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

Abstract

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 ∘ = 0.1 MPa ) 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 - Δ c U m ∘ ( cr ) / ( kJ · mol - 1 ) - Δ f H m ∘ (cr ) / ( kJ · mol - 1 ) - Δ f H m ∘ ( g ) / ( kJ · mol - 1 ) 4-Ethoxybenzoic acid 4362.7 ± 1.3 605.5 ± 1.8 482.2 ± 2.0 4-Butoxybenzoic acid 5677.3 ± 2.0 647.1 ± 2.5 518.1 ± 2.6 4-(Octyloxy)benzoic acid 8269.0 ± 3.3 767.9 ± 3.9 604.9 ± 4.1 From 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.