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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

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journal contribution
posted on 08.10.2002, 00:00 by Mario C. Foti, L. Ross C. Barclay, K. U. Ingold
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 α2H 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 (α2H = 0.670) and 3, 5-dichlorophenol (α2H = 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 β2H values; see Abraham et al. J. Chem. Soc. Perkin Trans. 2 1990, 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 × 104 M-1 s-1 for 3,5-di-tert-butyl catechol and 2 × 106 M-1 s-1 for 4-methoxy-1,8-naphthalene diol, but only 7.4 × 103 M-1 s-1 for α-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, Δν, increases linearly as the HBA activity of the additive increases, e.g., for 3,5-di-tert-butylcatechol, Δν/cm-1 = 33.8 β2H (R2 = 0.986). This may provide a new and simple method for determining β2H values.