Answer:
The ring strain in cyclopropane
Step-by-step explanation:
The ring strain in cyclopropane is the factor responsible for a more excellent heat of combustion per CH_2 for cyclopropane than the heat of combustion per CH_2 for cyclohexane. Cyclopropane has a higher heat of combustion per CH_2 unit than cyclohexane due to its higher ring strain.
Cyclopropane is a three-membered ring, and its C-C-C bond angles are approximately 60 degrees, which deviates significantly from the ideal tetrahedral angle of 109.5 degrees. This deviation causes increased ring strain and higher potential energy in cyclopropane than cyclohexane, which has a more stable six-membered ring with C-C-C bond angles close to 109.5 degrees.
The higher ring strain in cyclopropane results in a more excellent heat of combustion per CH_2 unit, as more energy is required to break the strained C-C bonds during combustion. On the other hand, cyclohexane has a lower heat of combustion per CH_2 unit due to its more stable structure and lower ring strain.