Is trans or cis less stable?

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Recently, while reading about isomers I learned that cis isomers are more stable than trans isomers.

I referred many books as well as searched around the internet but I didn't find a suitable explanation.

Could someone please explain this to me?

asked Dec 6, 2014 at 17:33

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As far as I am aware, and from a quick browse though some textbooks, the trans isomer is generally more stable than the cis isomer. This is due to the reduced steric hindrance of the substituents in the trans configuration versus the cis configuration. For example trans-but-2-ene is more stable the cis-but-2-ene because there is less steric interference between the two methyl groups either side of the double bond. However, there are exceptions to this rule which I couldn't find a quick explanation for. The rule applies to both cyclic and acyclic compounds although there are some obvious cyclic exceptions such as cyclic alkenes, where the cis isomer is more stable, especially for smaller rings, due to the large ring strain caused by the orientation of the trans isomer. This page has a good illustration of this:

answered Dec 6, 2014 at 18:54

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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.

2. 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 $38.5\ce{kJ.mol^{-1}}$. 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

answered Dec 6, 2014 at 19:38

Yomen AtassiYomen Atassi

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cis is more stable than trans as long as no. of constituent C atoms is less than 11. For more than 11 and for acyclic , trans is more stable....the reason is that like radicals and charges repel however for less than11 the repulsion will be balanced best by cis

answered Dec 28, 2016 at 5:54

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Trans alkenes are more stable than cis alkenes because they have fewer steric interactions.

Explanation:

In cis-but-2-ene, there is steric hindrance, because the methyl groups are on the same side of the double bond.

Because of this steric crowding, there are van der Waals repulsive forces between the electron clouds of the groups.

This decreases the stability of the cis alkene.

In trans-but-2-ene, the methyl groups are on opposite sides of the double bond, and the repulsion does not occur.

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