ochem cis vs trans mp

[rmm30]

so im going through some ochem problems. The book im using says that the trans isomer has higher mp than cis bc it is more symmetrical and packs into crystal better. could someone explain how trans is more symmetrical? thanks

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[TXKnight]

so im going through some ochem problems. The book im using says that the trans isomer has higher mp than cis bc it is more symmetrical and packs into crystal better. could someone explain how trans is more symmetrical? thanks

Try the other forum. MCAT Q&A?

 

[NYR56]

Imagine the different molecules attached to the carbons have different weights. A trans molecule could be balanced right in the center of the C=C, whereas the cis molecule cannot. This isn't exactly why a trans molecule has a higher BP, but it may help you visualize how it is more symmetrical.

 

[Zedor]

http://en.wikipedia.org/wiki/Cis–trans_isomerism


If you look about half way down the page at the three dimensional structures, you can see the how trans is more symmetrical. Because of this, it is more easily stacked to form a solid. Cis has the higher boiling point due to its dipole.

 

[Rolling]

Just as was said before, you can imagine that :

As was noted, for the same substituents on the double bond, ones that are opposite one another will cancel out each other's dipoles resulting in no dipole moment, whereas in the cis case, given the asymmetrical charge distribution, you'll end up with a polar molecule. Greater intermolecular forces --> greater BP.


the trans isomer will have a higher MP than the cis isomer. You can "imagine" like you can "stack" the trans easier on themselves because of their structure. While the "cis" isomer is infact polar therefore leading to the idea that intermolecular forces would be higher, in the case of a solid, the structures will be quite packed, and the "trans" allows for "better packing" and therefore intermolecular forces will be greater because the molecules are much more packed/closer together in the case of trans over cis. If it helps, try actually drawing the cis isomers and seeing how they would "fit" with one another. In the trans case, it's much easier.

Infact, I believe this idea of the geometry of the molecules making it easier to pack is a large contributing factor as to why trans unsaturated fats are worse for your body than cis unsaturated fats. However, if I am wrong in this, please correct me.

I hope this helps.

 

[didanawisgi]

Just as was said before, you can imagine that :

As was noted, for the same substituents on the double bond, ones that are opposite one another will cancel out each other's dipoles resulting in no dipole moment, whereas in the cis case, given the asymmetrical charge distribution, you'll end up with a polar molecule. Greater intermolecular forces --> greater BP.


the trans isomer will have a higher MP than the cis isomer. You can "imagine" like you can "stack" the trans easier on themselves because of their structure. While the "cis" isomer is infact polar therefore leading to the idea that intermolecular forces would be higher, in the case of a solid, the structures will be quite packed, and the "trans" allows for "better packing" and therefore intermolecular forces will be greater because the molecules are much more packed/closer together in the case of trans over cis. If it helps, try actually drawing the cis isomers and seeing how they would "fit" with one another. In the trans case, it's much easier.

Infact, I believe this idea of the geometry of the molecules making it easier to pack is a large contributing factor as to why trans unsaturated fats are worse for your body than cis unsaturated fats. However, if I am wrong in this, please correct me.

I hope this helps.

This. It was going to be my explanation - imagine a long hydrocarbon chain such as fat having a double bond in it. If it is a trans isomer, the chains still be relatively straight and stack neatly giving ample opportunity for van der waals forces. If they are cis, there will be a lot of kinks and they won't stack nearly as neatly.

 

[rmm30]

thanks so much for the help. all really great answers. So can the statement that trans is more symmetrical than cis be confirmed by looking for mirror planes of symmetry? Kind of like chirality tests (even though there are no chiral centers).

 

[Rolling]

I don't know if I agree. Assuming a basic structure...there's symmetry in both the cis and the trans