ELECTROSTATIC SEPARATION OF REACTANTS WITHIN A MACROSCOPICALLY HOMOGENEOUS SOLUTION: AN INTEGRATED PHYSICAL ORGANIC CHEMISTRY EXPERIMENT
Part of the efforts being made in developing the physical chemistry curriculum focuses in the improvement of experiments for the physical chemistry laboratory, which should better reflect the range of activities found in current physical chemistry research (1). This is reflected, in part, by the inclusion of modern instrumentation and computational methods but also by the choice of topics. For instance, integrated laboratories related with environmental interests, biological or supramolecular systems, etc., are thought to be closely models the way that real (or daily) chemistry is done, providing stronger motivation for students by the believe that this type of integrated approach of interest in.
Here we report on an integrated physical-organic chemistry experiment, suitable for undergraduate students taking kinetic or catalysis courses, aimed to attract their attention to altering chemical reactivity by employing surfactant solutions. The physical organic chemistry experiment proposed involves a kinetic study of the formation of an azo dye, obtained from coupling an arenediazonium ion, ArN2+, with a naphthoxide ion (R-O-) in the presence of sodium dodecyl sulfate (SDS) micelles.
Surfactant solutions may spontaneously form aggregates creating a highly anisotropic interfacial region, which forms the boundary between the highly polar aqueous and nonpolar oil regions, imparting new chemical and physical properties to the system. Micelles, as well as other association colloids, are dynamic aggregates of amphiphilic molecules that are composed of nonpolar hydrocarbon tails attached to polar, non ionic, zwitterionic, or ionic head groups.
Micelles may act as microreactors concentrating, separating or diluting reactants and thereby they may have dramatic effects on chemical reactivity. Since the surface of SDS micelles is negatively charged, addition of cationic (ArN2+) and anionic (R-O-) reactants results in a micellar induced separation of those chemicals within a macroscopically homogeneous solution and therefore SDS micelles inhibit the coupling reaction. The association constant between the diazonium salt and the SDS micelle is estimated from the observed coupling rate constants in the presence of different amounts of SDS.
Discussion on their utility in controlling chemical reactivity and their advantages and inconveniences will be presented. Alternative experiments specifically designed for undergraduate students taking introductory courses of chemical kinetics are proposed.
1) Abrash, S. A. “Modern Development in the Physical Chemistry Laboratory”, in “Advances in TEaching Physical Chemistry”, Ellison, M. D., Schoolcraft, T. A., American Chemical Society, 2008