ORGN 797 |
| Rate-surfactant concentration profiles for the alkaline hydrolysis of 2-(3-iodophenoxy)quinoxaline 1 depend on substrate concentration. At low substrate concentration (5.0 x 10-6 M), and in dilute aqueous solutions of cationic surfactants (C16H33NR3Cl, R = Me, n-Pr, and n-Bu), rate constants for the hydrolysis of 1 go through a single maximum with increasing surfactant concentration and with added NaCl. Second-order rate constants, kM, for reactions of 1 in the micellar pseudophase increase with head group size. With a ten-fold increase on substrate concentration, rate-surfactant profiles show double rate maxima. In the rate-surfactant profiles the first rate maximum is ascribed to the reaction mediated by submicellar aggregates and the second to reaction in micelles. Rates are larger in premicelles than in micelles. This result is consistent with a very strong interaction between substrate and surfactant in premicelles, and a relatively loose organization of substrate in the water-rich interfacial region of a cationic micelle. There are recent reports about the formation of premicellar aggregates and their ability to mediate reactions involving hydrophobic substrates. Premicelles have a significant rate effect, because if these assemblies were kinetically less effective than micelles, rates will increase monotonically to a single rate maximum as surfactant concentration increases and premicelles are converted into micelles. Reaction of 1 with OH- was also followed in solutions of nonmicellizing tri-n-octylalkylammonium mesylates (alkyl = Me, Et). The tetralkylammonium salt speeds the basic hydrolysis of 2-(3-iodophenoxy)quinoxaline 1. This result provides evidence of the existence of premicellar particles that are able to associate with the substrate and increase reaction rates. For the alkaline hydrolysis of compound 1 submicellar assemblies activate the substrate toward nucleophilic substitution but micelles assist reaction by concentrating both reactant in the small volume of the Stern layer.
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Total Synthesis, Materials, Molecular Recognition, Process R&D, and Physical Organic Chemistry
8:00 PM-10:00 PM, Wednesday, 13 September 2006 Moscone Center -- Hall D, Poster
Division of Organic Chemistry |
