124700-28-3Relevant academic research and scientific papers
Unexpected acid catalysis in reactions of peroxyl radicals with phenols**
Valgimigli, Luca,Amorati, Riccardo,Petrucci, Silvia,Pedulli, Gian Franco,Hu, Di,Hanthorn, Jason J.,Pratt, Derek A.
, p. 8348 - 8351 (2009)
Weak organic acids in millimolar concentrations increase the reactivity of peroxyl radicals with common phenolic antioxidants dramatically. This counterintuitive phenomenon relies on a substantially different reaction mechanism from that in the absence of an acid: rate-determining electron transfer occurs from the hydrogen-bonded phenol to the hydroperoxide cation radical present in equilibrium with the peroxyl radical under these conditions (see scheme).
Effect of solvation on the reaction rate constants of the diphenylaminyl radical with phenols and hydroquinones
Antonov,Denisov,Nadtochenko,Varlamov
, p. 91 - 96 (2008/02/13)
The kinetics of the reactions of the diphenylaminyl radical with unhindered phenols and hydroquinones in acetonitrile at 294 K was studied by laser flash photolysis. The rate constants k range from 104 to 106 L mol-1 s-1 and depend on the nature of substituents in phenols and hydroquinones. The k values also depend on the solvent nature and decrease by 6.5 and 50 times, on the average, when decane is replaced by toluene and acetonitrile, respectively. The logk values in decane, toluene, and acetonitrile depend linearly on the dissociation energies D OH of OH bonds in phenols and hydroquinones. Taking into account this dependence and using the experimental k value in acetonitrile, the estimate D OH = 346.6 kJ mol-1 was obtained for 2,5-dichlorohydroquinone. The thermodynamic parameters of formation of H-bonded complexes of phenols and hydroquinones with toluene and acetonitrile were calculated in the framework of the multiplicative approach to thermodynamic description of H-bonding. The rate constants k (free) of the reactions of the diphenylaminyl radical with free phenols and hydroquinones in toluene and acetonitrile were estimated. Comparison of the k (free) values with each other and experimental k values in decane suggests that the changes in k upon solvent replacement is mainly caused by the formation of H-complexes of phenols and hydroquinones with the solvent. Springer Science+Business Media, Inc. 2007.
Hydrogen-bonding effects on the properties of phenoxyl radicals. An EPR, kinetic, and computational study
Lucarini, Marco,Mugnaini, Veronica,Pedulli, Gian Franco,Guerra, Maurizio
, p. 8318 - 8329 (2007/10/03)
The effect of 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) on the properties of phenoxyl radicals has been investigated. HFP produces large variations of the phenoxyl hyperfine splitting constants indicative of a large redistribution of electron spin density, which can be accounted for by the increased importance of the mesomeric structures with electric charge separation. The conformational rigidity of phenoxyl radicals with electron-releasing substituents is also greatly enhanced in the presence of HFP, as demonstrated by the 2 kcal/mol increase in the activation energy for the internal rotation of the p-OMe group in the p-methoxyphenoxyl radical. By using the EPR equilibration technique, we have found that in phenols the O-H bond dissociation enthalpy (BDE) is lowered in the presence of HFP because it preferentially stabilizes the phenoxyl radical. In phenols containing groups such as OR that are acceptors of H-bonds, the interaction between HFP and the substituent is stronger in the phenol than in the corresponding phenoxyl radical because the radical oxygen behaves as an electron-withdrawing group, which decreases the complexating ability of the substituent. In phenols containing OH or NH2 groups, EPR experiments performed in H-bond accepting solvents showed that the interaction between the solvent and the substituent is much stronger in the phenoxyl radical than in the parent phenol because of the electron-withdrawing effect of the radical oxygen, which makes more acidic, and therefore more available to give H-bonds, the OH or NH2 groups. These experimental results have been confirmed by DFT calculations. The effect of HFP solvent on the reactivity of phenols toward alkyl radicals has also been investigated. The results indicated that the decrease of BDE observed in the presence of HFP is not accompanied by a larger reactivity. The origin of this unexpected behavior has been shown by DFT computations. Finally, a remarkable increase in the persistency of the α-tocopheroxyl radical has been observed in the presence of HFP.
