Kinetic Solvent Effects on Proton and Hydrogen Atom Transfers from Phenols. Similarities and Differences

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TypeArticle
Journal titleJ. Am. Chem. Soc
ISSN00027863
Volume128
Issue4
Pages11721182; # of pages: 11
AbstractBimolecular 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' values ranged from 0.261 (2-MeO-phenol) to 0.728 (3,5-Cl2-phenol), and the solvents' values from 0.44 (MeCN) to 1.00 (HMPA), where and 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 , 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 increases, slopes of plots of log kS against increase as increases). However, in some solvents, kS values deviate systematically from the least-squares log kS versus correlation line (e.g., in THF and MeCN, kS 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) scale of solvent HBA activities than with . The scale, therefore, does not solely reflect solvents' HBA activities but also contains contributions from anion solvation.
Publication date
Linkhttp://pubs3.acs.org/acs/journals/doilookup?in_doi=10.1021/ja0548081
AffiliationNational Research Council Canada; NRC Steacie Institute for Molecular Sciences
Peer reviewedNo
Identifier10072378
NPARC number12327720
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Record identifiera319c502-4f2d-4a89-b0d7-27461c5c95a4
Record created2009-09-10
Record modified2016-05-09
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