Sharp difference in the change in molar volumes and enthalpy of dissolution of 2D and 3D molecules in solvents illustrated by the Diels-Alder reaction

A sharp difference in the change in the molar volume during the transition from the solid phase to the solution for a number of 2D molecules of reagents compared with 3D molecules of the Diels-Alder reaction adducts was observed. The dissolution of 2D crystal molecules is accompanied by a significant increase in volume (10–15%) with a dissolution enthalpy close in magnitude to the fusion enthalpy. A similar transition for 3D crystal molecules is often accompanied by an unexpected decrease in volume and even negative enthalpy of dissolution in inert solvents. The similar change in these parameters is also considered for a number of 3D molecules: 1,3,5-triphenylbenzene, hexaphenylbenzene, 9,10-diphenylanthracene, congressane, and C-60 fullerene. This can be explained by the fact that the surface of large 3D molecules in a solution can be much more thoroughly surrounded by solvent molecules than in its own environment. These results indicate that the values of activation and reaction volumes and their ratio (ΔV≠S/ΔVr-n, S) may be weakly related to the mechanism of non-polar cycloaddition reactions.