Abstract:

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.