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本文用升华量热法、落入量热法和差热分析方法,精密测定了邻位、间位和对位取代的三个溴代苯甲酸的升华焓、熔化焓及三相点温度,邻-溴、间-溴和对-溴苯甲酸在298.15K的标准升华焓分别为:(95.94±0.41),(99.20±0.18)和(103.08±0.59)kJmol~(-1)。三个化合物在相应升华温度下的饱和蒸气压也通过升华实验同时被测定出来。邻位、间位和对位异构体的三相点温度为:(422.37+0.01),(429.68±0.01)和(527.61±0.02)K;其熔化焓分别为(24.54±0.07),(21.27±0.09)和(28.70±0.08)kJmol~(-1)。 根据文献数据确定的苯环、酸基和溴功能团的能量贡献,计算出了对-溴代苯甲酸的升华焓,计算值与实验结果相符。比较三个异构体升华焓的差别,指出和解释了邻位异构体分子内氢键的存在。
In this paper, sublimation calorimetry, falling calorimetry and differential thermal analysis were used to determine the sublimation enthalpies, enthalpies of melting and three-phase temperatures of ortho, meta and para substituted bromobenzoic acids, The standard sublimation enthalpies of bromine, meta - bromo and para - bromobenzoic acid at 298.15K were (95.94 ± 0.41), (99.20 ± 0.18) and (103.08 ± 0.59) kJmol -1, respectively. The saturated vapor pressures of the three compounds at the corresponding sublimation temperatures were also determined by sublimation experiments. (422.37 +0.01), (429.68 ± 0.01) and (527.61 ± 0.02) K, respectively. The enthalpies of fusion for the ortho, meta and para isomers were (24.54 ± 0.07) and (21.27 ± 0.09) and (28.70 ± 0.08) kJmol -1. The enthalpies of sublimation of p-bromobenzoic acid were calculated based on the energy contributions of benzene rings, acid groups and bromine groups determined by literature data. The calculated values agree well with the experimental results. The difference of sublimation enthalpies between the three isomers was compared and the existence of intramolecular hydrogen bonds in the ortho-isomer was pointed out and explained.