Sublimation thermodynamic parameters for 5-fluorouracil and its 1-methyl and 1,3-dimethyl derivatives from vapor pressure measurements

The vapor pressures for 5-fluorouracil, 1-methyl-5-fluorouracil, and 1,3-dimethyl-5-fluorouracil were measured by a torsion method. The following linear equations, representative of the temperature dependence in the covered temperature ranges, were selected: 5-fluorouracil, log(p/kPa) = (15.18 +/- 0.20) - (7860 +/- 100)K/T, (421-483) K; 1-methyl-5-fluorouracil, log(p/kPa) = (12.64 +/- 0.20) - (6083 +/- 100)K/ T, (381-423) K; 1,3-dimethyl-5-fluorouracil, log(p/kPa) = (15.16 +/- 0.40) - (6286 +/- 200)K/T, (338-473) K. From these equations the standard sublimation enthalpies and entropies, Delta(sub)Hdegrees (298 K) = 154 +/- 5, 116 +/- 2, and 119 +/- 4 kJ(.)mol(-1) and Delta(sub)Sdegrees (298 K) = 259 +/- 8, 203 +/- 4, and 252 +/- 8 J(.)K(.)(-1)mol(-1), respectively, for 5-fluorouracil, 1-methyl-5-fluorouracil, and 1,3-dimethyl-5-fluorouracil were determined. The vaporization behavior for these compounds was compared with that for uracil and corresponding methyl derivatives.