THERMODYNAMIC AND NMR-STUDY OF THE INTERACTIONS OF CYCLODEXTRINS WITHCYCLOHEXANE DERIVATIVES

Equilibrium constants and standard molar enthalpies of reaction have b een determined by titration calorimetry for a series of cyclohexane de rivatives (cis-1,2-cyclohexanediol, cis,cis-1,3,5-cyclohexanetriol, tr ans-1,2-cyclohexanediol, cyclohexanol, cyclohexanone, 2-methylcyclohex anone, 2,5-piperazinedione, and delta-valerolactam) with alpha-cyclode xtrin and beta-cyclodextrin. For the reactions involving cyclohexanol, standard molar heat-capacity changes were also determined from calori metric measurements performed at several temperatures. The equilibrium constants for the reactions of these substances with beta-cyclodextri n are in all cases larger than those for the corresponding reactions w ith alpha-cyclodextrin. It was also found that the equilibrium constan ts for the reactions of these substances with gamma-cyclodextrin were too small (<4) to measure with this method. An important feature of th ese reactions is that while the standard molar enthalpies of reaction are approximately the same for both cyclodextrins, the standard molar entropy changes for the reactions of these substances with beta-cyclod extrin are substantially more positive than for the reactions with alp ha-cyclodextrin. The hydrophilic nature of the groups on the cyclohexa ne ring as well as steric effects was found to influence the thermodyn amics of these reactions. The standard molar enthalpy of transfer of t he cyclohexane derivatives from the alpha-cyclodextrin complex to the beta-cyclodextrin complex was found to be a linear function of the sta ndard molar entropy change for this same transfer reaction. NMR result s indicate that while the proximity of these cyclohexane derivatives t o the walls of the alpha-cyclodextrin and that to the walls of the bet a-cyclodextrin are comparable, the cyclohexane derivatives penetrate d eeper into the larger beta-cyclodextrin cavity than into the smaller a lpha-cyclodextrin cavity.