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, respectively, by UV-irradiation with mercury in the gas phase. The enthalpies of combustion Δ H c ° of 1, 2, and tri- tert -butylmethane ( 3 ) when measured calorimetrically were Δ H c ° = −9467.2 ± 2.3, −9491.4 ± 2.3, and −8825.0 ± 4.3 kJ·mol - 1, respectively. The enthalpies of vaporization of 1 and 2 and the enthalpy of sublimation of 3 were derived from the temperature dependence of their vapor pressures. The latter were determined by a flow method. The resulting standard enthalpies of formation were Δ H f ° (g) = −265.5 ± 2.6, −248.3 ± 2.4, and −235.2 ± 4.3 kJ·mol -1 for 1, 2, and 3, respectively. They correspond to strain enthalpies ( H s ) of 118.1, 177.2, and 156.1 kJ·mol - 1, respectively. These values provide valuable test cases for semiempirical and empirical calculations.