Branched alkanes like isobutane and neopentane are more stable than their straight chain isomers, n-butane and n-pentane (by 2.8 and 5.5 kcal/mol, respectively) due to the greater number of stabilizing 1,3-interactions. For example, there are three 1,3-interactions in isobutane but only two in n-butane. However, such attractive 1,3-interactions (which we call “protobranching”) also stabilize all n-alkanes relative to ethane (which has none). There is one in propane, two in n-butane, three in n-pentane, etc. “protobranching,” is not considered in conventional evaluations, e.g., of the strain energy of rings and of hyperconjugation. When protobranching is taken into account, the ring strain of cyclopropane is reduced from 27.7 kcal/mol (based on propane, which is stabilized by one protobranch) to 19.2 kcal/mol (based on ethane). Correction for torsional strain reduces this value further, by 3-6 kcal/mol. Values for hyperconjugation, +5.3 and +7.7 kcal/mol for alkenes and alkynes respectively, are obtained by adding the protobranching corrections.
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