Now we want to go into a little bit more detail and what it means for a reaction to be kinetically
controlled or thermodynamically controlled.
The reaction called conjugated hydrohalogenation is a really good example of a reaction that
has these different types of control.
It’s not the only one but it's a great reaction to use as an example to explain this concept.
If you guys recall, with this reaction, hot reaction conditions favor the formation of
the thermodynamic product which we call the 1,4-product due to the orientation of the
hydrogen and the halogen that goes with it.
We learned that cold reaction conditions favor the kinetic product, which we call the 1,2-product
for the same reasons.
What I’m here to do is show you guys on an energy diagram what these words – kinetic
and thermodynamic – actually relate to.
First of all, I just want to say a disclaimer here.
Notice that I put the words simplified energy diagram here.
That's because these energies in the diagram isn’t perfectly drawn.
It’s simplified to make it as easy for me to teach you as possible.
But this is not the official diagram.
In fact, any diagram of a reaction that has a carbocation should actually have two humps.
Notice that here you only have one hump drawn.
That's because in order for you to understand what this is about, you don't need the full
reaction diagram.
You just need to see kind of the intermediate and then the product.
That's going to help us to really determine why it's called kinetic or thermodynamic.
First of all, let’s just start at the beginning.
Notice that I'm starting off, this is my energy reaction coordinate at the bottom.
This is my x-axis.
My y-axis is in free energy or delta G. What we notice is that in the reaction coordinate,
we're starting off with a diene.
The diene energy level is starting off right here.
Regardless of the type of reaction conditions, we’re always starting at the same energy.
Notice that there's two different pathways that we can take from the get-go.
We notice that I have these two activation energies.
I have an activation energy that's called activation energy K or kinetic, and an activation
energy that is thermodynamic.
What we notice is that the intermediates that are created in these two different reactions
actually have different energies because if you'll notice, remember kinetic means 1,2
right?
Remember that thermodynamic means 1,4.
What we see when we're looking at the different types of intermediates is that the 1,2 intermediate
is a secondary carbocation.
Whereas the 1,4 intermediate is a primary carbocation.
Do you guys remember which one is more stable?
Would a primary or a secondary carbocation be more stable?
Secondary.
What that means is that the preferred intermediate is actually going to be the secondary or the
1,2 product.
The 1,2 product is going to flow-through an intermediate that is more stable.
This intermediate is more stable.
This intermediate is less stable.
That's the first part.
But notice that as we move to the products, things kind of change because what we see
is that even though the secondary intermediate is more stable, look at the reaction product
that it produces.
We wind up getting an alkyl halide that looks like this.
Whereas for my thermodynamic product or my 1,4, I get an alkyl halide that looks like
this.
The alkyl halides have equal energies, very similar energy.
Halogens do not benefit from being primary or secondary.
That's not the important part.
What we do see though is that double bonds have different levels of stability depending
on how many R-groups are surrounding them.
I'm not sure if you guys recall the concept of hyperconjugation which said that carbocations
and double bonds are stabilized through R-groups.
That's why a tetra-substituted double bond is much more stable than a mono-substituted
double bond.
This is a concept that we borrow from elimination reactions way back in the day when we were
just talking about elimination reactions.
We learned that the most stable double bonds are the ones with the most R-groups around
them.
Check out these two products.
Which of these two products is the more stable one?
For my kinetic product, I have a mono-substituted product.
Whereas for my 1,4 product or my thermodynamic, I have a di-substituted.
Notice that these things are in conflict with each other because my thermodynamic intermediate
was the least stable but the thermodynamic product is the most stable.
It’s exactly the opposite for my kinetic product.
My kinetic intermediate was most stable but my kinetic product was least stable.
What does this mean in terms of the temperature control?
Where does that come into play?
Basically, temperature control allows us to overcome the very high activation energy of
the thermodynamic product.
Because eventually all the molecules, all the products want to look like this.
They want to be the thermodynamic product.
That's the best one.
However, if they don't have enough energy, they're not going to be able to overcome the
very steep activation energy to become a thermodynamic 1,4 product.
If we want to favor the most stable product, we use lots of heat to jack up the energy
of the reagents so that we can overcome this very high activation energy and make the most
stable product.
Whereas if we want to favor the less stable product, then we make the reaction conditions
very cold so that the ambient energy is very low so that it’s only going to be able to
cross the threshold of less activation energy.
It's never going to be able to form the less stable intermediate because it's so cold that
it can’t overcome the very steep activation energy of the thermodynamic product.
That has to do with the names kinetic and thermodynamic.
Kinetic means that it’s the easiest one to make and it's the one that is formed the
fastest.
Whereas the thermodynamic product is going to be the one that is overall the most stable
at the end.
The thermodynamic product is usually going to require heat to make it possible because
of the extra energy that you need to put in for the activation energy.
If we were to summarize this, what we could say is that the kinetic pathway has the more
stable intermediate.
Let’s actually just write that in.
It has the more stable intermediate, the secondary, but it has the less stable product, the mono-substituted.
Whereas the thermodynamic pathway has the less stable intermediate, primary, but the
most stable product, which would be di-substituted.
Mono-substituted and di-substituted.
Secondary and primary.
These things are in conflict with each other and it allows us, it gives us the opportunity
to use temperature to control the formation of whichever product
that you want which is really cool.
Again, this isn't the only reaction we learned in Orga 1 and 2 that has this idea of thermodynamic
versus kinetic control but it’s one of the best examples.
Anyway guys, remember that if anything you could always just remember that your thermodynamic
product, the hot one, is the higher number (1,4).
But now at least hopefully you understand more why this even exists.
Awesome.
Let's go ahead and move on to the next topic.