| Abstract
| - The gas-phase O−H bond dissociation enthalpy, BDE, in phenol provides an essential benchmark for calibratingthe O−H BDEs of other phenols, data which aids our understanding of the reactivities of phenols, such astheir relevant antioxidant activities. In a recent review, the O−H BDE for phenol was presented as 90 ± 3kcal mol-1 (Acc. Chem. Res.2003, 36, 255−263). Due to the large margin of error, such a parameter cannotbe used for dynamic interpretations nor can it be used as an anchor point in the development of more advancedcomputational models. We have reevaluated the existing experimental gas-phase data (thermolyses and ionchemistry). The large errors and variations in thermodynamic parameters associated with the gas-phase ionchemistry methods produce inconsistent results, but the thermolytic data has afforded a value of 87.0 ± 0.5kcal mol-1. Next, the effect of solvent has been carefully scrutinized in four liquid-phase methods for measuringthe O−H BDE in phenol: photoacoustic calorimetry, one-electron potential measurements, an electrochemicalcycle, and radical equilibrium electron paramagnetic resonance (REqEPR). The enthalpic effect due to solvation,by, e.g., water, could be rigorously accounted for by means of an empirical model and the difference inhydrogen bond interactions of the solvent with phenol and the phenoxyl radical. For the REqEPR method, asecond correction is required since the calibration standard, the O−H BDE in 2,4,6-tri-tert-butylphenol, hadto be revised. From the gas-phase thermolysis data and three liquid-phase techniques (excluding theelectrochemical cycle method), the present analysis yields a gas-phase BDE of 86.7 ± 0.7 kcal mol-1. TheO−H BDE was also estimated by state-of-the-art computational approaches (G3, CBS-APNO, and CBS-QB3) providing a range from 86.4 to 87.7 kcal mol-1. We therefore recommend that in the future, and untilfurther refinement is possible, the gas-phase O−H BDE in phenol should be presented as 86.7 ± 0.7 kcalmol-1.
|
