Strain Energy of Small Ring Hydrocarbons. Influence of C−H Bond Dissociation Energies

Ab initio calculations at the G2, G3, and CBS-Q levels of theory have been applied to the question of the origin of ring strain in a series of unsaturated hydrocarbons. In addition to the angular ring strain germane to all three-membered ring hydrocarbons, a general trend is in evidence that suggests that the increased ring strain (SE) of unsaturated small ring alkenes may be attributed in part to their relatively weak allylic C-H bonds. The high strain energy of cyclopropene (54.1 kcal/ mol) is attributed largely to angular strain. The anomalously low SE of cyclobutene relative to cyclobutane (DeltaSE = 4 kcal/mol) is a consequence of normal C-H bond dissociation energies for cyclobutane (100.6 kcal/mol) and very strong vinyl C-H bonds (111.9 kcal/mol) and a relatively strong pi-bond energy (63.5 kcal/mol) for cyclobutene. The greater SE of methylenecyclopropane (39.5 kcal/ mol), relative to methylcyclopropane (29.8 kcal/mol), can be attributed to the strong ring C-H bonds of methylcyclopropane (110.5 kcal/mol) and relatively weak allylic C-H bonds (99.3 kcal/mol) of methylenecyclopropane. The increased SE of 1-methylcyclopropene relative to isomeric methylenecyclopropane is ascribed to its weak ring C-H bonds and to angular strain. The relative thermodynamic stability of a series of small ring alkenes is determined by a measure of their hydrogenation enthalpies. Independent confirmation of the SEs of a series of substituted cyclopropenes is provided by their dimerization/combination with cyclopropane to form a six-membered ring reference compound.