Q51 - Organic Chemistry (Mcmurry)

Question

Q 51 Normally, a trans alkene is more stable than its cis isomer trans-Cyclooctene, however, is less stable than cis-cyclooctene by 38.5 kJ/mol. Explain.

Step-by-Step Solution

Answer

Answer

Trans alkene is more stable than cis alkene due to the less steric strain present in the trans-isomers.

Step 1: Stability of alkene

Trans alkene is more stable than cis 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 an eight-membered ring, cis form is more stable than trans-form due to more flexibility in the ring.

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

After the eight-membered ring, the flexibility of the ring increases, so both the isomers of cyclooctene exist. But trans cyclooctene is less stable due to a large amount of strain when the present in the trans-form as compared to cis form

Isomers of cyclooctene have large energy differences due to a large amount of steric strain present in the isomers