Q7-52E - Organic Chemistry (Mcmurry)

Question

Trans-Cyclooctene is less stable than cis-cyclooctene by 38.5 kJ/mol, but trans-cyclononene is less stable than cis-cyclononene by only 12.2 kJ/mol. Explain.

Step-by-Step Solution

Answer

a

Step 1: Stability of alkene

Cis alkene is more stable as compared to trans alkene due to the less steric strain present in the trans isomers. But this concept is not valid only above eight membered ring systems.

Above eight membered ring cis form are more stable as compared to trans form due to more flexibility in the ring.

Step 2: Explanation for trans-Cyclononene is less stable than cis-cyclononene

In cyclononene less amount of steric present in the nine membered ring and cyclononene can also be present in the puckered form. Hence the isomers of cyclononene have less energy difference.

On the other hand the isomers of cyclooctene have large energy difference due to large amount of steric strain present in the molecule.