Less stable product is major product??

[SaintJude]

Oh man, Kaplan got me on this one.

The question asks for the "best reaction profile graphs" of the chemical reaction with two product with different yields Product A (20%) and Product B (80%). So I (incorrectly) assumed that Product B had a higher product yield because it was more stable. But that was wrong!

Instead Kaplan's explains that "Product B is the major product and thus must have a lower activation energy than Product A". So, is the only absolute indicator of a major product its activation energy? Does yield have very little to do with it?


I attached the entire question as a picture.
View attachment Activation Energy.jpg

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[Ssina]

Well product A is a conjugated system while B is not so A is more stable, and the more stable product is formed through the higher activation energy... this has to do with kinetic vs therm products which will dictate the yield... so I wonder if you had any information on the temperature or.other conditions

 

[SaintJude]

But since when does the higher activation energy lead to the more stable product? I thought that a lower transition state indicated a more stable product. In fact, if anything this problem suggest to me that activation energy isn't related to stability. Or is it?

But yeah, in retrospect, the passage also stated that the heat of hydrogenation of product B was 0.5 kcal greater than that of A. According to Kaplan, lower heat of hydrogenation is an absolute indicator of higher stability.

 

[dranet]

There is another way to look at this. You can say that product A is theThermodynamic product whilst product B is the Kinetic product. B forms very quickly it is taking the fastest route to complete the rxn. Product B on the other hand is very "hot" and needs more time to settle down to its stable product by doing things like carbocation rearrangements- so this guy has the highest energy Transition state. Just as a Tip this is what enzymes do in biological systems they lower the energy of the transition state which translates into lower Ea.
This q is typical of the MCAT, we are taught that for the most part the most stable product is the one most formed quantitatively and then they throw a curve ball turning it into the opposite. So if you remember Thermo=HOT transition molecule and Kinetic= Kool transition molecule it will bat the curveball in your favour.

 

[Ssina]

So it's the kinetic and thermo product, where by increasing temperature, basically adding more energy, the reaction goes through a higher activation energy, potentially less stable intermediate, but forms the more stable (thermo) product

 

[MedPR]

But since when does the higher activation energy lead to the more stable product? I thought that a lower transition state indicated a more stable product. In fact, if anything this problem suggest to me that activation energy isn't related to stability. Or is it?

But yeah, in retrospect, the passage also stated that the heat of hydrogenation of product B was 0.5 kcal greater than that of A. According to Kaplan, lower heat of hydrogenation is an absolute indicator of higher stability.

Just from my orgo experience, higher activation energy typically leads to the more stable product (thermodynamic product), while the kinetic product is produced more rapidly (lower activation energy). Thus, in the absence of heat the kinetic product will usually predominate because there isn't enough energy to reach the higher transition state to produce the more stable, thermodynamic product.

 

[Erythropoietin]

How much something is produced (thermochemistry) can never be identified from the activation energy (kinetics).

 

[SaintJude]

I see--this is a topic I do vaguely remember. Thanks. I just found a good link I'll have to read over:http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch10/ch10-3-3.html

 

[dranet]

So it's the kinetic and thermo product, where by increasing temperature, basically adding more energy, the reaction goes through a higher activation energy, potentially less stable intermediate, but forms the more stable (thermo) product

Yes you are onto it. The Ea for the Thermodynamic product is higher than the Ea for the Kinetic product. It is about collisons. The Thermodynamic intemediate is the result of many collisons, breaking up and recolliding, its saving it up till its happy. The Kinetic product makes less collisons stablises and moves on quickly to make the Kinetic product.They found that in Enzyme products which you could say produce Kinetic products. When an enzyme binds a substrate it puts it out of action, it has to move on, it doesn't get time to recollide, reform, recollide.

 

[Ssina]

How much something is produced (thermochemistry) can never be identified from the activation energy (kinetics).

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Yes you are onto it. The Ea for the Thermodynamic product is higher than the Ea for the Kinetic product. It is about collisons. The Thermodynamic intemediate is the result of many collisons, breaking up and recolliding, its saving it up till its happy. The Kinetic product makes less collisons stablises and moves on quickly to make the Kinetic product.They found that in Enzyme products which you could say produce Kinetic products. When an enzyme binds a substrate it puts it out of action, it has to move on, it doesn't get time to recollide, reform, recollide.

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