Kinetic solvent effects on hydroxylic hydrogen atom abstractions are independent of the nature of the abstracting radical. Two extreme tests using vitamin E and phenol

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DOIResolve DOI: http://doi.org/10.1021/ja00145a005
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TypeArticle
Journal titleJournal of the American Chemical Society
ISSN0002-7863
Volume117
Issue40
Pages99669971; # of pages: 6
Subjectalpha tocopherol; phenol; radical; article; chemical reaction kinetics; oxidation; reaction analysis
AbstractRate constants have been measured at 25°C in 13 solvents (S) for abstraction of the phenolic hydrogen atom from α-tocopherol (TOH) by tert-butoxyl (BO .) k(TOH/BO)(S), and by 2,2-diphenyl-1-picrylhydrazyl (DPPH .), k(TOH/DPPH)(S), and in eight solvents for abstraction of the phenolic hydrogen atom from phenol by cumyloxyl, k(PhOH/CumO)(S), and DPPH .k(PhOH/DPPH)(S). Over the range of solvents examined k(TOH/BO)(S) and k(TOH/DPPH)(S) vary by a factor of ca. 65 and k(PhOH/CumO)(S) and k(PhOH/DPPH)(S) vary by a factor of ca. 120. In accordance with a prediction, the kinetic solvent effect is essentially identical for the same substrate and is independent of the attacking radical. That is, for almost any pair of solvents, A and B (k(TOH/BO)(A)/k(TOH/BO)(B))/(k(TOH/DPPH)(A)/k(TOH/DPPH)(B)) ~ 1.0. The same applies with phenol as the substrate. Exceptions to this 1:1 relationship occur when one of the reactions becomes partly diffusion controlled and in the solvent tert-butyl alcohol in which DPPH . shows a larger reactivity than would be expected. The absolute magnitudes of the alkoxyl and DPPH rate constants in the same solvent differ by a factor of over 1000000 (10 6) for α-tocopherol and by 10000000000 (10 10) for phenol! We have therefore confirmed, under extreme conditions, a new, unifying principle for free radical chemistry in solution.
Publication date
LanguageEnglish
AffiliationNational Research Council Canada (NRC-CNRC)
Peer reviewedYes
NPARC number21276775
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Record identifier3a87bcfa-becd-4bd2-9906-85caac0a2fac
Record created2015-10-13
Record modified2016-05-09
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