Still Confused about Branching & MP

[jwnichols21]

Hey Guys,

Okay so there is so much uncertainty regarding this question and two of the leading sources in DAT prep contradict eachother. Can we just settle this once and for all?

Branching and its relation to MP

Kaplan Says: Increase Branching, Decrease MP
Chad Says: Increase Branching, Increase MP

WHICH ONE IS IT?!!!!!

Thanks.

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[dentist911]

Hey Guys,

Okay so there is so much uncertainty regarding this question and two of the leading sources in DAT prep contradict eachother. Can we just settle this once and for all?

Branching and its relation to MP

Kaplan Says: Increase Branching, Decrease MP
Chad Says: Increase Branching, Increase MP

WHICH ONE IS IT?!!!!!

Thanks.

More branching increases melting point while it decreases the boiling point.

so when it comes to branching see if the question is asking for boiling point or melting point.

 

[jwnichols21]

Could you explain why? Also, why does Kaplan say otherwise?

 

[dentist911]

Could you explain why? Also, why does Kaplan say otherwise?

Not 100% sure why that's the case but I think it's safe, for the DAT purposes, to just stick with:

more branching = higher MP = lower BP

symmetry is important for melting points...for example trans allows for better packing (less steric) and thus explains the higher melting point...

Why BP goes down?
BP goes down with branching because the molecules have less surface area touching each other than if they were straight chain (i.e. fewer london forces/van der waals forces holding them together = lower BP)


not a chem major and maybe someone else can explain this in more depth?

 

[jwnichols21]

The BP one is pretty intuitive, so lets focus on MP.

So you're saying if it's branched and trans, then it will have higher MP? What if it was branched but cis? Or branched only on one side vs. branched all over? Or disproportionate branching? The possibilities are endless, so I'm just really confused with what the go-to rule would be.

 

[dentist911]

The BP one is pretty intuitive, so lets focus on MP.

So you're saying if it's branched and trans, then it will have higher MP? What if it was branched but cis? Or branched only on one side vs. branched all over? Or disproportionate branching? The possibilities are endless, so I'm just really confused with what the go-to rule would be.

dude, just stick with this (or at least i'm gonna do it):

more branching = higher MP = lower BP

and have this in mind:

trans isomer has the higher melting point than cis
cis isomer has the higher boiling point than trans

Why?
Trans isomers pack better than cis isomers. The "U" shape of the cis isomer doesn't pack as well as the straighter shape of the trans isomer. (looked it up online)


don't worry about anything more than this...they won't ask anything more "complicated" than what is mentioned already.

 

[OChemTA]

dude, just stick with this (or at least i'm gonna do it):

more branching = higher MP = lower BP

and have this in mind:

trans isomer has the higher melting point than cis
cis isomer has the higher boiling point than trans

Why?
Trans isomers pack better than cis isomers. The "U" shape of the cis isomer doesn't pack as well as the straighter shape of the trans isomer. (looked it up online)


don't worry about anything more than this...they won't ask anything more "complicated" than what is mentioned already.

buddy, you're wrong...

in general, increased branching lowers melting point.

Here's the reasoning... There are no strong intermolecular forces in hydrocarbons. Therefore melting points and boiling points are determined by the weak intermolecular forces (VDW, london ,etc...). When these forces are strong enough to overcome the kinetic energy of the molecules (heat), they change phase (ie. liquid to solid aka melting point).

The stacking of these molecule depend on the weak intermolecular forces and whatever will weaken these intermolecular interaction... and one of them is steric hindrance.

Branching increases steric hinderance so that the molecules can't be in as close a proximity as non-branched, therefore, less VDW and london forces.... which means it takes a lower temp for a phase change from liquid to solid to occur. (lower mp)

Alternatively, you can think of it this way, if something stacks really well, like lego pieces, then you have to put more energy in to break it apart. (higher mp) If something stacks poorly, like cheetos, it takes less energy to break it apart (lower mp)

This rule, in general, is correct.

It's the reason why saturated fatty acids are solid (lard) while unsaturated fatty acids are liquid (oils)


It's also the same theory behind how cells maintain homeostatsis in cold temps by manipulating the amount of cholesterol in the bilayer ( more cholesterol, more steric hinderance, more fluid the membrane even in cold temps - don't want the membrane to gel up)

 

[dentist911]

http://masterorganicchemistry.com/2010/07/09/chemical-tetris/

"Better stacking, higher melting point. Case closed. Right? Not quite. It's also about surface area."

 

[OChemTA]

http://masterorganicchemistry.com/2010/07/09/chemical-tetris/

"Better stacking, higher melting point. Case closed. Right? Not quite. It’s also about surface area."

Uhh... did you not see where I wrote:

"and one of them is steric hindrance"

Let me give you some advice, when we hear hooves, we don't look for zebras.

The question OP asked was the effect of branching on MP. OP didn't specify whether it was n-alkane vs branched alkane OR branched alkane vs more branched alkanes.

and let's be honest here... in the general sense of the word, the OP and the DAT means the former definition, not the latter. Don't OVER analyze the question, you don't have the time to do so and you won't come up with the correct answer. there are so many variables to this question when it's not specific... ie length of branch, shape of branch (aka cyclic?), branching on branch, length of n-alkane etc. Don't provide a specific answer in a specific case to a general question.

To the OP, my advice is to respond with "increased branching decreases mp" if the DAT only asks "how branching affects MP"...but hey, what do I know? I only TAed Ochem, and got admitted to both pharmacy and dentistry...

 

[LetsGo2DSchool]

Nevermind.

 

[AlbinoPolarBear]

To the OP, my advice is to respond with "increased branching decreases mp" if the DAT only asks "how branching affects MP"...but hey, what do I know? I only TAed Ochem, and got admitted to both pharmacy and dentistry...

+1

Let's not get too complicated mixing branching up with cis and trans alkenes here. The DAT won't get that complicated.

 

[UCSD1984]

Uhh...so does branching increase or decrease melting point? Lmao, I just read through all of this and still don't know the answer, when I was pretty sure before I read this post that branching INCREASES MP and DECREASES BP.

 

[dentist911]

Uhh...so does branching increase or decrease melting point? Lmao, I just read through all of this and still don't know the answer, when I was pretty sure before I read this post that branching INCREASES MP and DECREASES BP.

lol same here...i had no idea people have different views over this...

 

[Gury410]

Here is my understanding of this...

More Branching =

DECREASES Boiling Point (branching does not allow for tight packing --> less intermolecular forces --> takes less energy to break molecules apart --> boiling occurs at a lower temperature)

INCREASES Melting Point (heres the thing: shorter chain + symmetry = crystalline structure which is more stable and requires more energy to break apart --> so, molecules resist getting separated from each other and thus the point at which the solid melts increases)

INCREASES Freezing Point (theoretically, it's the same point as the melting point but in the opposite direction of phase change --> liquid to solid. With short, symmetrical hydrocarbons, a crystalline structure wants to form because the molecules will gain some stability in such an orientation --> so, a branched/symmetrical compound freezes at a higher temperature. I believe jet fuel has UNBRANCHED hydrocarbons added to it to lower the freezing point and prevent the fuel from freezing in space)

INCREASES Heat of Combustion (simply put... more substituted carbon --> more stable --> releases more energy as its bonds are broken --> therefore you get a higher heat of combustion with greater branching)

As far as comparing goes...
For example:
- A n-decane VS a 2-methyl-decane, the crystalline structure will probably not easily form and the unbranched decane will melt at a higher temp due to its greater surface area/van der waals/packing/etc.

- A 2,2-dimethypropane VS. n-pentane, the crystalline structure will likely form due to symmetry of the branched molecule, thus gaining stability and melting at a higher temp.

I doubt you will get asked to compare stuff that would be hard to judge, like the 2,2-dimethylpropane VS n-decane, since I'm sure that at a certain point it just comes down to experimental data and what not...

Generally, I would just think:
-Less branching = tighter packing = more intermolecular forces = harder to break stuff apart
-BUT, if symmetry is possible = also hard to break stuff apart (which can make it confusing!)
-Also, the chance of forming a hydrocarbon crystalline structure decreases as the compound gets larger (more carbons).

I hope all this makes sense!

 

[jwnichols21]

I love how after all of this, a clear answer has still not arisen. Everything you guys say when you make your claim makes sense in its own way. What are we supposed to believe?! This is ridic

 

[OChemTA]

Here is my understanding of this...

More Branching =


INCREASES Melting Point (heres the thing: shorter chain + symmetry = crystalline structure which is more stable and requires more energy to break apart --> so, molecules resist getting separated from each other and thus the point at which the solid melts increases)

INCREASES Freezing Point (theoretically, it's the same point as the melting point but in the opposite direction of phase change --> liquid to solid. With short, symmetrical hydrocarbons, a crystalline structure wants to form because the molecules will gain some stability in such an orientation --> so, a branched/symmetrical compound freezes at a higher temperature. I believe jet fuel has UNBRANCHED hydrocarbons added to it to lower the freezing point and prevent the fuel from freezing in space)

Wrong for the same reasons as dentist911... you're only understanding one aspect of melting points, but not the bigger picture.

I love how after all of this, a clear answer has still not arisen. Everything you guys say when you make your claim makes sense in its own way. What are we supposed to believe?! This is ridic

It's only not clear because you're not reading the posts... if mine's too confusing, read the link dentist911 provided:

http://masterorganicchemistry.com/2010/07/09/chemical-tetris/

In general, the question you asked, "Branching and its relation to MP" is vague as to how much branching... Colloquially speaking, the question you're asking is n-alkanes (linear) vs any branching and in this case, increased branching decreases melting point.

Look at this picture, the unbranched (linear aka n-alkane) has the HIGHEST mp. You increase branching, regardless of symmetry and/or branching and the mp DECREASES. Only within the case of "branching vs more branching" does increased branching increase mp

 

[shinyMIRROR]

so this is what im understanding from this thread..

MORE BRANCHING= HIGHER MELTING POINT which means it is the SMALLEST number since it is a negative number. so for example, if there was a number -135 and -180, the -180 would indicate the HIGHER melting point since it is the SMALLEST number. RIGHT??

I think I initially got confused because higher means larger, but in negative numbers, higher is smaller... so that's what im understanding. HIGHER melting point= smallest number because they are negative. Please someone let me know if im completely on the wrong track.

 

[OChemTA]

so this is what im understanding from this thread..

MORE BRANCHING= HIGHER MELTING POINT which means it is the SMALLEST number since it is a negative number. so for example, if there was a number -135 and -180, the -180 would indicate the HIGHER melting point since it is the SMALLEST number. RIGHT??

I think I initially got confused because higher means larger, but in negative numbers, higher is smaller... so that's what im understanding. HIGHER melting point= smallest number because they are negative. Please someone let me know if im completely on the wrong track.

*face palm*

you're completely on the wrong track...

Which number is bigger... -180 or -135?































the answer is -135



Melting points are just temperatures...

since n-heptane mp is -95 while 2-methyl pentane's mp is -154... branching causes a DECREASE in mp.

 

[AlbinoPolarBear]

*face palm*

you're completely on the wrong track...

Which number is bigger... -180 or -135

Welcome to the DAT boards.

Keep in mind, not everyone is an OChem TA 😉

 

[OChemTA]

I'm just trying to help...

besides, shinyMIRROR specifically asked "Please someone let me know if im completely on the wrong track"

sorry if I was taking it too literally...

I was like...


must.


resist..



















facepalm...


but the urge was too strong 😛

 

[SN1]

so this is what im understanding from this thread..

MORE BRANCHING= HIGHER MELTING POINT which means it is the SMALLEST number since it is a negative number. so for example, if there was a number -135 and -180, the -180 would indicate the HIGHER melting point since it is the SMALLEST number. RIGHT??

I think I initially got confused because higher means larger, but in negative numbers, higher is smaller... so that's what im understanding. HIGHER melting point= smallest number because they are negative. Please someone let me know if im completely on the wrong track.

NO. -180 would indicate lower MP. -180 is smaller than -130.

 

[JohnnnyD]

so this is what im understanding from this thread..

MORE BRANCHING= HIGHER MELTING POINT which means it is the SMALLEST number since it is a negative number. so for example, if there was a number -135 and -180, the -180 would indicate the HIGHER melting point since it is the SMALLEST number. RIGHT??

I think I initially got confused because higher means larger, but in negative numbers, higher is smaller... so that's what im understanding. HIGHER melting point= smallest number because they are negative. Please someone let me know if im completely on the wrong track.

Read over what the TA said again. Its not a matter of "branching always increases MP," or "branching always decreases MP." You have to have a reference point.

There are 2 different comparisons that can be made here:

1) comparing an unbranched alkane to a branched alkane.
2) comparing a branched alkane with a "more branched" alkane.


In the first type of comparison, when comparing an unbranched alkane with a branched alkane, the branched alkane will have a lower MP. Thus, branching DECREASES MP.

In the second type of comparison, when comparing a branched alkane with a "more branched" alkane, the "more branched" alkane will have a higher MP. Thus, branching INCREASES MP.

just a summary of what the TA said, but maybe it will clear some things up

 

[JohnnnyD]

Here is my understanding of this...

More Branching =


INCREASES Heat of Combustion (simply put... more substituted carbon --> more stable --> releases more energy as its bonds are broken --> therefore you get a higher heat of combustion with greater branching)

Not to thread jack by any means, but I feel the branching and MP topic should be pretty cleared up by meow.

I have a question about this now tho. wouldnt less branched alkanes have higher heats of combustion? if the heat of combustion (deltaH) is the difference in the energies of the reactants and products, wouldnt the less stable compound release more energy than the more stable compound?

I mean, we use octane as gasoline, not 2,2,3,3 tetramethylbutane or something like that haha

 

[Gury410]

Wrong for the same reasons as dentist911... you're only understanding one aspect of melting points, but not the bigger picture.



It's only not clear because you're not reading the posts... if mine's too confusing, read the link dentist911 provided:

http://masterorganicchemistry.com/2010/07/09/chemical-tetris/

In general, the question you asked, "Branching and its relation to MP" is vague as to how much branching... Colloquially speaking, the question you're asking is n-alkanes (linear) vs any branching and in this case, increased branching decreases melting point.

Look at this picture, the unbranched (linear aka n-alkane) has the HIGHEST mp. You increase branching, regardless of symmetry and/or branching and the mp DECREASES. Only within the case of "branching vs more branching" does increased branching increase mp

Did you finish reading my post or just skim it?
Read the comparison part and tell me if that's different from what you're trying to say...

What I'm saying is branching will increase MP only if creates symmetry which favors a crystalline structure, otherwise, branching will generally decrease MP.

 

[shinyMIRROR]

thanks johnny D... it actually cleared up a lot.. i was confused with higher, lower.. in terms of dealing with negative numbers. Thanks!

ochemTA.. thanks for the face palm. i face palm myself everyday studying for the dat... its so frustrating. and yes i know which one the out of the two is the smallest/largest number.. im not THAT stupid.

Thanks guys.. this is one topic I can cross of my list of things I need to understand.

 

[jwnichols21]

http://masterorganicchemistry.com/2010/07/09/chemical-tetris/

"Better stacking, higher melting point. Case closed. Right? Not quite. It’s also about surface area."

This solidified everything. Thank you so much for clarifying this confusing topic for me. I am just so surprised that Kaplan and Chad both gloss over such a difficult/confusing topic that has so many ways to look at.

 

[UCSD1984]

...so there is no spoon?