Thermochemistry of conjugation of simple cyclopropane derivatives

Heats of vaporization of cyclopropyl cyanide and cyclopropylbenzene have been measured by vaporization calorimetry, and combined with liquid state heats of formation to give gaseous state heats of formation. When these compounds, cyclopropane, 1,1-dimethylcyclopropane, and cyclopropylamine are involved in (hypothetical) isodesmic reactions with ethane to give the isopropyl derivative and propane, the enthalpies of reaction are −116.0 ± 1.3 kJ mol −1 , except for cyclopropylbenzene (−103.7 kJ mol −1 ). The latter is less exothermic, in part, because of slight steric destabilization of the product, isopropylbenzene, but mainly due to 8 kJ mol −1 stabilization of cyclopropylbenzene, which is the only of these cyclopropyl derivatives showing appreciable thermochemical stabilization.