Kinetic Thermodynamic

by Kiley Lynch

Here’s a choice you’ve probably faced at some point. You need to buy something for your apartment – a can opener, let’s say. You have two options. You can buy the 99 cent can opener that you know is probably going to get through about 3 cans before breaking down, or you can buy the $6.99 model that is going to be a lot more reliable. Which one do you choose? 
It’s an incomplete analogy, but it’s a little like a situation encountered in organic chemistry where two different products can form depending on the reaction conditions. This is called kinetic and thermodynamic control.

A common example is in additions of acids (like HCl) to dienes, such as butadiene.

The cost of the can opener is similar to the activation energy required to start the reaction. The activation energy  is related to the stability of the carbocation that’s formed. Recall that carbocations become more stabilized as the number of attached carbons increases. So the top example (with a secondary carbocation) is more stable than the bottom example (primary carbocation), and is formed faster.This is the kinetic product. 

The reliability of the can opener is similar to the thermodynamic stability of the alkene that is formed. Alkenes increase in stability with the number of attached carbons. So the “1,4” product (where H and Cl add to C-1 and C-4 in our example) is more stable than the “1,2” product (where H and Cl add to C-1 and C-2) since the alkene has two carbon substituents. This is the thermodynamic product. 

And money is analogous to the available energy (i.e. temperature). At low temperatures, when not much energy is available, the kinetic product will dominate. At high temperatures the thermodynamic product will be the major product.

Tomorrow: let’s talk about the “homes” where electrons live.

Thanks for reading!
James

P.S. I wrote about this – “Can Opener Economics” – (Kinetic and Thermodynamic control)