Thermochemistry of some alkylsubstituted anthracenes

Abstract

The standard ( p ∘ = 0.1 MPa) molar enthalpies of combustion, Δ c H m ∘ , for crystalline 9-methylanthracene, 2-ethylanthracene, and 9,10-dimethylanthracene were determined, at the temperature 298.15 K, using a static bomb combustion calorimeter. For these compounds, the standard molar enthalpies of sublimation, Δ cr g H m ∘ , at T = 298.15 K were derived by the Clausius-Clapeyron equation, from the temperature dependence of the vapour pressures of these compounds, measured by the Knudsen-effusion technique. The results were as follows: - Δ c H m ∘ ( cr ) / ( kJ · mol - 1 ) Δ cr g H m ∘ / ( kJ · mol - 1 ) 9-Methylanthracene 7714.7 ± 3.4 101.8 ± 1.0 2-Ethylanthracene 8357.3 ± 3.2 107.6 ± 0.6 9,10-Dimethylanthracene 8367.2 ± 3.8 113.0 ± 1.7 These values were used to derive the standard molar enthalpies of formation of the compounds in their crystalline and gaseous phases, respectively. The derived standard molar enthalpies of formation, in the gaseous state, are analysed in terms of enthalpic increments and interpreted in terms of molecular structure. Using estimated values for the heat capacity differences between the gas and the crystal phases of the studied compounds, the standard ( p ∘ = 0.1 MPa) molar enthalpies, entropies, and Gibbs energy of sublimation, at T = 298.15 K, were derived. Additionally, standard enthalpies of formation were estimated by computations based on density-functional theory. The estimated values are in excellent agreement with experimental data.