Influence of remote intramolecular hydrogen bonds on the stabilities of phenoxyl radicals and benzyl cations
| Autor: | Derek A. Pratt, Mario C. Foti, Keith U. Ingold, Riccardo Amorati, Gian Franco Pedulli, Carmelo Daquino |
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| Přispěvatelé: | Foti M. C., Amorati R., Pedulli G. F., Daquino C., Pratt D. A., Ingold K. U |
| Jazyk: | angličtina |
| Rok vydání: | 2010 |
| Předmět: |
chemistry.chemical_classification
Models Molecular Intramolecular reaction Molecular Structure Hydrogen bond Radical Organic Chemistry Benzylammonium Compounds Hydrogen Bonding Electron acceptor Photochemistry Hydrogen atom abstraction Dissociation (chemistry) Reaction rate constant chemistry Phenols Intramolecular force Cations |
| Zdroj: | Journal of organic chemistry 75 (2010): 4434–4440. doi:10.1021/jo100491a info:cnr-pdr/source/autori:Foti MC; Amorati R; Pedulli GF; Daquino C; Pratt DA; Ingold KU/titolo:Influence of remote intramolecular hydrogen bonds on the stabilities of phenoxyl radicals and benzyl cations./doi:10.1021%2Fjo100491a/rivista:Journal of organic chemistry/anno:2010/pagina_da:4434/pagina_a:4440/intervallo_pagine:4434–4440/volume:75 |
| Popis: | Remote intramolecular hydrogen bonds (HBs) in phenols and benzylammonium cations influence the dissociation enthalpies of their O-H and C-N bonds, respectively. The direction of these intramolecular HBs, para --meta or meta --para, determines the sign of the variation with respect to molecules lacking remote intramolecular HBs. For example, the O-H bond dissociation enthalpy of 3-methoxy-4-hydroxyphenol, 4, is about 2.5 kcal/mol lower than that of its isomer 3-hydroxy-4-methoxyphenol, 5, although group additivity rules would predict nearly identical values. In the case of 3-methoxy-4-hydroxybenzylammonium and 3-hydroxy-4-methoxybenzylammonium ions, the CBS-QB3 level calculated C-N eterolytic dissociation enthalpy is about 3.7 kcal/mol lower in the former ion. These effects are caused by the strong electron-withdrawing character of the -O(*) and -CH(2)(+) groups in the phenoxyl radical and benzyl cation, respectively, which modulates the strength of the HB. An O-H group in the para position of ArO(*) or ArCH(2)(+) becomes more acidic than in the parent molecules and hence forms stronger HBs with hydrogen bond acceptors (HBAs) in the meta position. Conversely, HBAs, such as OCH(3), in the para position become weaker HBAs in phenoxyl radicals and benzyl cations than in the parent molecules. These product thermochemistries are reflected in the transition states for, and hence in the kinetics of, hydrogen atom abstraction from phenols by free radicals (dpph(*) and ROO(*)). For example, the 298 K rate constant for the 4 + dpph(*) reaction is 22 times greater than that for the 5 + dpph(*) reaction. Fragmentation of ring-substituted benzylammonium ions, generated by ESI-MS, to form the benzyl cations reflects similar remote intramolecular HB effects. |
| Databáze: | OpenAIRE |
| Externí odkaz: |
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