Saturated/Unsaturated fatty acids

[cc609]

Would someone be able to confirm what I understand of this unsaturated and saturated fatty acid boiling point and freezing/melting points?

So I know that saturated has no kinks so it can tightly pack, and is solid at room temperature. Thus it has a high boiling point.

And unsaturated has kinks and liquid at room temperature, so it has a low boiling point because branched molecules have low boiling points.

So far is this okay?
What about freezing point/melting point? So since saturated is already solid at room temperature, i would assume high melting/freezing point, but since unsaturated is branched, shouldn't i also have high m.p/fp? Is it high but not AS high as saturated?

If i have messed something up please correct me! Also, this is assuming they all have same number of carbons. Also if someone has more details on the affect of trans vs cis for unsaturated and their relative b.p/m.p and f.pp...i would appreciate it so much. I would compare them side by side and it still confuses me.

So in summary
1) first off, I know this would be good to know but would DATs go in to this detail, especial with trans cis do you think?
2) Is my boiling point concept correct (relative terms between unsaturated and saturated) and how does the melting point explanation go?
3) how do melting points and freezing points compare for trans and cis unsaturated acids?

Thank you so so much!

---

Members don't see this ad.

 

[FeralisExtremum]

For fatty acids...forget boiling point, that's not what we're interested in or what is relevant biologically. The trends of fatty acids we look at are with respect to melting/freezing point, the solid/liquid phase transition. More accurately:

Saturated fatty acids have higher melting/freezing points. Two ways to look at this:
- There are no kinks, so interactions between the fatty acid chains are maximized. More London forces = more heat necessary to melt it = higher melting point.
- There is no kinks to disrupt formation of the 3D structure (solid form) as you lower temperature. Therefore, you don't need to cool it down as much to get it into a solid form = higher freezing point.

Unsaturated fatty acids have lower melting/freezing points.
- Kinks in the structure disrupt interactions between the fatty acid chains. Less Van der Waals forces = less heat necessary to melt it = lower melting point.
- Kinks in the structure disrupt formation of the 3D structure (solid form) as you lower temperature. We need to cool it down more (lower temperature) to get it to solid form.

Don't mix this up with branching, it's not necessarily the same thing. Branching will lower the boiling point because it reduces surface area = less London forces between molecules = less heat needed to make it boil. But highly symmetrical branching will actually raise the melting point because highly symmetrical means the molecule fits into the 3D structure more easily, so we don't need to cool it down as much to get it to solid form.

For your last question:
-In alkenes, cis isomers tend to have higher boiling points because they have more of a net dipole, so there's stronger forces between them. In trans isomers the dipoles cancel out, so the boiling point is lower.
-In alkenes, the trans isomers tend to have higher melting points because the trans isomer is easier to fit into a 3D structure. If you want to think of this in terms of fatty acids above...the trans isomer is less "kinked" than the cis isomer.