一些烷氯代烃标准蒸发焓的测定

在进行有机氯化物分子内非键合相互作用的热化学研究中, 为了排除分子间的相互作用, 需要准确的标准蒸发焓或升华焓, 以便把凝聚态的标准生成焓转变为气态的数据。文献中多氯代烷烃的蒸发焓数据不多, 其中由直接量热法得到的准确数据更少, 因此我们测量了一些多氯代烷烃的标准蒸发焓, 并对其规律作了一些探讨。

ENTHALPIES OF VAPORIZATION OF SOME MUITICHLORO-ALKANES

By using an LKB 8721-3 vaporization calorimeter, the standard enthalpiea of vaporization of some multichloro-alkanes have been determined as the following: 1, 2-dichloroethane, 35.12±0.05; 1,1-dichloroethane, 30.57±0.05; 1,2-dichloropropane, 36.14±0.05; 1,3-dichloropropane, 40.61±0.10; 1,2,3-trichloropropane, 47.75±0.10; 1,4-dichlorobutane, 46.36±0.03; 1,2-dichlorobutane, 40.16±0.12 kJ mol~(-1) respectively. A linear equation, ΔH_ν~0=21.33+0.1589 t_b, can be used to fit the experimental data of dichloroalkanes, where t_b is the normal boiling point of compounds. A comparison of the ΔH_v~0 of dichloroalkanes and their molecular structures shows that (1) when a Cl atom on the primary carbon isomerizes onto the secondary carbon, the ΔH_v~0 of the isomer will decrease; (2) when the number of carbon atoms linked between the two chlorine atoms increases, the ΔH_v~0 of the isomer will increase in the order: 2,2-<1,1-<1,2-<1,3-<1,4-≈1,5-dichloroalkanes. This can be considered as a result of the Cl…Cl interaction in the molecules which makes a decrease in the depole moment of C-Cl bonds or in the formal charge on the Cl atoms, and so in the intermolecular electroatatic interaction energy. The decreae in the latter is estimated from the difference between calculated value from the equation, ΔH_v~0 (a,b-Cl_2C_nH_(2n))=ΔH_v~0 (a-ClC_nH_(2n+1))+ΔH_v~0 (b-ClC_nH_(2n+1))-ΔH_v~0 (C_nH_(2n+2)), and experimental result as the following: for 1,1-≈2,2-dichloroalkanes, ~7; 1,2-dichloroalkanes, ~4; 1,3-dichloroalkanes, ~1.5; 1,4-≈1,5-dichloroalkanes, ~0 kJ mol~(-1), respectively.