Experimental standard molar enthalpies of formation of some 4-alkoxybenzoic acids

The present work is part of a research program on the energetics of the linear 4-n-alkoxybenzoic acids, aiming the study of the enthalpic effect of the introduction of an alkoxy chain in the position 4- of the benzoic acid ring. In this work, we present the results of the thermochemical research on 4-n-alkoxybenzoic acids with the alkoxy chain length n = 2, 4, and 8.The standard (p∘=0.1MPa) molar enthalpy of formation of crystalline 4-ethoxybenzoic acid, 4-butoxybenzoic acid, and 4-(octyloxy)benzoic acid was measured, at T = 298.15 K, by static-bomb calorimetry. These values, combined with the values of standard molar enthalpies of sublimation, were used to derive the standard molar enthalpies of formation in the gaseous phase.4-Alkoxybenzoic acid-ΔcUm∘(cr)/(kJ·mol-1)-ΔfHm∘(cr)/(kJ·mol-1)-ΔfHm∘(g)/(kJ·mol-1)4-Ethoxybenzoic acid4362.7 ± 1.3605.5 ± 1.8482.2 ± 2.04-Butoxybenzoic acid5677.3 ± 2.0647.1 ± 2.5518.1 ± 2.64-(Octyloxy)benzoic acid8269.0 ± 3.3767.9 ± 3.9604.9 ± 4.1Full-size tableTable optionsView in workspaceDownload as CSVFrom those experimental values, the standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, are interpreted in terms of structural contributions to the energetics of the substituted benzoic acids and compared with the same parameters estimated from the Benson’s Group Method.