Thermochemistry of methyl-D-glucopyranosides and methyl-D-galactopyranosides

ISSN： 0021-9614

Source： Journal of Chemical Thermodynamics, Vol.32, Iss.6, 2000-06, pp. : 767-775

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Abstract

A 1230 Parr static bomb calorimeter was modified to reduce and stabilize the heat exchange between the calorimetric vessel and its surroundings. Simultaneously, the temperature sensor has been coupled to an automatic data collection system to capture time–temperature data during a combustion experiment. These improvements have permitted measurements of the energies of combustion of compounds containing C, H, and O with a precision better than 0.01 per cent. With this calorimetric system, the standard molar enthalpies of combustion and formation in the condensed state for methyl- α -d-glucopyranoside, methyl- β -d-glucopyranoside, methyl- α -d-galactopyranoside and methyl- β -d-galactopyranoside were measured. From experimental results, the following thermodynamic functions for the different pyranosides at T = 298.15 K were determined:Methyl- α -d-Methyl- β -d-Methyl- α -d-Methyl- β -d-glucopyranosideglucopyranosidegalactopyranosidegalactopyranoside∆_cU_m^o(cr, 298.15K) / (kJ · mol^{- 1}) - 3514.99 ± 0.37 - 3512.87 ± 0.41 - 3516.96 ± 0.30 - 3504.51 ± 0.35∆_cH_m^o(cr, 298.15K) / (kJ · mol^{- 1}) - 3516.23 ± 0.37 - 3514.11 ± 0.41 - 3518.20 ± 0.30 - 3505.75 ± 0.35∆_fH_m^o(cr, 298.15K) / (kJ · mol^{- 1}) - 1239.15 ± 0.88 - 1241.27 ± 0.86 - 1237.18 ± 0.91 - 1249.63 ± 0.89∆_fusH_m(T_{f us}) / (kJ · mol^{- 1})35.11 ± 0.0927.03 ± 0.1027.95 ± 0.0833.18 ± 0.07T_fus / K436 ± 0.04384.9 ± 0.06397.62 ± 0.08450.9 ± 0.73The values of the standard molar enthalpy of formation in the condensed state show less relative stability of the axial isomers, due to a steric hindrance between the axial methoxyl group and the neighboring hydrogens. The enthalpies of fusion measured by d.s.c. reflect a different number and strengths of intermolecular hydrogen bonds in each of the isomers.