Question

trans-Cyclooctene has been resolved, and its enantiomers are stable at room temperature. trans-Cyclononene has also been resolved, but it racemizes with a half-life of \(4 \mathrm{~min}\) at \(0^{\circ} \mathrm{C}\). How can racemization of this cycloalkene take place without breaking any bonds? Why does trans- cyclononene racemize under these conditions but trans-cyclooctene does not? You will find it especially helpful to examine the molecular models of these cycloalkenes.

Short answer

Answer: The key factor contributing to trans-cyclononene's racemization compared to trans-cyclooctene is the presence of steric strain in its molecular structure. This strain causes the molecule to adopt a less stable conformation, making it more susceptible to undergo racemization without breaking any bonds. In contrast, trans-cyclooctene possesses a more stable conformation with less steric strain, preventing racemization under the same conditions.

Step-by-step solution

Understand racemization

Racemization is a process in which optically active compounds convert to a 50:50 mixture of its enantiomers, leading to an optically inactive mixture. It occurs when a molecule undergoes a conformational change and exchanges positions with its enantiomer.

Examine molecular models

In order to explain the difference in racemization behavior between trans-cyclooctene and trans-cyclononene, you should examine their molecular models. This will help you identify the unique structural features contributing to the stability of each compound and their susceptibility to racemization.

Identify steric strain in trans-cyclononene

A critical factor contributing to the racemization of trans-cyclononene is the presence of steric strain, meaning that atoms in the molecule experience repulsive forces due to close atomic contacts between non-bonded constituents. This strain causes the molecule to adopt a less stable conformation, making it more susceptible to undergo racemization.

Compare the steric strain of both cycloalkenes

Comparing the structure of trans-cyclooctene and trans-cyclononene, trans-cyclooctene has a more stable conformation thanks to its less strained structure. This difference in steric strain and stability between the two cycloalkenes can be linked to the absence of racemization in trans-cyclooctene.

Explain racemization without breaking bonds

Trans-cyclononene is susceptible to racemization without breaking any bonds due to the presence of steric strain, which destabilizes the structure of the molecule. The repulsive forces between non-bonded constituents allow the molecule to "flip" its conformation without breaking any bonds, facilitating the racemization.

Concluding remarks

In conclusion, trans-cyclononene racemizes with a half-life of \(4 \mathrm{~min}\) at \(0^{\circ} \mathrm{C}\) due to the steric strain in its molecular structure. This strain leads to a less stable conformation, allowing the molecule to undergo racemization without breaking any bonds. In contrast, trans-cyclooctene does not racemize under the same conditions because it possesses a more stable conformation with less steric strain. Examining the molecular models of these cycloalkenes is essential to understanding their differences in stability and racemization behavior.