Enthalpies of fusion and solid-to-solid transition,entropies of fusion for urea and twelve alkylureas

Enthalpies and temperatures of fusion have been measured by differential scanning calorimetry for urea and a number of its mono- and di-alkyl derivatives. Enthalpies obtained are: urea, 14.79 kj mol⁻¹; monomethylurea, 15.75 kJ mol⁻¹ ; monoethylurea, 13.94 kJ mol⁻¹ ; monopropylurea, 14.63 kJ mol⁻¹ ; monoisopropylurea, 17.40 kJ mol⁻¹ ; monobutylurea, 14.55 kJ mol⁻¹ ; monotertbutylurea, 33.13 kJ mol⁻¹ ; dimethyl-1,1 urea, 29.61 kJ mol⁻¹ ; dimethyl-1,3 urea, 13.62 kJ mol⁻¹; diethyl-1,1 urea, 16/78 kJ mol⁻¹ ; diethyl-1,3 urea, 12.46 kJ mol⁻¹ ; dibutyl-1,3 urea, 14.87 kJ mol⁻¹; trimethyl-1,1,3 urea, 14.30 kJ mol⁻¹. Entropies of fusion have been derived from the experimental results.By temperature scanning starting from r.t. some solid-to-solid transitions for four alkylureas have also been detected, all hitherto unreported. Temperatures and enthalpies of transition are: for monoisopropylurea, 375.5 K and 2.31 kJ mol ⁻¹ ; for monobutylurea (two transitions), 313.1 K and 7.02 kJ mol⁻¹ , 344.9 K and 0.88 kJ mol⁻¹ ; for diethyl-1,3 urea, 339.4 K and 1.87 kJ mol⁻¹ ; for dibutyl-1,3 urea, 311.5 K and 11.10 kJ mol⁻¹.