Source:http://linkedlifedata.com/resource/pubmed/id/12392436
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| Predicate | Object |
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| rdf:type | |
| lifeskim:mentions | |
| pubmed:issue |
43
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| pubmed:dateCreated |
2002-10-23
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| pubmed:abstractText |
Catechols and 1,8-naphthalene diols contain one "free" hydroxyl and one intramolecularly H-bonded hydroxyl group. The "free" hydroxyls are strong hydrogen-bond donors (HBDs) with alpha2H values (Abraham et al. J. Chem. Soc., Perkin Trans. 2 1989, 699) ranging from 0.685 to 0.775, indicating that these compounds have similar HBD properties to those of strongly acidic phenols such as 4-chlorophenol (alpha2H = 0.670) and 3, 5-dichlorophenol (alpha2H = 0.774). Kinetic effects on H-atom abstractions from the diols in HB acceptor (HBA) solvents can be quantitatively accounted for over at least 50% of the available range of solvent HBA activities (as measured by their beta2H values; see Abraham et al. J. Chem. Soc. Perkin Trans. 2 1990, 521) on the basis of a single reactive OH group, the "free" OH. This free OH group is an outstanding H-atom donor in poor HBA solvents; e.g., in hexane rate constants for reaction with the DPPH* radical are 2.1 x 104 M-1 s-1 for 3,5-di-tert-butyl catechol and 2 x 106 M-1 s-1 for 4-methoxy-1,8-naphthalene diol, but only 7.4 x 103 M-1 s-1 for alpha-tocopherol (vitamin E). The diols are much more reactive than simple phenols because the O-H bond dissociation enthalpy of the "free" OH group is weakened by 5-9 kcal/mol by the intramolecular H-bond. The IR spectra of all the diols in CCl4 show two fairly sharp O-H stretching bands of roughly equal intensity separated by 42-138 cm-1. Addition of a low concentration of DMSO, a strong HBA, causes the band due to the intramolecularly H-bonded OH group to decrease in intensity to roughly half the extent that the "free" OH band loses intensity. The latter forms an intermolecular H-bond with the DMSO, the former does not. What has been overlooked in earlier work is that as the DMSO concentration is increased the band due to the intramolecularly H-bonded OH group first broadens and then evolves into a new, lower frequency (by 19-92 cm-1) band. The magnitude of the shift in the frequency of the intramolecular OH band caused by H-bonding of HBAs to the "free" OH group, Deltanu, increases linearly as the HBA activity of the additive increases, e.g., for 3,5-di-tert-butylcatechol, Deltanu/cm-1 = 33.8 beta2H (R 2 = 0.986). This may provide a new and simple method for determining beta2H values.
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| pubmed:language |
eng
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| pubmed:journal | |
| pubmed:status |
PubMed-not-MEDLINE
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| pubmed:month |
Oct
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| pubmed:issn |
0002-7863
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| pubmed:author | |
| pubmed:issnType |
Print
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| pubmed:day |
30
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| pubmed:volume |
124
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| pubmed:owner |
NLM
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| pubmed:authorsComplete |
Y
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| pubmed:pagination |
12881-8
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| pubmed:dateRevised |
2008-1-17
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| pubmed:year |
2002
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| pubmed:articleTitle |
The role of hydrogen bonding on the h-atom-donating abilities of catechols and naphthalene diols and on a previously overlooked aspect of their infrared spectra.
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| pubmed:affiliation |
Istituto di Chimica Biomolecolare del CNR - Sezione di Catania, Via del Santuario 110, I-95028 Valverde (CT), Italy. foti@issn.ct.cnr.it
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| pubmed:publicationType |
Journal Article
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