Question

Why and how trans isomer is more stable than cis isomer?

Answer 1

Contrary to the generally accepted belief, many cis isomers of olefinic compounds are more stable than their trans isomers. The stable cis form of substituted olefins such is 1–fluoro–1, 3–butadiene and l,4–difluoro–1,3–butadiene as being due to intramolecular van der Waals forces between the substituents [ Angew. Chem. , 75 , 793 (1963) ]. These forces are strong enough to hold the molecules in a cis–preferred configuration despite steric hindrance effects; these effects have been the basis for assuming that the trans form of such molecules is the more stable. But when the substituent atoms are so bulky that they overcome the intramolecular van der Waals forces, then the trans form becomes the more favored structure.

cis cycloalkenes in general are more stable than their trans isomers. The trans double bond causes strong twisting of the ring. Because of the resulting high ring strain small trans cycloalkenes have not been observed and cis isomers show considerable ring strain. However, the latter are sufficiently stable in order to exist. To form a trans isomer the cycloalkene ring must contain at least eight carbons. The energy difference between cis- and trans-cyclooctene is approximately 38.5kJ⋅mol−138.5kJ⋅molX−1. Eventually, trans isomers become more stable than cis isomers once the ring contains more than eleven carbons. For more details, please see also: http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/stereochemie/cyclische_stereoisomere.vlu/Page/vsc/en/ch/2/oc/stoffklassen/systematik_struktur/cyclische_verbindungen/carbocyclisch/cycloalkene/physikalische_eigenschaften.vscml.html