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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?

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  • $\begingroup$ just as like Charges repell each other and the unlike charges attract each other......Keep this rule in mind and think that cis isomers are Same and Near to each other So the they repell each other As well as the trans isomers are far away from each other and Opposite so not to repell and remaiNs Stable....... $\endgroup$
    – user33883
    Commented Aug 17, 2016 at 19:39
  • $\begingroup$ This might be helpful: en.wikipedia.org/wiki/Steric_effects :) $\endgroup$
    – getafix
    Commented Aug 17, 2016 at 19:48
  • $\begingroup$ I'd like to see the scientific basis behind this rule of yours. $\endgroup$
    – M.A.R.
    Commented Aug 17, 2016 at 20:01

3 Answers 3

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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

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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:

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  • $\begingroup$ What about two OH groups that are cis to each other? Would they be more stable than if they were trans due to hydrogen bonding? $\endgroup$
    – Melanie Shebel
    Commented Nov 30, 2015 at 2:59
  • $\begingroup$ @MelanieShebel Yes, intramolecular H-bonding would have a stabilising effect in cis-1,2-diols. $\endgroup$
    – bon
    Commented Nov 30, 2015 at 10:20
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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

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