Experimental thermochemical study of fluoro-, chloro-, and bromo-derivatives of uracil

The values of the standard molar enthalpies of formation, of 5-fluorouracil, 5-chlorouracil, 5-bromouracil, and 6-chlorouracil, in the crystalline phase, at T = 298.15 K, were derived from the standard massic energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry.The combination of the derived values of the standard molar enthalpies of formation, in the crystalline phase, and the literature values of the standard molar enthalpies of sublimation, allowed the calculation of the standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, which are interpreted in terms of the influence of the halogen atoms upon the enthalpic stability of the halogen derivatives of uracil.

► Combustion calorimetry was used to determine ΔfHm∘(cr) of halo derivatives of uracil.
► Gas-phase ΔfHm∘ of the studied compounds have been derived.
► The influence of the halogen atoms on the enthalpic stability of the halo-uracils is discussed.