Experimental standard molar enthalpies of formation of some methylbenzenediol isomers

Abstract

The present work is part of a research program on the energetics of formation of alkyl substituted benzenediols, aiming the study of the enthalpic effect of the introduction of methyl substituents into benzenediols. In this work we present the results of the thermochemical research on 2-methylresorcinol, 3-methylresorcinol, 4-methylresorcinol, and methylhydroquinone. The standard ( p ∘ = 0.1 MPa ) molar enthalpies of formation, in the crystalline phase, at T = 298.15 K, of the compounds mentioned above were derived from their standard massic energies of combustion, measured by static-bomb combustion calorimetry, while the standard molar enthalpies of sublimation of those compounds were obtained by the temperature dependence of their vapour pressures determined by the Knudsen effusion technique. - Δ c U m ∘ ( cr ) / ( kJ · mol - 1 ) Δ f H m ∘ ( cr ) / ( kJ · mol - 1 ) Δ cr g H m ∘ / ( kJ · mol - 1 ) 2-Methylresorcinol (cr) 3499.3 ± 0.8 −393.6 ± 1.2 99.2 ± 2.3 4-Methylresorcinol (cr) 3491.3 ± 1.1 −401.6 ± 1.4 107.3 ± 3.0 5-Methylresorcinol (cr) 3491.4 ± 1.1 −401.5 ± 1.4 102.9 ± 3.5 Methylhydroquinone (cr) 3492.7 ± 1.4 −400.2 ± 1.7 108.4 ± 3.9 From those experimental values, the standard molar enthalpies of formation of the studied methylbenzenediols in the gaseous phase, at T = 298.15 K were then derived. The results are interpreted in terms of structural contributions to the energetics of the substituted benzenediols and compared with the same parameters estimated from the Cox Scheme. Moreover, the standard ( p ∘ = 0.1 MPa ) molar enthalpies, entropies, and Gibbs energies of sublimation, at T = 298.15 K, were derived for the four isomers of methylbenzenediols.