In this video, we're going to look at the reaction energy diagram for the E2 reaction.
But first, a quick review of the E2 reaction as I teach in my Substitution Elimination
Reactions video series linked below which you can find on my website leah4sci.com/substitution-elimination.
The E stands for elimination and the 2 tells us that this is a bi-molecular reaction.
It doesn't mean we have two steps, rather we have 2 molecules reacting at the very same
time in one single step.
And let's not forget the beta for beta elimination telling us that this reaction happens between
the hydrogen and leaving group that are beta or right next to each other.
Let's take a look at the reaction between 2-bromopropane reacting with sodium methoxide
in a heated methanol solution to give us propene our final product.
Since this is a one step reaction that involves two molecules the rate for an E2 reaction
is equal to some constant K times our two molecules.
The substrate or the carbon chain holding the leaving group and the base, the molecule
that attacks.
Let's take a look at the mechanism so we know what to look for on the diagram.
Our carbon chain has more than just bromine on carbon number two, don't forget we have
invisible hydrogens everywhere on this carbon skeleton.
I'm only showing the one that will participate in the reaction so that the base CH3O- can
come in and attack.
Where did I get that negative charge?
don't forget that anytime you have a metal like sodium or potassium followed by something
like an Oxygen, OH or OCH3, that sodium is a spectator cation and the group next to it
is a negative ion.
In this case CH3O-.
Oxygen will use its lone electron pair to attack the beta hydrogen.
This will cause the electrons to collapse in the direction of the leaving group which
will kick out the leaving group breaking bromine away from the molecule.
I explain the why and how behind this mechanism in much more detail in the E2 video linked
below.
Since everything collided at the same time we don't have any intermediates, instead we
go right to the product.
What happened as a result of all these arrows?
The carbon skeleton remains the same, the hydrogen that came off is now attached to
the OCH3 giving us another methanol in solution.
The electrons that bound hydrogen to carbon are now sitting as a pi bond between the alpha
and beta carbon and of course we have Br minus floating somewhere around in solution.
Before we look at the energy diagram, some important things to recognize in this reaction,
we had two molecules reacting at the same time, that means we had no intermediate.
We went right from the reactant to the product.
The attacker, the OCH3- is strong and reactive.
That means compared to the slow E1 reaction, this is a very fast reaction.
For all the drama that took place, the energy diagram is relatively simple.
What we're looking at is what happens to the energy of the entire system.
The reactants, the products, the transition state or intermediate if we have one as the
reaction progresses.
Therefore we measure energy on the Y-axis.
And the X-axis which tells us what is happening throughout the reaction is called the reaction
coordinate but I like to think of it as time.
What happens to the energy of the system as time goes on.
Notice that I didn't put any numbers on this diagram because I'm not looking at a specific
reaction as much as the family of E2 reactions where a different substrate or a different
base will give you something slightly higher or lower but the concept meaning the relative
shape of the graph will be similar.
We have our starting molecule, 2-bromopropane, it's a stable molecule at a relatively good
energy minding its own business until it comes across a base.
In the presence of enough energy to anger that base, to cause that base to attack.
As the energy of the system goes up, and up, and up, and up, the base now has enough energy
to attack that hydrogen and start pulling the electron away to start kicking out the
bromine which is what we'll see at the top of this mountain known as the transition state.
What we have here is an in-between of everything that's about to happen.
Our carbon skeleton remains the same and the bromine is sitting there attached but not
quite perfectly attached because if you look at what happened with hydrogen and with the
OCH3 you’ll see exactly what's going on.
The OCH3 is in the process of grabbing that hydrogen atom.
The bond between oxygen and hydrogen is not quite fully formed but it's already starting
to establish.
As a result, the bond between carbon and hydrogen is also partially established because while
the hydrogen is still partially attached, oxygen is already starting to yank it away.
That means the electrons that bound hydrogen to carbon are now starting to form that pi
bond, that pi bond isn't quite fully formed but we can tell the direction in which it's
going and yes you guessed it!
The bond between carbon and bromine is also starting to disintegrate.
Let's consider the charges of the players involved.
The starting molecule is completely neutral, the attacker was negative.
If Oxygen is still in the process of attacking, it doesn't quite have its full negative charge
but it's not quite neutral giving it a partial negative.
Hydrogen always remains bound to one atom so its charge doesn't change and same thing
with the carbon skeleton.
Unlike the SN1 and E1 reaction, we don't have a carbocation intermediate and therefore we
have no charges on carbon, not even at the transition state.
So where did that negative charge go?
Bromine is collapsing away from carbon with an extra pair of electrons.
It's about to get a negative charge and so during the transition state it has a partial
negative while it's still pulling the electrons towards itself.
If you think this transition state looks messy you are absolutely correct and that is why
it is not something that can be isolated rather something that is understood to happen at
that maximum energy level.
What do I mean by maximum energy?
We started out at this level and then the heat and energy that went into the system
so that we can have this mess of a transition state happening, this is called the activation
energy.
Regardless of how quick this E2 reaction happens, it still requires an input of energy to get
to that unfavorable transition state.
The difference and energy between the reactant and that transition state is called the activation
energy and that is the energy required to make this thing possible.
But that transition state doesn’t last for too long, in fact it's so fleeting because
as soon as the bonds are properly formed and broken, the system will release so much energy
so that the final products have lower energy than the starting molecule.
In fact, if you compare the energy of the starting molecule to the energy of the final
product, you'll see that the overall energy of the system went down.
We have a net negative energy in the system because it became so much more stable, telling
us that this is a spontaneous reaction.
Let's review the reaction in context of what we see to make sure that you follow.
We started with a relatively stable starting molecule in the presence of a CH3O- . As we
put more and more energy into the system, the CH3O- has enough energy to start grabbing
the hydrogen which breaks away from carbon which collapses the electrons towards the
leaving group which kicks out the leaving group.
This transition state is fleeting.
The system quickly releases all of the energy to give us our stable low energy final products.
Remember the 2 stands for bi-molecular which means the two molecules react at the same
time which tells us absolutely no isolated intermediate.
Just the reactant that undergoes some crazy transition that we cannot isolate to give
us our final product for a quick exothermic spontaneous reaction.
A quick note about E2 reactions, notice that because this molecule has 2 hydrogens on its
terminal carbon, it doesn't exist as Cis or Trans.
However, if our starting molecule looked something like this where we had bromine on a secondary
carbon, as well as a beta hydrogen to pull from a secondary carbon, we have the option
of getting two different products.
A Cis-alkene and a trans-alkene.
As I teach in the E2 videos, the trans alkene is the major product because it's more stable.
And if you were to draw this on the reaction energy diagram, you would show the trans alkene
at a much lower energy than the Cis-alkene to demonstrate the fact that trans is definitely
more stable and therefore lower energy.
For even more on E2 reactions along with everything substitution elimination, first give this
video a thumbs up and subscribe to my channel to make sure you don't miss out on any new
videos.
Then, visit my website, leah4sci.com/substitution-elimination for my entire SN1 SN2 E1 E2 video series,
practice quiz, cheat sheet, and so much more.
The link again, leah4sci.com/substitution-elimination.