melting point confusion

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I'm confused about why branching/symmetry increases the melting point. I know it has something to do with how they pack together or something. I have in my notes however that high IMF leads to high MP. And since branching reduces the SA which means less London Dispersion which lowers IMF, which is the same reason why branching lowers boiling points. So why are melting points different?

Aren't unsaturated fats (like olive oil) liquid at room temp, and saturated fats (animal grease) solid at room temp? Which seems like the opposite here, since the unsaturated fats are branched and they have a lower melting point. Since branching adds "kinks" to the whole thing it makes it more fluid. Right? I'm confused

Originally Posted by TXpredent

~ unsaturated fats (like olive oil) liquid at room temp
~saturated fats (animal grease) solid at room temp? W

hich seems like the opposite here why?,
since the unsaturated fats are branched (not neccesarily branched, but they may have kinks where a double bond should or shouldnt be.) and they have a lower melting point. Since branching adds "kinks" to the whole thing it makes it more fluid. Right?
so if branching adds kinks, and it's more fluid, then would you expect the more fluid unsaturated fat to be liquid or solid? It has a lower IMF due to its kinks, and a lower melting point, so at lower temperatures they have melted from solid to a liquid and will remain a liquid at temperatures where saturated fats are solid...

seems like youre over thinking it..

Right, so what I'm confused about is why does branching INCREASE the melting point? When in our example of the olive oil it DECREASED it. You're right about the double bonds though, I totally forgot about that!

But I always thought melting point was proportional to the IMF, so the more IMF, the higher the melting point. But since branching reduces the IMF, it should reduce the melting point, not increase it. I saw in one of the solutions of the Destroyer that it said that symmetrical branching allows it pack tighter, thus making it harder (increasing the MP) to be melted. Can anyone explain this further? Because to me, it seems like branching would make it pack less tightly.

Well, I'm not sure if this will answer your question, but if you look at Kaplan Test #1, question #80 in the Natural Sciences, it helped me understand the differences in decreasing/increasing melting/boiling points...

I'll try to attach a photo but I'm not quite sure if it'll work

yup, this was the question that caused the confusion to begin with! Why are branched molecules able to pack tighter?? I can kinda see why the trans isomer in that case can pack tighter though, it's not bent like the cis one and so they can fit together better.

http://forums.studentdoctor.net/arch.../t-111078.html

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