A practical integrated approach to supramolecular chemistry III. Thermodynamics of inclusion phenomena

This is the third in a series of experiments aimed at introducing undergraduate students into the field of supramolecular chemistry. The first of these integrated works ( J. Chem. Educ.1999, 76, 419) allows measurement of the equilibrium constant of the formation of the inclusion complex of an azo-dye (guest molecule) in α-cyclodextrin (host molecule). In a second experiment ( J. Chem. Educ.1999, 76, 422), the kinetics of the formation of the inclusion complex was studied. In the present article, we propose an experiment aiming to familiarize students with the thermodynamics of inclusion phenomena. With this in mind, a spectrometric study of the inclusion of the azo-dye Biebrich Scarlet in α-cyclodextrin has been designed. A model implying that the guest molecule is buried within the cavity of α-cyclodextrin through a two-step mechanism is used: the dye (substrate, S) reacts with the α-cyclodextrin, C, in a rapid equilibrium to form the intermediate (CS)*, which then gradually forms the final complex, CS. This experiment should help students: (i) to determine the thermodynamic parameters corresponding to the inclusion reactions, (ii) to calculate some activation parameters, and (iii) to gain insight into the "driving forces" behind inclusion reactions. Additional results obtained with other azo-dyes, which might be useful in practical work with students, are also offered.