Enthalpies of formation of methoxybenzoic acids and their dependence on structure

Abstract

The energy of combustion of 2,5-dimethoxybenzoic acid has been determined using a static bomb calorimeter. The vapor pressures of the compound have been measured over a 18 K temperature interval by the Knudsen effusion technique. Heat capacity measurements between T = 270 K and T = 338 K were carded out by DSC, From these experimental results the standard molar enthalpies of combustion, sublimation, and formation in the crystalline and gaseous state at the temperature 298.15 K have been derived. With this compound, the series of mono-and dimethoxybenzoic acids have been completed. TheirAfH~, values were expressed by an additive relationship, taking into account the number of methoxy groups and the number of all 1,2 interactions: an accuracy of 3.3 kJ.mol -) was achieved. In an alternative approach the substituent effect of the methoxy groups was evaluated within the framework of isodesmic reactions. The effect of disubstitution was referred to mono derivatives and the excess energy--the so-called buttressing effect--was evaluated (2-24 kJ'molS ~ for individual bis derivatives). These values were explained in terms of the conformation of the methoxy group around the C~r--O bond.

Vapour pressures, enthalpies and entropies of sublimation of trans -cinnamic acid and of nine methoxy and dimethoxycinnamic acids

“…( , for benzoic acid and methoxy substituted benzoic acids ( ) and for trans-cinnamic acid and methoxy substituted cinnamic acids (•). 1, benzoic acid; (6) 2, 2-methoxybenzoic acid; (10) 3, 3-methoxybenzoic acid; (10) 4, 2,5-dimethoxybenzoic acid; (11) 5, 2,3-dimethoxybenzoic acid; (12) 6, 4-methoxybenzoic acid; (10) 7, 2,4-dimethoxybenzoic acid; (12) 8, 3,5-dimethoxy-benzoic acid; (12) 9, 3,4-dimethoxybenzoic acid; (12) of Porto (research unit number 81, CIQ-UP/L 5). DMH thanks the PRAXIS XXI for the award of a research grant (BD/9349/96) under Programa Ciência.…”

Standard molar enthalpies of combustion of the three trans -methoxycinnamic acids

“…The −596.9 ± 1.5 (24) −595.0 ± 2.8 −1.9 ± 3.1 two methoxyl groups; for 1,2-dimethoxybenzene, a value of (16.9 ± 3.8) kJ · mol −1 was observed.…”

“…(16.9 ± 4.6) kJ · mol −1 , and (6.1 ± 4.4) kJ · mol −1 , respectively. In order to understand these observations the following data, presented in tables 6 and 7, have been considered: the experimental values of f H o m (g, T = 298.15 K) for the disubstituted methoxy-benzene isomers (3) and the corresponding estimated values calculated from the standard molar enthalpy of formation in the gaseous phase for benzene plus the enthalpy increments for methoxy substitution in the benzene molecule; the experimental values of f H o m (g, T = 298.15 K) for the mono-and di-substituted methoxybenzoic acids, (22)(23)(24) and the respective estimated values calculated from the standard molar enthalpy of formation in the gaseous phase for benzene plus the enthalpy increments for the methoxy and carboxyl substitution in the benzene molecule.…”

Interplay of thermochemistry and Structural Chemistry, the journal (volume 21, 2010) and the discipline

“…The compounds were also studied by using ab initio and DFT methods. It would be interesting to compare the enthalpy of formation of these two species with those of 2,4-dimethoxybenzoic acid and that recently found for its 2,5-isomer [246].…”