AAMC Section Bank C/P #20

[AWolfman]

I am aware that you can differentiate between the thermodynamic enolate vs. the kinetic enolate, but how does that pertain to this question? I really do not understand the logic behind their answer either.

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[aldol16]

The images are too small and they can't be made bigger. If this is the vesicle one, the AAMC doesn't understand the difference between kinetic and thermodynamic control. That's what happens when you get biologists to write MCAT passages.

 

[AWolfman]

Yeah that's the one. What point are they trying to get across?

This is the part of the passage it is referring to:
Liposomes generated from oleic acid (Compound 2, C17H33CO2H) gave similar results to those shown in Figure 1 only at pH 8.5. At pH 10 or above and pH 7 and below, liposomes of Compound 2 did not form. The researchers mixed liposomes of different sizes and observed that those formed from Compound 1 were stable to mixing, but mixing those from Compound 2 formed new liposomes with an average size expected for the effective final lipid concentration.

Question:
What does the behavior of liposomes prepared from compounds 1 and 2 upon mixing indicate about the energetics of their transformations? Liposomes prepared from:

Answer:
Compound 1 is kinetic, Compound 2 is thermodynamic.

 

[aldol16]

There was a thread about this about a week ago. I don't know what they're trying to get at, but it's not correct in chemical terms. Compound 1 must be at a local energy minimum because it is stable to mixing. Compound 2 cannot be at a local minimum because it is unstable to mixing and thus will roll downhill to form the stable mixture. That means that Compound 2 must be a kinetic product because once you supply the system with enough energy via mixing, it will roll downhill to form the thermodynamic product. Conversely, compound 1 must be a thermodynamic product because it is stable to mixing.

 

[AWolfman]

There was a thread about this about a week ago. I don't know what they're trying to get at, but it's not correct in chemical terms. Compound 1 must be at a local energy minimum because it is stable to mixing. Compound 2 cannot be at a local minimum because it is unstable to mixing and thus will roll downhill to form the stable mixture. That means that Compound 2 must be a kinetic product because once you supply the system with enough energy via mixing, it will roll downhill to form the thermodynamic product. Conversely, compound 1 must be a thermodynamic product because it is stable to mixing.

That was my original thought process. The kinetic product generally has to rearrange to become thermodynamically stable, whereas the thermodynamic product does not rearrange because it is already in the most stable form.

 

[To be MD]

**bump** I just got this question wrong on SB. I had the exact same thought process. Is @aldol16 's reasoning correct?

 

[bobeanie95]

Could someone confirm this? I originally agreed with aldol's reasoning, yet I've come across threads that claim B is correct since thermodynamic reactions are reversible (compound 2), while kinetic is irreversible (compound 1).

 

[sbspftw]

bump? same question, same initial reasoning. I picked C, because I thought Compound 1 was "already" at thermodynamic equilibrium, and that compound two, needed to rearrange upon mixing to get to the thermo equilibrium. Got it wrong, and was honestly kinda lost....

 

[DaAlienist]

So

There was a thread about this about a week ago. I don't know what they're trying to get at, but it's not correct in chemical terms. Compound 1 must be at a local energy minimum because it is stable to mixing. Compound 2 cannot be at a local minimum because it is unstable to mixing and thus will roll downhill to form the stable mixture. That means that Compound 2 must be a kinetic product because once you supply the system with enough energy via mixing, it will roll downhill to form the thermodynamic product. Conversely, compound 1 must be a thermodynamic product because it is stable to mixing.

does the AAMC make "errors" like this often? Do they correct them, or do we just accept their reasoning, no matter how dubious since they make the test?

 

[aldol16]

does the AAMC make "errors" like this often? Do they correct them, or do we just accept their reasoning, no matter how dubious since they make the test?

Not often but to my knowledge, this problem hasn't been corrected. You also don't know what the correct answers are on the exams themselves, so you are completely subject to their reasoning, no matter how erroneous. They're doctors, not chemists.

 

[deleted647690]

There was a thread about this about a week ago. I don't know what they're trying to get at, but it's not correct in chemical terms. Compound 1 must be at a local energy minimum because it is stable to mixing. Compound 2 cannot be at a local minimum because it is unstable to mixing and thus will roll downhill to form the stable mixture. That means that Compound 2 must be a kinetic product because once you supply the system with enough energy via mixing, it will roll downhill to form the thermodynamic product. Conversely, compound 1 must be a thermodynamic product because it is stable to mixing.

Hey aldol, would you mind explaining how this relates to temperature change? All I know from thermodynamics is that the thermodynamic product is more stable and formed at higher temperature because of a higher activation energy, while the kinetic product is less stable and favored at lower temperature due to lower activation energy. How does this relate to being "stable to mixing" or "unstable to mixing"?

 

[UBLI-EINSTEIN]

This has been long time so I may not be correct. However here's my reasoning.

I think compound 1 is kinetic and compound 2 is thermodynamic.

The characteristics of kinetic compound - has lower activation energy (lower requirements), and thus is formed faster than thermodynamic one.
In the passage, the range of pH suitable for compound 1 comes from 2-12. This suggest compound 1 can adopt a lower requirement of formation.
The characteristics of thermodynamic compound - has higher activation energy (higher requirements, more restrictions) + is formed slower than kinetic one, + once formed, must be the most stable one.
According to the passage, compound 2 requires a specific pH 8.7 for its formation (pH serves as a restriction in this case).
pH 8.7 in this case is just like a specific activation energy, which you need to achieve so that to form a specific product. Therefore, compound 2 is under thermodynamic control for liposome generations.

 

[aldol16]

Hey aldol, would you mind explaining how this relates to temperature change? All I know from thermodynamics is that the thermodynamic product is more stable and formed at higher temperature because of a higher activation energy, while the kinetic product is less stable and favored at lower temperature due to lower activation energy. How does this relate to being "stable to mixing" or "unstable to mixing"?

Well, mixing is a source of energy input so typically, when you input energy into a system by mixing, you can move the whole system towards the thermodynamic product. So typically, the thermodynamic product will be stable to mixing because it's the stablest point in the system whereas the kinetic product will not be stable to mixing.

 

[deleted647690]

This has been long time so I may not be correct. However here's my reasoning.

I think compound 1 is kinetic and compound 2 is thermodynamic.

The characteristics of kinetic compound - has lower activation energy (lower requirements), and thus is formed faster than thermodynamic one.
In the passage, the range of pH suitable for compound 1 comes from 2-12. This suggest compound 1 can adopt a lower requirement of formation.
The characteristics of thermodynamic compound - has higher activation energy (higher requirements, more restrictions) + is formed slower than kinetic one, + once formed, must be the most stable one.
According to the passage, compound 2 requires a specific pH 8.7 for its formation (pH serves as a restriction in this case).
pH 8.7 in this case is just like a specific activation energy, which you need to achieve so that to form a specific product. Therefore, compound 2 is under thermodynamic control for liposome generations.

Well, mixing is a source of energy input so typically, when you input energy into a system by mixing, you can move the whole system towards the thermodynamic product. So typically, the thermodynamic product will be stable to mixing because it's the stablest point in the system whereas the kinetic product will not be stable to mixing.

Hm, thanks for the answers! I am following both of you guys, but I guess this just wasn't a good question by the aamc

 

[Bigbang_333]

I am way late! But maybe it can benefit someone else.
The key here is looking at the last sentence of the last paragraph where it says that compound 2 forms an average size of products!
What we should know about kinetic and thermodynamic control is that, Kinetic control--> happens fast--> stable intermediate, BUT a less stable product
Thermodynamic control--> happens slower--> less stable intermediate BUT a more stable product.
With that we know that thermodynamic is the one with a stable, average product thus, compound 2 produces an average product so its thermodynamic.

 

[goldenladle]

I am way late! But maybe it can benefit someone else.
The key here is looking at the last sentence of the last paragraph where it says that compound 2 forms an average size of products!
What we should know about kinetic and thermodynamic control is that, Kinetic control--> happens fast--> stable intermediate, BUT a less stable product
Thermodynamic control--> happens slower--> less stable intermediate BUT a more stable product.
With that we know that thermodynamic is the one with a stable, average product thus, compound 2 produces an average product so its thermodynamic.

How do we make that jump in equating 'average size' to being stable (and thus under thermodynamic control)?

 

[treeclimbingmonkey]

How do we make that jump in equating 'average size' to being stable (and thus under thermodynamic control)?

I am way late! But maybe it can benefit someone else.
The key here is looking at the last sentence of the last paragraph where it says that compound 2 forms an average size of products!
What we should know about kinetic and thermodynamic control is that, Kinetic control--> happens fast--> stable intermediate, BUT a less stable product
Thermodynamic control--> happens slower--> less stable intermediate BUT a more stable product.
With that we know that thermodynamic is the one with a stable, average product thus, compound 2 produces an average product so its thermodynamic.

I think what bigbang is trying to say is that you can have multiple less stable "kinetic products" but you can only have one very stable thermodynamic product. In the passage it states that compound 1 produces a series of products and upon mixing (adding energy) doesn't change its vesicle size indicating a kinetic control due to a multitude of less stable products. For compound two upon mixing (adding energy), the vesicle equilibrate to a single average size indicating the formation of a single thermodynamically stable product that was favored. Thus compound two is under thermodynamic control.
See diagram below.

 

[Cap_Falcon10]

I would like to know how this average size jump was also made @goldenladle and @Bigbang_333