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

By means of spectrophotometry, equilibrium constants for the formation of 1:1 inclusion complexes of cyclomaltohexaose (α-cyclodextrin, α-CD) or cyclomaltoheptaose (β-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, pKa values have been determined for the phenols bound to the α- and β-CD cavities. These phenols, which are accommodated in the α-CD cavity, have been found to be stronger acids than the free, uncomplexed ones, except for 4-methoxyphenol. On the other hand, 4-cyanophenol, 3-nitrophenol, and 4-methoxyphenol bound to the β-CD cavity are weaker acids than the uncomplexed ones, although 3-cyanophenol, 4-nitrophenol, and 4-bromophenol bound to β-CD show the same trend as those bound to α-CD. The different influences of α- and β-CDs on the pKa values are likely due to the difference in the magnitudes of the induced dipole moments of the guest caused by α- and β-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.