Thermodynamics vs Kinetics

[Josh138]

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Hi everyone,

I just had a quick question about thermodynamic vs kinetics in a reaction.

On page 270 on the first half of the TBR Orgo Book 1 series, it says that "As a general rule slower reactions are more selective than faster reactions because reactants have more time to select the best site for reacting." Reason being is that "As the temperature increases, the reaction proceeds faster, and is therefore less selective."

This led me to believe that at higher temperatures, that kinetics has a larger effect on a reaction than thermodynamics.

However then for one of the reading passages on page 320 it states in the passage that "The percentage of the secondary alcohol formed increases as the temperature of the hydration reaction increases. This is attributed to a shift from kinetic control to thermodynamic control."

Can anyone explain why thermodynamic control is greater than kinetic control at higher temperatures?

Thanks in advance!

 

[gwjib04]

Can anyone explain why thermodynamic control is greater than kinetic control at higher temperatures?

Consider one reaction where R -> P₁ or P₂
This reaction is exothermic so R-> P_{1 or 2} + heat

At low temperature, there is less energy around for the reaction to use so it goes straight from reactants to products to form the easiest product (one with lowest E_act) very quickly. That would be the reaction which has the faster rate constant k which I'm calling P₁. This happens so fast, rearrangements can't even occur. Therefore, its not the most stable product.

When temperature is increased, the reaction of R->P₁ + heat "shifts left" and favors the reactants which we can explain with Le Chatlier's principle.

So now P₁ isn't necessarily favored like it was at low temp so the product that results will be the one with the more favorable equilibrium constant K at the new high temperature. That will be P₂ because it is more stable. At high temp, the reaction is not forming the products right away, its flipping back and forth form reactants to products (dynamic equilibrium) which takes longer but makes a more stable product to stick around in the end.

So the transition state of the thermodynamic product requires more energy but its product is more stable. You can even bring in the equation dG^o=-RT lnK_eq and Hammond's Postulate here to help explain. Bottom line, the reaction with the more higher energy TS has a more stable product.

At low temp. its a competition between rate constants and activation energy. At high temp, its a competition between equilibrium constants and the product that's more stable. More substituted hydrocarbons are more stable.

I think its analogous to say something like: If your boss made you do a task really fast, you'd find the easiest way to just get it done. But if your boss gave you more time and investment to do something, you'd think about it for a while, try a few different things and eventually come up with a better result.

To me this is a challenging topic but its great to study because it requires you to unite some hard topics (kinetics, equilibrium, thermodynamics and orgo) all at once.

Recommend you check out the UCalgary chem site. That's my resource for all things O chem be/c it has crystal clear explanations:
http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch10/ch10-3-3.html

BerkReviewTeach will probably clarify this better than me

 

[Josh138]

Fantastic explanation gwjib04, that makes a lot of sense. I think the part where I am still a bit shaky on is the wording on TBR's explanation between thermodynamic and kinetics specifically the part where they state " "As the temperature increases, the reaction proceeds faster, and is therefore less selective." In case context matters, they wrote this when they were explaining why chlorination selects for tertiary over secondary carbons when temperature increases.

To me it seems that thermodynamics is more "selective" than kinematics because it is carefully trying to minimize the free energy of the system while kinetics just seems to do it all really quickly without caring about what products are formed, as long as it's going the route of the reaction with less activation energy.

Additionally when they say that the reaction is "faster" shouldn't a reaction under a dominant kinetic control be faster than thermodynamic? Because as you mentioned, the reaction that favors kinetics is trying to drive the reaction that has the faster rate constant.

Thank you again for your response. I really appreciate it!

 

[gwjib04]

I think the part where I am still a bit shaky on is the wording on TBR's explanation between thermodynamic and kinetics specifically the part where they state " "As the temperature increases, the reaction proceeds faster, and is therefore less selective." In case context matters, they wrote this when they were explaining why chlorination selects for tertiary over secondary carbons when temperature increases.

I looked at TBR OC1 pg 270. I see what you mean about their discussion of free radical chlorination. My understanding is that Chlorine doesn't behave like most others so look to Bromine's behavior for the rule. It looks like they don't give a real explanation why they're so different. Its amazing to me how similar those 2 halides are and how different they can behave. I was just reading other posts about differences between Cl and Br as Nuc: / LG and now this!

Check out das video link and all should become more clear. The example he uses is 1,2 /1,4 conjugated dienes which is a Mech that confused me as well so its a nice clarification.
What I learned is that time and temp are both separate and the same when it comes to these reactions.
This guy Michael Evans has a lot of good chem videos but they can get pretty deep😱

http://youtu.be/DrToRsXmjP4

 

[Endure]

Can anyone explain why thermodynamic control is greater than kinetic control at higher temperatures?

Thanks in advance!

The way I understood it - and I'm not sure if this is correct - is that the thermodynamic product is favored when the system has sufficient energy (i.e., the activation energy) to allow both reaction pathways to proceed. When these conditions are present, the product that is more stable will form once the kinetic product reaches equilibrium.