Standard molar enthalpies of formation for crystalline vanillic acid, methyl vanillate and acetovanillone by bomb calorimetry method

Abstract

The massic energies of combustion for three crystalline monosubstituted guaiacols such as vanillic acid, methyl vanillate and acetovanillone were determined by using bomb calorimeter method and were found to be − (21,070.3 ± 8.6), − (23,352.3 ± 7.4) and − (26,732.2 ± 5.2) J g−1, respectively. The standard molar enthalpy of combustion and standard molar enthalpy of formation in the crystalline state at 298.15 K were calculated. They are − (3542.9 ± 1.5) and − (748.5 ± 1.8) kJ mol−1 for vanillic acid, − (4255.4 ± 1.7) and − (715.3 ± 2.1) kJ mol−1 for methyl vanillate, − (4444.7 ± 0.9) and − (526.1 ± 1.5) kJ mol−1 for acetovanillone, respectively. The basic contribution in a value of standard molar enthalpy of formation for oxygen-containing aromatic compounds provides the replacement of the hydrogen atom in the benzene ring to the oxygen-containing group. The calculation of these contributions gives the following values for the crystalline state at 298.15 K for substituents CHO − (150 ± 6), COCH3 − (201 ± 4), COOCH3 − (392 ± 6) and COOH − (428 ± 4) kJ mol−1. The differences in contributions for crystalline and gaseous states were estimated for groups CHO, COOCH3, COCH3 − (26 ± 3) kJ mol−1 in monosubstitute benzene derivatives, 4-substituted anisoles and guaiacols, − (35 ± 4) kJ mol−1 in 4-substituted phenols and for group COOH − (50 ± 3) kJ mol−1 in all abovementioned groups of compounds. More higher differences in contributions found for 4-substituted phenols and benzenecarboxylic acids are attributed to the hydrogen bonds formation in crystalline state of these derivatives.