HEAT-CAPACITIES OF CHLOROCYCLOHEXANE AND BROMOCYCLOHEXANE BETWEEN THE TEMPERATURES 10 K AND 300 K, AND PHASE-TRANSITIONS IN THE CRYSTALLINE STATE FOR CHLOROCYCLOHEXANE

Heat capacities of chlorocyclohexane (CAS[542-18-7]), c-CHCL(CH2)(5), and bromocyclohexane, (CAS[108-85-0]), c-CHBr(CH2)(5), were measured with adiabatic calorimeters in the temperature range between T = 8 K and T = 300 K and the standard thermodynamic functions were evaluated. For chlorocyclohexane, temperatures, molar enthalpies, and molar entropies were determined to be(220.24 +/- 0.03) K, (8.11 +/- 0.02) kJ . mol(-1), and(36.8 +/- 0.1) J . K-1. mol(-1), respectively, for the crystal-to-(orientationally disordered crystal) (denoted by II-to-I) phase transition; and (228.01 +/- 0.05) K, (2.02 +/- 0.01) kJ . mol(-1), and (8.9 +/- 0.1) J . K-1. mol(-1), respectively, for fusion. Moreover, two other heat-capacity anomalies were found at T = (118.6 +/- 0.1) K and T = (148 +/- 1) K and are suggested to be successive (denoted by IV-to-III and III-to-II, respectively) phase transitions of the superstructural commensurate-to-incommensurate-to-(norma commensurate) phase-sequence type. Molar entropies of the IV-to-III and III-to-II phase transitions were estimated to be (0.7 +/- 0.1) J . K-1. mol(-1) and (0.05 +/- 0.01) J . K-1. mol(-1), respectively. Large orientational disorder amounting to S-m(o) approximate to 4.6 J . K(-1)mol(-1) was suggested to exist even in phase II of the ''globular molecule'' chlorocyclohexane. Bromocyclohexane, on the other hand, displayed only fusion at T = (216.87 +/- 0.03) K. The molar enthalpy and molar entropy of fusion were determined to be (10.79 +/- 0.03) kJ . mol(-1) and (49.8 +/- 0.1) J . K-1. mol(-1), respectively. (C) 1995 Academic Press Limited.