OrangeMed

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Hey guys,

How is the boiling point of saturated and cis- trans-unsaturated fatty acids related? I know that cis-unsaturated fatty acids have greater boiling point due to dipole-dipole interaction, but how do the boiling point of saturated FA and trans-unsaturated FA relate? What is the correct order for all the 3 types of FA?

Thanks!
 

aldol16

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You can think about it this way - saturated fatty acids pack on top of each other and are fully exposed to vdW interactions with a huge surface area. Unsaturated fatty acids are not as good at packing because they have kinks in their chain. So do you think trans or cis fatty acids would pack better relative to each other? And how do you think the degree of intermolecular interactions relates to boiling point?
 

OrangeMed

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You can think about it this way - saturated fatty acids pack on top of each other and are fully exposed to vdW interactions with a huge surface area. Unsaturated fatty acids are not as good at packing because they have kinks in their chain. So do you think trans or cis fatty acids would pack better relative to each other? And how do you think the degree of intermolecular interactions relates to boiling point?
I know that intermolecular force is directly proportional to boiling point, greater the IMF, greater the BP. Doesn't trans FA pack better than cis unsaturated FA? Then by that criteria, trans FA should have greater BP, but that's not the case than cis.
I thought we look at IMF when considering BP and not "how well it packs."
Both saturated FA and trans-FA pack well.
 

aldol16

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I know that intermolecular force is directly proportional to boiling point, greater the IMF, greater the BP. Doesn't trans FA pack better than cis unsaturated FA? Then by that criteria, trans FA should have greater BP, but that's not the case than cis.

Uhhh you're referring to the same thing here. Intermolecular forces here are a function of packing. The only significant intermolecular forces you'll get here are van der Waals forces - not from a single interaction but rather from all the interactions acting over a huge surface area. There's no significant dipole moment created in a C=C double bond. And vdW interactions are a function of packing.

Trans fats do have higher boiling points than cis fats. Look at the melting point trend. The same factors come into play there. Your trans fats are going to be solid at room temp while your cis fats are usually liquid at room temp. Which do you think is harder to boil - something that's still solid or something that's already liquid?
 
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If the number of C are equal, then saturated > unsaturated trans > unsaturated cis. In saturated FAs vs. unsaturated FAs, there are more intermolecular bonds to break due to more surface area (more C-H bonds). In trans vs. cis, trans FAs have stronger intermolecular bonding (thus able to "pack" better) than cis FAs.
 
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aldol16

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If the number of C are equal, then saturated > unsaturated trans > unsaturated cis. In saturated FAs vs. unsaturated FAs, there are more bonds to break in a saturated FA. In trans vs. cis, trans FAs have stronger intermolecular bonding (thus able to "pack" better) than cis FAs.

Could you clarify the italicized part? You don't break any bonds when you're boiling something because that's just a phase change and not a chemical reaction.
 
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Could you clarify the italicized part? You don't break any bonds when you're boiling something because that's just a phase change and not a chemical reaction.

Sorry, fixed it. I meant that there are more C-H bonds in saturated FAs so there's more room for intermolecular bonding (van der Waals). It's 2 AM where I'm at and today was a bio day so I'm quite exhausted haha. Good catch!
 

OrangeMed

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Uhhh you're referring to the same thing here. Intermolecular forces here are a function of packing. The only significant intermolecular forces you'll get here are van der Waals forces - not from a single interaction but rather from all the interactions acting over a huge surface area. There's no significant dipole moment created in a C=C double bond. And vdW interactions are a function of packing.

Trans fats do have higher boiling points than cis fats. Look at the melting point trend. The same factors come into play there. Your trans fats are going to be solid at room temp while your cis fats are usually liquid at room temp. Which do you think is harder to boil - something that's still solid or something that's already liquid?

Solid would be harder to boil than liquid, therefore, trans-FA should have greater boiling point than cis. However, its a fact that cis isomers have greater dipole moment and so have greater boiling point than trans isomers because of stronger IMF. Does this concept not apply here? In this case, you're right that IMF is a function of packing, but that is only true for vdW's forces.
I agree with the MP trend of saturated > trans unsaturated > cis unsaturated. I don't think its the same trend for BP though.
 
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The melting point and boiling point trend is the same.

In this image, you can see that saturated FAs stack quite easily because everything is able to rotate and line up as such. In cis FAs, the kinks created by the cis double bond force the rest of the chains to angle in different directions, making it quite difficult for nearby chains to stack. Although trans FAs aren't shown, their stacking would be more similar to saturated FA stacking because the hydrogens (thus the rest of the chains to which the double bond is attached to on each side) are on opposing sides.
upload_2016-1-10_11-44-32.png
 

OrangeMed

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The melting point and boiling point trend is the same.

In this image, you can see that saturated FAs stack quite easily because everything is able to rotate and line up as such. In cis FAs, the kinks created by the cis double bond force the rest of the chains to angle in different directions, making it quite difficult for nearby chains to stack. Although trans FAs aren't shown, their stacking would be more similar to saturated FA stacking because the hydrogens (thus the rest of the chains to which the double bond is attached to on each side) are on opposing sides.
View attachment 199431
I understand your explanation, but can you also please talk about the presence of dipole-dipole force in cis unsaturated FA and how/if that would affect the BP. Shouldn't that cause the BP of cis to be greater than trans unsaturated FA because of greater strength of dipole interactions?
If I consider only the packing aspect of FA alignment then I agree with your response, but its the dipole-dipole interaction in cis unsaturated FA that's troubling me.
 
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For cis fatty acids, you have the 2 large groups on the same side which causes greater steric hinderance and thus it has a lower boiling point than the trans form. If you want to look at it from the point of view of dipole-dipole interactions, there would be greater number of interactions in the trans form and thus trans having a higher boiling point.

 
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I understand your explanation, but can you also please talk about the presence of dipole-dipole force in cis unsaturated FA and how/if that would affect the BP. Shouldn't that cause the BP of cis to be greater than trans unsaturated FA because of greater strength of dipole interactions?
If I consider only the packing aspect of FA alignment then I agree with your response, but its the dipole-dipole interaction in cis unsaturated FA that's troubling me.

The dipole-dipole interactions actually inhibit the ability of cis FAs to pack well. Sure, its interactions with another chain might be stronger than if a trans FA were interacting with another trans FA but the cis FA will only be limited to a strong interaction on that one other chain. In comparison, a trans FA can have weaker dipole-dipole interactions but more of them because it packs easier. Cis bonds cause a kink in the FA so the rest of the chain beyond the double bond will have a hard time lining up with other beyond-the-cis-double-bond-chains.
 

aldol16

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Solid would be harder to boil than liquid, therefore, trans-FA should have greater boiling point than cis. However, its a fact that cis isomers have greater dipole moment and so have greater boiling point than trans isomers because of stronger IMF. Does this concept not apply here? In this case, you're right that IMF is a function of packing, but that is only true for vdW's forces.
I agree with the MP trend of saturated > trans unsaturated > cis unsaturated. I don't think its the same trend for BP though.

It is in fact the same trend. There is nothing different in accounting for a phase transition from solid to liquid and liquid to gas. Why do you think it's different?

I understand your explanation, but can you also please talk about the presence of dipole-dipole force in cis unsaturated FA and how/if that would affect the BP. Shouldn't that cause the BP of cis to be greater than trans unsaturated FA because of greater strength of dipole interactions?
If I consider only the packing aspect of FA alignment then I agree with your response, but its the dipole-dipole interaction in cis unsaturated FA that's troubling me.

Stop talking about the "dipole-dipole interaction." We're talking about C-C and C-H bonds here. The difference between a cis and trans fatty acid in terms of dipole-dipole interactions is very very small because the electronegativity difference is very very small. The key difference that determines mp and bp trends here is all the vdW interactions. They, when combined, are the strongest intermolecular forces here.
 
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