ENTHALPIES OF FORMATION OF HEXAETHYLETHANE, OCTAMETHYLHEXANE, AND TRI-TERT-BUTYLMETHANE - EXTREMELY STRAINED HYDROCARBONS

3,3,4,4-Tetraethylhexane (1) and 2,2,3,3,4,4,5,5-octamethylhexane (2) were synthesized from 3-ethylpentane and 2,2,3-trimethylbutane, respec tively, by UV-irradiation with mercury in the gas phase. The enthalpie s of combustion Delta H-c degrees of 1, 2, and tri-tert-butylmethane ( 3) when measured calorimetrically were Delta H-c degrees = -9467.2 +/- 2.3, -9491.4 +/- 2.3, and -8825.0 +/- 4.3 kJ.mol(-1), respectively. T he enthalpies of vaporization of 1 and 2 and the enthalpy of sublimati on of 3 were derived from the temperature dependence of their vapor pr essures. The latter were determined by a flow method. The resulting st andard enthalpies of formation were Delta H-f degrees (g) = -265.5 +/- 2.6, -248.3 +/- 2.4, and -235.2 +/- 4.3 kJ.mol(-1) for 1, 2, and 3, r espectively. They correspond to strain enthalpies (H-s) of 118.1, 177. 2, and 156.1 kJ mol(-1), respectively. These values provide valuable t est cases for semiempirical and empirical calculations.