Inclusion effects of cyclomalthoexa- and heptaose (α- and β-cyclodextrins) on the acidities of several phenol derivatives

By means of spectrophotometry, equilibrium constants for the formation of I:1 inclusion complexes of cyclomaltohexaose (alpha-cyclodextrin, alpha-CD) or cyclomaltoheptaose(beta-CD) in aqueous solutions have been evaluated for neutral and anionic species of 3-cyanophenol, 4-cyanophenol, 3-nitrophenol, 4-nitrophenol, 4-bromophenol, and 4-methoxyphenol. Using the equilibrium constants of the neutral and anionic species, pK(a), values have been determined for the phenols bound to the alpha- and beta-CD cavities. These phenols, which are accommodated in the alpha-CD cavity, have been found to be stronger acids than the free, uncomplexed ones, except for Lt-methoxyphenol. On the other hand, 4-cyanophenol, 3-nitrophenol, and 3-methoxyphenol bound to the beta-CD cavity are weaker acids than the uncomplexed ones, although 3-cyanophenol, 4-nitrophenol, and 4-bromophenol bound to beta-CD show the same trend as those bound to alpha-CD. The different influences of alpha- and beta-CDs On the pK(a), values are likely due to the difference in the magnitudes of the induced dipole moments of the guest caused by alpha- and beta-CDs; depending on the magnitude of the induced dipole moment, the inclusion complexes are stabilized through the dipole-dipole interaction between the host and guest. (C) 1997 Elsevier Science Ltd. All rights reserved.