Why do trans isomers pack more efficiently than cis isomers?

[kenny1999]

Trans and Cis isomer model??

Hello It's my first time here. I absolutely "know" that trans-isomer has a higher melting point than that of cis-isomer because it is more "symmetric" than cis-isomer and can pack more "efficiently"

However, I only "know" the fact but I don't "understand". I wish to understand this. I have been searching for a good model to explain how it can pack better than "cis-isomer" on google but I hardly find any, I hope someone will introduce me some good models (best with 3D) to show this because I hope to understand rather than memorizing it. Thanks

Thank you

Kenny

 

[aroc91]

If you look at the Cis-trans isomerism wiki page, note the bottom 2 pictures on the right side, of oleic acid and elaidic acid.

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

Oleic acid, because of the cis C=C bond, doesn't stack neatly with other oleic acid molecules because of its bent shape, while elaidic, because it's completely flat, does stack neatly, like bricks. You have to think about how they lay against each other and how hindered they are.

 

[epenguin]

As the OP requested, it would be better with 3-D models of the crystal structures. It is not that obvious, I am not that persuaded, that |||| stacks any better than <<<< .

I'd hazard not a prediction but a guess that in the crystal both might fit staggered by half a molecule. Then the polar groups could be farther apart. And that there might be some exceptions to this rule.