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Total rewrite as was a panic-stricken 2am question.

Unfortunately I wrote on an application that VSEPR theory is a reason of Cis-Trans isomerism (specifically in fatty acids however I doubt that would make any difference). Although in my head it seemed to work at the time, I appreciated about a week after that I was actually thinking of the wrong type of thing.

My question is,

How can VSEPR and/or the electron configurations in an unsaturated fatty acid explain how cis and trans isomerism arises?

As far as I'm aware, the Pi bond prevents (resists?) rotation around the C=C link, but I feel as if I'm missing something.

Also,

Does the geometry caused by the repulsion of H's and alkyl electrons on either side of the C=C bond affect the isomers differently depending on whether or not they are Cis or Trans?


One of the below posters also highlighted a good question, does anybody have any insight on this?: "I think the main question is whether there's a preference for cis or trans isomers in a fatty acid based on steric/VSEPR considerations" (Thank you for your response)

Thanks for your help

Bonus related question:

Is there any preference for a specific stereoisomer in the dehydrogenation of a fatty acid's alkyl chain? If so, what are they and why?

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    $\begingroup$ What difference did you want to rationalize? $\endgroup$ – Dissenter Nov 13 '14 at 21:38
  • $\begingroup$ While I think VSEPR theory is not suitable for many things (it is a little bit too simple and outdated), I think it is especially not suited for this problem. I also backup @Dissenter's comment as I am not quite sure what you want to know. Maybe you can edit your question, to make it a little clearer. Do you want to rationalize the energy difference between cis/trans isomers or the structural differences in the surrounding environment of the double bond? $\endgroup$ – Martin - マーチン Nov 13 '14 at 23:09
  • $\begingroup$ I think the main question is whether there's a preference for cis or trans isomers in a fatty acid based on steric/VSEPR considerations. In practice, I think both isomers are found in biological materials. $\endgroup$ – Geoff Hutchison Nov 14 '14 at 0:49
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Answered my own bonus question (I believe): In case anybody was interested,

Trans seems to be favoured due to the methyl group on either side of a Cis bond repelling eachother (due to to both the methyl groups wanting to retain their tetrahedral structure, hydrogens are repelling one another and hence distorting things), and hence the trans is more energetically favourable.

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I have a feeling that looking at the question from the perspective of evolutionary biochemistry is more important than looking at it from purely bonding model perspective.

Cis fatty acids tend to have lower melting and boiling points than saturated fatty acids.
The cis geometry puts a "kink" or bend in the chain which makes efficient packing difficult, reducing the strength of the intermolecular forces holding the molecules to each other. The weaker forces result in lower melting points.

Most living things produce a mixture of different saturated and unsaturated fatty acids, and this mixture has been refined by evolution to suit the needs of the species.

In most species of plants, we tend to see 25% or less saturated fat, and the rest as a mixture of different unsaturated fats. This is not the case for palm kernel and coconut oils, which can have as much as 80% saturated fat. Reference

In contrast, land animals like pigs and cows have 40%-50% of their fats as unsaturated. Ducks, in contrast have only about 30% of their fat as saturated.

Individuals that produce these lower-melting fats had a competitive advantage over those that didn't, so the genes for the production of a greater percentage of unsaturated (cis) fats spread through the population.

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