'A' Level Chemistry Problem Analysis: Stability of Cycloalkene Isomers

QN: For cycloalkenes, are cis or trans geometric isomers more stable?

Thought process:

Firstly, you must appreciate why trans alkenes are generally more stable than cis alkenes, the reason being that destabilizing steric strain arises within the cis isomer which is caused by van der Waals repulsions between the 2 bulky alkyl groups on the same side of the double bond.

Secondly, for cycloalkenes possessing less than 8 carbon atoms, only the cis geometric isomer is viable, because the trans geometric isomer is simply far too unstable to exist - this due to severe ring strain arising from significant angle strain (ie. significant deviations from the ideal bond angles as predicted by VSEPR orbital hybridization theory, which is to maximimze thermodynamic stabilities by minimizing electron pair repulsions).

Thirdly, combining the 2 points above (ie. considering both opposing factors), the cis geometric isomers of cycloalkenes with 8 to 11 carbon atoms are therefore more stable than the trans geometric isomers. For cycloalkenes with 12 caron atoms and above, the trans geometric isomers are on the other hand more stable.

PS: One may wish to consult this LINK for additional reading.


The above content is contributed by Mr Heng, owner and 'A' Level Chemistry tutor at Bedok Funland JC. He also goes by the handle UltimaOnline on various online popular homework forums.


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