Thermochemistry of organic molecules: The way to understand energy–structure relationships

Abstract

The combination of experimental calorimetric measurements, particularly of the standard energies and enthalpies of combustion and formation, and theoretical examination of model molecules constitutes a powerful tool for the understanding of the conformational and chemical behavior of organic molecules.In this article, several examples are provided where the synergy between experiment and theory made possible the comprehension of various fundamental interactions in oxygenand sulfur-containing six-membered heterocyclic compounds, the determination of the strain energy in two C 8 H 8 derivatives, dimethyl cubane-1,4-dicarboxylate and dimethyl cuneane-2,6-dicarboxylate, and the calculation of the enthalpies of formation of the parent compounds, cubane and cuneane, and the study of the energy-structure relationship in barbituric acid.that are responsible for the enthalpy of formation of each isomer. As an example, we can consider the enthalpies of formation of the three isomers of formula C 5 H 12 , pentane, 2-methylbutane, and 2,2-dimethylpropane determined and published by Good [1] and given in Table 1.

Experimental and Theoretical Thermochemistry of the Isomers 3- and 4-Nitrophthalimide

“…For isomers, it is possible to determine the relative stability from the standard enthalpies of formation in the gas phase of each isomer. 70 As can see in Table 9, this enthalpy is more negative for 4NFT than that for 3NFT; this means that the isomer with the nitro group in position 4 is more stable than that with the nitro group in position 3. The enthalpic difference, which equals (−21.2 ± 7.6) kJ•mol −1 , corresponds to the enthalpy of isomerization, that is, to the enthalpy due to the change in position of the nitro group from position 3 to 4.…”

“…For isomers, it is possible to determine the relative stability from the standard enthalpies of formation in the gas phase of each isomer . As can see in Table , this enthalpy is more negative for 4NFT than that for 3NFT; this means that the isomer with the nitro group in position 4 is more stable than that with the nitro group in position 3.…”