The role of hydrogen bonding on the H-atom-donating abilities of catechols and naphthalene diols and on a previously overlooked aspect of their infrared spectra

Catechols and 1,8-naphthalene diols contain one "free" hydroxyl and one intramolecularly H-bonded hydroxyl group. The "free" hydroxyls are strong hydrogen-bond donors (HBDs) with alpha(2)(H) values (Abraham et al. J. Chem. Soc., Perkin Trans. 2 1989, 699) ranging from 0.685 to 0.775, indicating that these compounds have similar HBD properties to those of strongly acidic phenols such as 4-chlorophenol (alpha(2)(H) = 0.670) and 3,5-dichlorophenol (alpha(2)(H) = 0.774). Kinetic effects on H-atom abstractions from the diols in HB acceptor (HBA) solvents can be quantitatively accounted for over at least 50% of the available range of solvent HBA activities (as measured by their beta(2)(H) values; see Abraham et al. J. Chem. Soc. Perkin Trans. 21990, 521) on the basis of a single reactive OH group, the "free" OH. This free OH group is an outstanding H-atom donor in poor HBA solvents; e.g., in hexane rate constants for reaction with the DPPH. radical are 2.1 X 10(4) M-1 s(-1) for 3,5-di-tert-butyl catechol and 2 x 10(6) M-1 s(-1) for 4-methoxy-1,8-naphthalene diol, but only 7.4 x 10(3) M-1 s(-1) for alpha-tocopherol (vitamin E). The diols are much more reactive than simple phenols because the O-H bond dissociation enthalpy of the "free" OH group is weakened by 5-9 kcal/mol by the intramolecular H-bond. The IR spectra of all the diols in CCl4 show two fairly sharp O-H stretching bands of roughly equal intensity separated by 42-138 cm(-1). Addition of a low concentration of DMSO, a strong HBA, causes the band due to the intramolecularly H-bonded OH group to decrease in intensity to roughly half the extent that the "free" OH band loses intensity. The latter forms an intermolecular H-bond with the DMSO, the former does not. What has been overlooked in earlier work is that as the DMSO concentration is increased the band due to the intramolecularly H-bonded OH group first broadens and then evolves into a new, lower frequency (by 19-92 cm(-1)) band. The magnitude of the shift in the frequency of the intramolecular OH band caused by H-bonding of HBAs to the "free" OH group, Deltav, increases linearly as the HBA activity of the additive increases, e.g., for 3,5-di-tert-butylcatechol, Deltav/cm(-1) = 33.8 beta(2)(H) (R-2 = 0.986). This may provide a new and simple method for determining beta(2)(H) values.