Structure and Energetics of a New Hydrate of 4′-Hydroxyacetophenone

A new hydrate of 4′-hydroxyacetophenone (HAP), triclinic, space group P1̅, was isolated and characterized. Both the structure determination by single crystal X-ray diffraction at 150 K and the results of thermogravimetry analysis carried out in the temperature range 263−363 K indicated the H2O/HAP molar ratio to be 1.5. The water molecules line up in chains that reside in lattice channels (channel hydrate) and are sustained by OH···O hydrogen bonds. The chains also interact with the OH (donor) and CO (acceptor) groups of the HAP molecules through OH···O hydrogen bonds. The results suggest that the OH and CH3CO groups of the HAP molecules in the hydrate are in a syn orientation. This conformation is the same as that observed for the previously described anhydrous form II (orthorhombic, P212121). The dehydration process generates, however, the form I HAP polymorph (monoclinic, P21/c), where the anti conformation is preferred. From drop-sublimation Calvet microcalorimetry experiments it was possible to conclude that Δf H°m(HAP·1.5H2O, cr) = −812.5 ± 3.1 kJ·mol−1, at 298.15 K. Finally, thermodynamic analysis of the temperature and relative humidity effects on the dehydration process to yield HAP(cr I) and H2O(g) indicated that loss of water becomes favorable (ΔrGm < 0) at 298.15 K, for a relative humidity of ∼66%. This value is compatible with routine laboratory observations of the HAP·1.5H2O(cr) stability at ambient temperature. The analysis also suggested that, independently of the relative humidity, the hydrate will tend to decompose above ∼331 K, a temperature that closely matches the high limit of the dehydration range observed in the TG experiments.