Micellar Effects on the Reaction between an Arenediazonium Salt and 6-O-Octanoyl-L-ascorbic Acid.Kinetics and Mechanism of the Reaction

摘要

The effects of 6-O-octanoyl-L-ascorbic acid,VC8,on the reaction with 3-methylbenzenediazonium,3MBD,tetrafluoroborate were investigated in the absence and in the presence of sodium dodecyl sulfate,SDS,cetyltrimethylammonium bromide,CTAB,and tetradecyltrimethylammonium bromide,TTAB,micelles at different pHs by employing a combination of UV-vis spectroscopy,high-performance liquid chroma-tography,HPLC,and differential pulse polarography techniques.VC8 behaves as a typical surfactant in aqueous solution giving rise to micellar aggregates at VC8 concentrations above its critical micelle concentration(cmc).This behavior is reflected in the variation of the observed rate constant,kappa_obs,with[VC8];&obs values increase smoothly upon increasing [VC8] up to a breakpoint at[VC8]approx=8X10~(-3)M after which further addition of VC8 makes &0bs values increase sharply.Polarographic titration of VC8 with 3MBD shows that the reaction between 3MBD and VC8 takes place through an inner sphere mechanism leading to the formation of an unstable Z-diazo ether intermediate.All evidence is consistent with a competitive reaction mechanism,that is,the thermal D_N+A_N dediazoniation and a rate-limiting decomposition of the Z-diazo ether "complex" formed from reaction between 3MBD and VC8~ ions in a rapid pre-equilibrium step.In presence of SDS,CTAB,or TTAB,at fixed pH and [VC8]cmc,kappa_obs values increase upon increasing [VC8] up to a maximum after which further addition of surfactant leads to a sharp(SDS)or smooth(CTAB,TTAB)decrease in kappa_obs values.Results are consistent with the predictions of the pseudophase model and are rationalized in terms of micellar-induced concentration-dilution effects(SDS)and reactant and coion(3MBD)incorporation into the micellar Stern layer of CTAB and TTAB micelles.HPLC analyses of the reaction mixtures in the presence of added surfactants show a significant micellar effect on product distribution.