Two samples of pure mezo- and d, l-2,3-dichlorobutanes were obtained from their mixture by fractional distn., and their enthalpies of vaporization at 25?were measured by using an LKB-8721-3 vaporization calorimeter, they are 38.41 ?0.07 (mezo-) and 39.73 ?0.05 (d, l-) kJ/mol, resp. A mixture sample with (mezo-/ d, l-) = 1.34/1 was used for combustion experiments, its enthalpies of combustion at 25?were determine by using a rotating bomb combustion calorimeter with a tantanum lined bomb, as -2519.74 ?1.06 kJ/mol. By using the literature's enthalpies of isomerism between the liquid meso- and d, l-2,3-dichlorobutanes, -1.13 ?0.21 kJ/mol, measured by the equilibrium method, enthalpies of combustion of meso- and d, l-2,3-dichlorobutanes were obtained. Then enthalpies of formation of both in gaseous state were derived as, -202.34 ?1.72 (meso-) and -202.15 ?1.72 (d, l-) kJ/mol, resp. According to the following equation: a,b-C4H8Cl2(g) + n-C4H10(g) = a-C4H9Cl + b-C4H9Cl + DHr; the enthalpies of reaction are derived as +5.3(meso-) and +5.5(d, l-) kJ/mol, which are closed with that of 1,2-dichlorobutane. The determine enthalpies of formation of the meso- and d, l-2,3-dichlorobutanes also coincide with the calculation results of the authors' "Electrostatic Bond Energy Scheme".

Key words: ENTHALPY OF FORMATION, ENTHALPY, CALORIMETRY