Trans alkenes more stable than cis? Even with internal H-Bonding?

[EECStoMed]

In general trans alkenes are more stable than cis alkenes but what if a cis molecule such as maleic acid (cis) compared to fumaric acid (trans) can intramolecularly hydrogen bond? Doesn't that make it more stable?

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[Dr OCD]

toughy. i would say that both trans and cis allow for equal amounts of hydrogen bonding, one is intra and the other intermolecular forces. however, its more likely that with intermolecular forces you would have a higher boiling point (an indicator of stability), so the trans confromation might have a greater boiling point and be more stable. but i doubt the mcat would ask you to split hairs this finely.

 

[SketchLazy]

toughy. i would say that both trans and cis allow for equal amounts of hydrogen bonding, one is intra and the other intermolecular forces. however, its more likely that with intermolecular forces you would have a higher boiling point (an indicator of stability), so the trans confromation might have a greater boiling point and be more stable. but i doubt the mcat would ask you to split hairs this finely.

I also agree that the MCAT would not ask you this because this seems like something only experimentation would solve. In terms of stability, you want to try to imagine the energy of the molecule. Unstable molecules are higher in energy than lower ones. So if there is a lot of steric hindrance than the molecule will be higher in energy because it will take a lot of energy to keep the molecules strained and less stable.

Edit:But if I had to guess. I would say the trans isomer is still the more stable one because 1)it is trans and 2)the oxygens with their large electron clouds are in close proximity in the cis formation. There is no possibility for rotation so it would require a lot of energy to keep them that close together making the molecule more unstable.

But concerning Doc's post about intermolecular forces; do intermolecular forces and boiling point have anything to do with stability? I thought a molecule's stability only had to do with things that related to a molecules internal energy. Higher boiling point only means there are more intermolecular forces keeping the molecules together meaning the substance is less volatile. The only case I think where you can base stability on volatility is that usually low-energy molecules are in a conformation that allows for a 'comfortable' distance between atoms, and this conformation is usually one that allows greater packing of molecules thus leading to a higher boiling point. Besides that, can you derive stability from volatility?

 

[BerkReviewTeach]

My two cents:

The problem with the intramolecular hydrogen-bonding associated with the cis isomer is that it would require a seven-membered ring where four atoms are sp2-hybridized, resulting in six co-planar atoms. The H-bond is not going to be as strong as typical H-bonds, because of the stressed overlap. If you make a model, you'll see the strain.

However, if you need to make a model to prove the answer, it is probably beyond the scope of a general MCAT question. It would seem that they would feed you the background needed to answer that.

 

[12thandSouth]

cis-alkenes have a higher boiling point because they have a dipole moment, and the additional intermolecular forces (due to the polarity) tend to raise the boiling point.

However, trans-alkenes have higher melting points than cis-alkenes because they have greater symmetry.

So what does this mean for stability? Don't know. Maybe it depends on the phase, no? That is to say, maybe trans-alkenes are more stable than cis when in the solid phase, and maybe cis-alkenes are more stable than trans when in the liquid phase.