kinetics vs thermodynamics

[vegan1]

I came across a couple of questions in a full length BS section that ask which compunds in a given set of experiments are "the product of kinetic control" and which is "the product of thermodynamic control." I understand what kinetics and thermodynamics are, but I don't really understand how something can be under "thermodynamic control" or "kinetic control" in an experiment. Can anyone offer some insight?

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[SoonerLuke]

The product under thermodynamic control will be the most stable/ lowest energy product. The product under kinetic control will the the least stable/ highest energy product. The kinetic product is the one with the lower activation energy, while the thermodynamic product has a higher activation energy, respectively. When you have sufficiently high temperature, the thermodynamic control product (most stable) is favored because the higher activation energy can be overcome. When the temperature is low, the reaction may have a hard time overcoming the higher activation energy required to achieve the thermodynamic product, so the kinetic product is often favored in that circumstance.

 

[MsUamuzi]

So in a nutshell, Kinetics prefers a lower transition state (energy-wise), lower activation energy, lower stability, cooler temperatures, and a bulky base which preferably attacks a primary or methyl group.

Thermodynamics prefers a higher activation energy, higher transition state, hotter temperatures, and a small base which preferably attacks a tertiary group.

 

[aldol16]

An easy way to think about it is that the kinetic product is more easily formed but less stable than the thermodynamic product. In terms of reaction coordinates (the way I like to think about it), take a look at the figure halfway down the page here: http://chemwiki.ucdavis.edu/Textboo...modynamic_Versus_Kinetic_Control_of_Reactions

Product 2 is the thermodynamic product and product 1 is the kinetic product. If you do the reaction under, say, low temp conditions, product 1 will form and very little of product 2 will form. However, at higher temperatures, the entire system can equilibrate and everything should settle into the product 2 form.

A good example is making enolates. You can make a kinetic enolate or a thermodynamic enolate, depending on your reaction conditions. The thermodynamic enolate is the more stable one and the more highly substituted one. This is a good tool to use in the case of Robinson ring annulations because you can tune the reactivity of your substrate to form the ring you want - if you want the less-substituted enolate, just do the reaction under lower temp conditions.