Influence of ‘Remote’ Intramolecular Hydrogen-Bonds on the Stabilities of Phenoxyl Radicals and Benzyl Cations

Download
  1. (PDF, 665 KB)
  2. Get@NRC: Influence of ‘Remote’ Intramolecular Hydrogen-Bonds on the Stabilities of Phenoxyl Radicals and Benzyl Cations (Opens in a new window)
DOIResolve DOI: http://doi.org/10.1021/jo100491a
AuthorSearch for: ; Search for: ; Search for: ; Search for: ; Search for: ; Search for:
TypeArticle
Journal titleJournal Of Organic Chemistry
Volume75
Issue13
Pages44344440; # of pages: 7
AbstractRemote 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 −CH2+ 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 ArCH2+ 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 OCH3, 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.
Publication date
LanguageEnglish
AffiliationNRCSteacie Institute for Molecular Sciences; National Research Council Canada
Peer reviewedYes
NPARC number17653054
Export citationExport as RIS
Report a correctionReport a correction
Record identifierca4a1eff-9da3-4d26-9b2a-7593d032e8ec
Record created2011-03-31
Record modified2016-05-09
Bookmark and share
  • Share this page with Facebook (Opens in a new window)
  • Share this page with Twitter (Opens in a new window)
  • Share this page with Google+ (Opens in a new window)
  • Share this page with Delicious (Opens in a new window)