Thermolabile Hydrocarbons, 33. Thermochemistry and Thermal Decomposition of 9,9′‐Bifluorenyl and 9,9′‐Dimethyl‐9,9′‐bifluorenyl – The Stabilization Energy of 9‐Fluorenyl Radicals

Abstract

AbstractFrom thermochemical measurements the heats of formation ΔH0f(g) forfluorene (1), 9‐methylfluorene (2), 9,9′‐bifluorenyl (5), and 9,9′‐dimethyl‐9,9′‐bifluorenyl (6) were determined. The homolytic cleavage of the dimers 5 and 6 to 9‐fluorenyl (3) and 9‐methyl‐9‐fluorenyl radicals (4), respectively, was studied in mesitylene with thiophenol as trapping agent and found to follow first‐order kinetics. The activation parameters ΔH≠(5) = 212.6 (±2.1) kJ mol−1, ΔS≠(5) = 70.3 (±4.2) J mol−1 K−1 ΔH≠(6) = 164.9 (±0.7) kJ mol−1, and ΔS≠(6) = 88.2 (±1.9) J mol−1 K−1 were obtained. For 3 and 4 the radical stabilization energies RSE(3) = 67 (±7) kJ mol−1 (relative to isopropyl) and RSE(4) = 64 (±8) kJ mol−1 (relative to tert‐butyl) were determined by a comparison with the activation parameters of the thermolysis of alkanes possessing the same strain enthalpy. The heats of formation for 3 and 4 and the C–H bond dissociation energies for the C–H bond in the 9–position of 1 and 2 were determined to be ΔH0f(g,3) = 300 kJ mol−1, Δ0f(g,4) = 268 kJ mol−1, BDECH(1) = 343 and BDECH(2) = 338 kJ mol−1 (estimated errors correspond to those of the RSE values). A good correlation between the heats of vaporization of 37 aromatic hydrocarbons and their calculated solvent‐accessible surfaces was demonstrated. Hence, a new method to estimate heats of vaporization was established.