Question

Show how octane-2,7-dione might cyclize to a cycloheptenone. Explain why ring closure to the cycloheptenone is not favored.

Short answer

The stability of cyclic compounds depend on the angle strain. Rings with large angle strain are less stable than rings with smaller angle strain.

The cyclic compound with more stability has less heat of combustionthan the cyclic compound with less stability.

Step-by-step solution

Ring Stability

The stability of cyclic compounds depend on the angle strain. Rings with large angle strain are less stable than rings with smaller angle strain.

The cyclic compound with more stability has less heat of combustionthan the cyclic compound with less stability.

 Cycloheptenone is not favorable

Octane-2,7-dione might cyclize to cycloheptanone which is a 7-membered ring, and its formation is unfavorable for entropy reasons as follows:

• The 7 membered ring will have high angle strain than straight chain heptane. Moreover larger the rings lower is the possibility of the reacting ends coming together
• The molecule has the possibility of forming a 5-membered ring or a 7-membered ring; it will almost always prefer to form the 5-membered ring

Thus cyclopentenone (7-membered ring) changes to form 1-(2-methylcyclopent-1-enyl)ethenone (5-membered ring).

Formation of 1-(2-methylcyclopent-1-enyl)ethanone