A precision vaporization calorimeter was used to measure the heats of evaporation of 2,2-dichlorobutane, 2,3-dichlorobutane, 1,2-dichlorobutane, 1,3-dichlorobutane, and 1,4-dichlorobutane. At 25? the heats of vaporization of dichlorobutanes are linearly related to their b.ps. (in 癈). When the no. of carbon atoms (Nc) between two chlorine atoms is <4 the experimental heat of evaporation decreases with the increasing Nc and is less than the value calculated by a group contribution model. When Nc < 4, the determine intermol. dipole-dipole interaction energy of a dichlorobutane decreases with the decrease of Nc. According to the mol. vol. values, the average distances between liquid dichlorobutane mols. are approx. equal to those between liquid monochlorobutane mols.

Key words: ISOMER