) ethanol) also enhance the rate of decomposition of 1 in benzene. Decomposition of the methoxy adduct of 1-methoxy- 2,4 -dinitronaphthalene (2) is enhanced to a smaller extent than that of 1. The catalysis is dependent upon the concentration of 1, surfactant, added water and dimethyl sulfoxide; and it is explicable in terms of solubilization of 1 in the polar cavity of the micelle where breaking of the carbon-oxygen bond takes place. Nucleophilic displacement of 4-nitropyridine-N-oxide by sodium methoxide in benzene is enhanced by Igepal CO-530 but not by dodecylammonium propionate (DAP) and Aerosol-OT. The catalytic mechanism is rationalized by the reduced solvation of the substrate in the micellar cavity. 1-(β-Hydroxyethoxy)-2,4,6 -trinitrobenzene forms a Meisenheimer complex in benzene in the presence of aggregated DAP and Igepal CO-530. This complex is stable in Igepal CO-530 but readily decomposes in DAP. The presence of a small amount of water affects both the rate of formation and decomposition in DAP. Indicators, bromophenol blue and 2,4-, 2,5- and 2,6-dinitrophenol, have been used to measure the CMC's of DAP, Aerosol-OT and Igepal CO-530 in benzene. Observed CMC's agree well with those established by other methods. Apparent pKa values of malachite green in water solubilized by 0.10 M Igepal CO-530 in benzene is lower than that in bulk water. Increasing the Igepal CO-530 concentration results in a gradual lowering of the pKa value until it reaches a minimum. Apparent pKa values of malachite green, bromophenol blue, bromophenol red and dinitrophenols in micellar DAP solubilized water in benzene differ from those in bulk water. The difference in pKa values between these two systems p arallels the pKa values of these dyes in bulk water. The carboxyl ate group of DAP is observed to be protonated by DAP solubilized acid in benzene forming a mixed micelle. The significance of these results is discussed critically and comparatively.