| Abstract
| - The PhCH2−H bond dissociation enthalpy in solution was determined for the first time as 380.5 ± 5.1 kJmol-1, from a time-resolved photoacoustic calorimetry study (TR−PAC) in toluene. The derived gas-phaseresult, 375.5 ± 5.0 kJ mol-1, allows the calculation of the enthalpy of formation of the benzyl radical asΔfH° (PhCH2•, g) = 208.0 ± 5.0 kJ mol-1, which is in excellent agreement with a value recently publishedin the literature, obtained from gas-phase experiments, and demonstrates the accuracy of TR−PAC as a toolfor the study of organic radical energetics. A detailed discussion of the methods and assumptions used toderive those values from the TR−PAC experiments was made. The study was then extended to ethylbenzeneand cumene. The results support the assumption that solvation enthalpies of the alkylbenzenes and theircorresponding radicals are similar. The trend of C−H bond dissociation enthalpies in those alkylbenzenes isin keeping with early gas-phase literature results, but it is in clear contrast with the values obtained from aproton affinity cycle using data from the NIST Chemistry WebBook.
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