Unsaturation boiling point

[Raiden2012]

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So would introducing unsaturation increase or decrease the boiling point?

On one hand, it is more planar, resulting in greater intermolecular interactions but at the same time doesn't the introduction of 'kinks' do the opposite like in phospholipid membranes?

 

[chrisharimlee]

I think that if youre comparing an alkane and an alkene the bp would increase with saturation, or decrease with unsaturation. Since the intermolecular forces will be weak there is more of a dependance on mass. Larger the mass more energy is required to vaporize it. The mass of an alkane is greater so i think its BP will also be greater.

 

[dmf2682]

Unsaturation makes molecules more stiff by introducing double bonds. Saturation allows for free rotation about single bonds. You can compare them to a pile of toothpicks versus a pile of strings. It will take more energy to untangle the strings and pull one out, than it would to pick a toothpick out of its pile, so you'd expect the unsaturated to have a lower boiling point.

 

[Raiden2012]

Thanks. I wasn't sure anymore because one of the answers to an online question stated that a compound with a benzene group conjugated with a C=C double bond had a higher boiling point than a straight chain alkane because of better stacking.

Does this mean they were referring to the benzene group?

 

[sciencebooks]

Thanks. I wasn't sure anymore because one of the answers to an online question stated that a compound with a benzene group conjugated with a C=C double bond had a higher boiling point than a straight chain alkane because of better stacking.

Does this mean they were referring to the benzene group?

I wonder if better stacking/higher BP is related to cyclic molecules or something?

 

[Raiden2012]

I wonder if better stacking/higher BP is related to cyclic molecules or something?

This was in tpr passage too

 

[sciencebooks]

This was in tpr passage too

Oh, TPR throwing out random things. Did you do the one on blackbody emitters? Lol. It wasn't even so much HARD as out of nowhere. I mean, they don't even mention blackbodies in the books...

 

[Raiden2012]

Oh, TPR throwing out random things. Did you do the one on blackbody emitters? Lol. It wasn't even so much HARD as out of nowhere. I mean, they don't even mention blackbodies in the books...

Haha yea that was totally random. I thought it was a trick answer then got it wrong lol

 

[shaboobly]

i skimmed through some responses, and saw that there was no mention of alkene substituents. an unsaturated molecule could have high polarity, thus creating high intermolecular dipole-dipole interactions and increasing the boiling point. saturated alkanes can have dipole moments, but isn't as significant as with the alkenes.

on the other hand, melting point of saturated alkanes is higher than that of unsaturated alkenes because of the stacking characteristic. it takes more energy to break apart the neatly organized lattice as if each molecule sits on each other.

 

[sciencebooks]

i skimmed through some responses, and saw that there was no mention of alkene substituents. an unsaturated molecule could have high polarity, thus creating high intermolecular dipole-dipole interactions and increasing the boiling point. saturated alkanes can have dipole moments, but isn't as significant as with the alkenes.

on the other hand, melting point of saturated alkanes is higher than that of unsaturated alkenes because of the stacking characteristic. it takes more energy to break apart the neatly organized lattice as if each molecule sits on each other.

I didn't even consider polarity, but you're right because if there was something highly polar, there are less things pulling away from the EN end of it.

*By the way, speaking of polarity, can I sneak a question in here really quick? 🙂 Say you're looking at HF versus NH3. Which is more polar? The single HF dipole is obviously bigger, but there are THREE smaller NH3 dipoles which are additive.

 

[shaboobly]

I didn't even consider polarity, but you're right because if there was something highly polar, there are less things pulling away from the EN end of it.

*By the way, speaking of polarity, can I sneak a question in here really quick? 🙂 Say you're looking at HF versus NH3. Which is more polar? The single HF dipole is obviously bigger, but there are THREE smaller NH3 dipoles which are additive.

you need to look at the differences in electronegativites for each of the bonds. HF has a difference of about 2 and NH has a difference of about 1 (x 3). however, when you add the dipoles of the NH bonds, you dont use the actual difference because the resultant is going to be pointed straight down and i think if you were to add them using a value less than 1 (because its not the full force of the dipole) then you would get something around 2 (idk exactly how much of the dipole contributes when you add because you gotta take into account the bond angles and how much the lone pair decreases the bond angles by.. probably something like this wont show up on the mcat because the polarities are too similar).

 

[sciencebooks]

you need to look at the differences in electronegativites for each of the bonds. HF has a difference of about 2 and NH has a difference of about 1 (x 3). however, when you add the dipoles of the NH bonds, you dont use the actual difference because the resultant is going to be pointed straight down and i think if you were to add them using a value less than 1 (because its not the full force of the dipole) then you would get something around 2 (idk exactly how much of the dipole contributes when you add because you gotta take into account the bond angles and how much the lone pair decreases the bond angles by.. probably something like this wont show up on the mcat because the polarities are too similar).

That's what I was thinking. It showed up on something TPR though so I was a little nervous, haha.

 

[imbackasd]

If we are comparing an alkene or alkane to a benzene, I mean shouldnt it be as simple as thinking that benzenes are locked in a cyclic conformation and saturated and thus it is harder to break that apart than any alkane, or alkene?

 

[shaboobly]

If we are comparing an alkene or alkane to a benzene, I mean shouldnt it be as simple as thinking that benzenes are locked in a cyclic conformation and saturated and thus it is harder to break that apart than any alkane, or alkene?

when you are melting something, you aren't combusting it as you suggest. "breaking apart" in the sense of something boiling is overcoming the intermolecular interactions between the molecules. the individual molecules need enough kinetic energy (temperature) to overcome the interactions and escape into the atmosphere (this is where vapor pressure comes in and relating it to the boiling point). the higher the interactions are, the higher the boiling point will be. also you mention benzene is saturated which is not the case. think about saturation as saturating the molecule with hydrogens, i.e. no double bonds.

the boiling point of benzene is lower than that of water because benzene has weaker interactions compared to water's hydrogen bonding; it doesn't matter if the benzene is cyclic. whenever you see a molecule that can hydrogen bond, the boiling point will be higher than you would normally suspect. also, you must take into account the polarity of the molecule (not individual bonds) when determining melting point as well. as it already suggests, hydrogen bonding is a type of dipole-dipole interaction, but is only available to highly electronegative atoms and hydrogen and is stronger than a normal dipole-dipole interaction.

molecular weight also plays a role in determining the boiling point because kinetic energy depends on the mass of individual molecules, which means the average kinetic energy (temperature) needs to be increased to get high molecular weight molecules to boil and escape the interactions.

the melting point of a molecule depends on how each individual molecule is packed. for instance in a saturated alkane, the zig zag conformation (like how you draw it) enables efficient packing and thus more energy to break each individual molecule out of its pattern). in an unsaturated alkene (a double bond), kinks in the molecules conformation don't allow for efficient packing, and thus result in a lower melting point compared to the saturated version.

 

[imbackasd]

Ah I see, thanks shaboobly

 

[nickelbackfan]

So would introducing unsaturation increase or decrease the boiling point?

On one hand, it is more planar, resulting in greater intermolecular interactions but at the same time doesn't the introduction of 'kinks' do the opposite like in phospholipid membranes?

Think of unsaturated fats in foods vs saturated fats....


Saturated fats include olive oil and other oils

unsaturated fats include fats that are solid at room temp such as animal lard



correct me if I'm wrong but this is how I remember it

 

[Raiden2012]

Think of unsaturated fats in foods vs saturated fats....


Saturated fats include olive oil and other oils

unsaturated fats include fats that are solid at room temp such as animal lard



correct me if I'm wrong but this is how I remember it

I think it's the other way round. Animal lard contains saturated fat; that's why it's unhealthy

 

[nickelbackfan]

I think it's the other way round. Animal lard contains saturated fat; that's why it's unhealthy

Oh right...well maybe you can use that analogy to compare melting points. Unsaturated fats (oils) have a lower melting point than saturated fats (animal lards) because at room temp they are a liquid while the animal lard is a solid?