Thermo vs. Kinetic favored product

[bluesTank]

In my MCAT class the other day we took an orgo passage, and I missed like 2-3 questions on it from problems dealing with thermo vs. kinetic favored products alone. For all of them, I just went with what I had read in EK etc. with the most stable carbocation etc. but all the questions tripped me up. I remember learning about that crap in orgo but that was 2 years ago, and nowhere in EK does it ever talk about it.

Does anyone know how to know when a question is dealing with thermodynamic vs. kinetic favored products? Like if it doesn't say anything about it, which do we assume? Just a little explanation would help, thanks!

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[primusrocks]

I believe the theremodynamic is the faster formed pdt and the kinetic is the more stable pdt if that is what you were asking.

 

[killinsound]

I believe the theremodynamic is the faster formed pdt and the kinetic is the more stable pdt if that is what you were asking.

that is correct. thermodynamic products usually deal with proximity while kinetic products deal with more substituted stability through conjugation or conjugation-esque effects.

kinetic products will dominate when less than ideal energy is put into a rxn.

thermodynamic products will dominate when energy put in is not a limiting factor.

 

[bluesTank]

But what specific cues in the question will dictate which will predominate? If it doesn't mention anything, just "what will be the predominant product?" which is it referring to?

 

[killinsound]

But what specific cues in the question will dictate which will predominate? If it doesn't mention anything, just "what will be the predominant product?" which is it referring to?

under those circumstances you have to take the reagants into account.

for example a very sterically hindered base (tert butoxide) with 2 chloro 1,4 dimethyl butane, wont result in the most thermo product, 1,4 dimethyl 2-butene, but will result in the kinetic product 1,4 dimethyl butene (1-butene).

 

[AnEyeLikeMars]

I believe the theremodynamic is the faster formed pdt and the kinetic is the more stable pdt if that is what you were asking.

I thought it was the exact opposite...

This is from an online chem glossary (UWisconsin)

Thermodynamically favored
When a substance is refered to as the thermodynamically favored product, this means that of the possible products for a reaction, it is energetically the most stable. Sometimes, the thermodynamically favored product is not observed because a less stable product is kinetically favored.

Kinetic
The study of the rates of chemical reactions is termed kinetics. The product which is formed the quickest is referred to as the kinetic product. The kinetic product need not be the most thermodynamically stable product. When a kinetic product is not the thermodynamic product, it will eventually convert to the more stable form.

 

[AnEyeLikeMars]

But what specific cues in the question will dictate which will predominate? If it doesn't mention anything, just "what will be the predominant product?" which is it referring to?

I think that when they ask for the predominant product without specifying anything else, they're asking for the most stable product.

 

[MDAdam]

I believe the theremodynamic is the faster formed pdt and the kinetic is the more stable pdt if that is what you were asking.

Err.. I thought it was the other way around.

"In kinetic reaction control the reverse reaction does not occur or is slow. In this way a product may be formed that is less stable but this product is formed faster because the activation energy for this reaction is lower. Kinetic control is favoured with milder and low temperature reaction conditions."

Wiki Click

 

[Nowaythisnameis]

In my MCAT class the other day we took an orgo passage, and I missed like 2-3 questions on it from problems dealing with thermo vs. kinetic favored products alone. For all of them, I just went with what I had read in EK etc. with the most stable carbocation etc. but all the questions tripped me up. I remember learning about that crap in orgo but that was 2 years ago, and nowhere in EK does it ever talk about it.

Does anyone know how to know when a question is dealing with thermodynamic vs. kinetic favored products? Like if it doesn't say anything about it, which do we assume? Just a little explanation would help, thanks!

are u referring to aamc 8 test passage for orgo? if so, that was easy

 

[killinsound]

are u referring to aamc 8 test passage for orgo? if so, that was easy

you are smart

 

[bluesTank]

are u referring to aamc 8 test passage for orgo? if so, that was easy

Thanks for your valuable input, but no, it was an in class passage.

 

[InTheMiddle73]

I always think of it in terms of what the words mean. "Kinetic" is talking about rate, so the kinetically controlled product is the one that is formed faster. The thermodynamic product, is not always slower to form, but it does require the reaction to be at equilibrium and is more stable than the kinetic product.

I also remember learning in orgo that the kinetic product is formed faster because its intermediate is more stable (like if there was a tertiary carbocation versus a secondary one). And that has to do with its activation energy being smaller to get to that intermediate.

Hope that helps!

 

[Barfalamule]

Kinetic product is the product with the least amount of Activation Energy, not the most stable product. The thermodynamic product is the most stable product but has a higher activation energy. See a textbook for pretty drawings, but this concept is key to the MCAT. Very important. Look at direct and conjugate addition in your orgo textbook for a good example.