title: Electronic and hydrogen bonding effects on the chain-breaking activity of sulfur-containing phenolic antioxidants.
abstract: A kinetic and thermodynamic investigation of phenols para-substituted with thiyl (SR), sulfinyl (SOR), and sulfonyl (SO(2)R) groups and ortho-substituted with thiyl groups is reported. The effect of the sulfur substituents on the O-H bond dissociation enthalpy values, BDE(O-H), was measured by means of the EPR radical equilibration technique and the reactivity toward peroxyl radicals, k(inh), of these phenolic antioxidants was determined by inhibited autoxidation studies. An inverse correlation between these two parameters was found. A p-SMe substituent decreased the BDE(O-H) value to a lesser extent than a p-OMe group (-3.6 vs -4.4 kcal/mol), whereas the effect of the same groups in an ortho position showed an opposite trend (-0.85 vs -0.2 kcal/mol). The latter result is explained in terms of the different strength of the intramolecular hydrogen bond between the OH proton and the sulfur or oxygen substituents in ortho derivatives. ESI-MS analysis of the products formed by reacting the sulfides with peroxyl radicals from the azoinitiator AIBN revealed the formation of a complex mixture of products, which may play an important role in determining the overall antioxidant activity of the parent compounds.
authors.affiliation: Dipartimento di Chimica Organica A. Mangini, Università di Bologna, Via S. Donato 11, 40126 Bologna, Italy.
authors.forename: Riccardo
authors.initials: R
authors.lastname: Amorati
authors.forename: Maria Grazia
authors.initials: MG
authors.lastname: Fumo
authors.forename: Stefano
authors.initials: S
authors.lastname: Menichetti
authors.forename: Veronica
authors.initials: V
authors.lastname: Mugnaini
authors.forename: Gian Franco
authors.initials: GF
authors.lastname: Pedulli
mesh_terms: D000114:Acetylene / Q000031:analogs & derivatives / Q000737:chemistry; D000975:Antioxidants / Q000737:chemistry*; D001554:Benzene / Q000737:chemistry; D004583:Electrons; D006860:Hydrogen Bonding; D008958:Models, Molecular; D015394:Molecular Structure; D037742:Nanotubes, Carbon / Q000737:chemistry; D010636:Phenols / Q000737:chemistry*; D013455:Sulfur / Q000737:chemistry*
chemical_list: D000975:Antioxidants; D037742:Nanotubes, Carbon; D010636:Phenols; C044736:phenylacetylene; D013455:Sulfur; D001554:Benzene; D000114:Acetylene