Kinetic solvent effects on proton and hydrogen atom transfers from phenols. Similarities and differences

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' \u3b12H values ranged from 0.261 (2-MeO-phenol) to 0.728 (3,5-Cl2-phenol), and the solvents' \u3b22H values from 0.44 (MeCN) to 1.00 (HMPA), where \u3b1 2H and \u3b22H 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 \u3b12H\u3b22H, 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 \u3b22H increases, slopes of plots of log kS against \u3b22H increase as \u3b12H increases). However, in some solvents, kS values deviate systematically from the least-squares log kS versus \u3b22H 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) \u3b2 scale of solvent HBA activities than with \u3b2 2H. The \u3b2 scale, therefore, does not solely reflect solvents' HBA activities but also contains contributions from anion solvation. \ua9 2006 American Chemical Society.Peer reviewed: YesNRC publication: Ye