Kinetic solvent effects on hydrogen atom abstraction from phenol, aniline, and diphenylamine. The importance of hydrogen bonding on their radical-trapping (antioxidant) activities

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DOIResolve DOI: http://doi.org/10.1021/jo951244i
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TypeArticle
Journal titleJournal of Organic Chemistry
ISSN0022-3263
Volume61
Issue4
Pages13161321; # of pages: 6
AbstractAbsolute rate constants for hydrogen atom abstraction by alkoxyl radicals from phenol, aniline, and diphenylamine have been measured in 14 solvents at room temperature by laser flash photolysis. For all three substrates the rate constants decline as the solvent becomes a stronger hydrogen-bond acceptor (HBA). Thus, on changing the solvent from CCl 4 to CH 3CN the rate constants decline by factors of 148, 7.1, and 4.7 for PhOH, PhNH 2, and Ph 2NH, respectively. The kinetic solvent effect for phenol correlates rather well with Abraham's β 2 H scale of relative HBA activities of the solvents we have employed as measured as solutes in CCl 4 solvent. This correlation is not quite so good with aniline and it is almost nonexistent for diphenylamine. With all three substrates the "deviant" solvents produce higher rate constants than would be expected from the β 2 H value of the solvent and, generally, these are the solvents in which steric hindrance to hydrogen-bond formation would appear probable. The kinetic data for the three substrates can be most readily interpreted in terms of specific hydrogen bonding rather than being due to some generalized weakly dipolar interaction. It is concluded, for example, that hydrogen bonding involves the π-electron cloud in benzene but the chlorine atom in chlorobenzene.
Publication date
LanguageEnglish
AffiliationNational Research Council Canada (NRC-CNRC)
Peer reviewedYes
NPARC number21276688
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Record identifier6c1b820d-5e65-4841-bb0e-93a6f292dda3
Record created2015-10-13
Record modified2016-05-09
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