| Abstract | Bimolecular rate constants for proton transfer from six phenols to the anthracene radical anion have been determined in up to eight solvents using electrochemical techniques. Effects of hydrogen bonding on measured rate constants were explored over as wide a range of phenolic hydrogen-bond donor (HBD) and solvent hydrogen-bond acceptor (HBA) activities as practical. The phenols' α2H values ranged from 0.261 (2-MeO-phenol) to 0.728 (3,5-Cl2-phenol), and the solvents' β2H values from 0.44 (MeCN) to 1.00 (HMPA), where α 2H and β2H are Abraham's parameters describing relative HBD and HBA activities (J. Chem. Soc., Perkin Trans. 2 1989, 699; 1990, 521). Rate constants for H-atom transfer (HAT) in HBA solvents, kS, are extremely well correlated via log kS = log K0 - 8.3 α2Hβ2H, where K0 is the rate constant in a non-HBA solvent (Snelgrove et al. J. Am. Chem. Soc. 2001, 123, 469). The same equation describes the general features of proton transfers (kS decreases as β2H increases, slopes of plots of log kS against β2H increase as α2H increases). However, in some solvents, kS values deviate systematically from the least-squares log kS versus β2H correlation line (e.g., in THF and MeCN, k S is always smaller and larger, respectively, than "expected"). These deviations are attributed to variations in the solvents' anion solvating abilities (THF and MeCN are poor and good anion solvators, respectively). Values of log kS for proton transfer, but not for HAT, give better correlations with Taft et al.'s (J. Org. Chem. 1983, 48, 2877) β scale of solvent HBA activities than with β 2H. The β scale, therefore, does not solely reflect solvents' HBA activities but also contains contributions from anion solvation. © 2006 American Chemical Society. |
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