Heats of vaporization of some monosubstituted cyclopropane, cyclobutane, and cyclopentane derivatives. Some observations on the enthalpies of isodesmic ring opening reactions of cyclobutane derivatives

The following heats of vaporization at 25 °C (ΔHv0) have been measured by vaporization calorimetry: cyclopropyl methyl ketone (39.41 ± 0.09 kJ mol−1). methyl cyclopropanecarboxylate (41.27 ± 0.06 kJ mol−1), cyclobutyl cyanide (44.34 ± 0.20 kJ mol−1), methyl cyclobutanecarboxylate (44.72 ± 0.09 kJ mol−1), ethylcyclobutane (31.24 ± 0.03 kJ mol−1), and cyclopentyl cyanide (48.12 ± 0.09 kJ mol−1). Values of ΔHv0 of the last four compounds have been combined with liquid state heats of formation to give new values of the gaseous state heats of formation (ΔHf(g)).When cyclobutyl cyanide, ethylcyclobutane, and cyclobutylamine are involved in hypothetical isodesmic reactions with ethane to give propane and isopropyl derivatives, the enthalpies of reaction are −102.1 ± 1.7 kJ mol−1 (−24.4 ± 0.4 kcal/mol), 8.3 kJ mol−1 (2.0 kcal/mol) less exothermic than the value for cyclobutane. Possible origins of this difference are discussed.