First, a quick review of the E1 reaction.
E1 specifically refers to a beta elimination reaction where the E tells us that it's elimination
and the 1 tells us that it's unimolecular.
This means we're looking at a reaction in which the beta groups, specifically hydrogen
and leaving group that are beta to each other, are eliminated from the molecule in a unimolecular
reaction where uni doesn't mean one step, instead uni tells us that we're reacting one
molecule at a time.
Let's take a look at what happens when 2-bromo-2-methylpropane reacts in a hot aqueous solution to give us
2-methylpropene.
I cover the mechanism in detail in the videos linked below.
But as a summary we have the bromine breaking away from the carbon chain without the use
of a catalyst, without anything kicking it out.
It breaks away on its own leaving us with a deficient carbon or a carbocation and a
Br- in solution.
This right here, the breaking away of the leaving group to give us a carbocation is
the slowest step in this reaction making it the rate determining step which is why the
rate of an E1 reaction is equal to some reaction constant k times the substrate.
The substrate being this molecule and nothing else.
This is what we mean by unimolecular, one molecule at a time where this molecule gives
us the rate determining step.
Once the carbocation forms, water will reach for the neighboring beta hydrogen which will
cause the electrons between hydrogen and carbon to collapse towards the positive charge, towards
the carbocation to form a pi bond in the process giving us the molecule we've drawn here for
the product.
Now that we know the steps, let's take a look at the energy diagram.
The E1 Reaction Energy Diagram takes a look at what happens to the energy of the system,
the change in energy between the molecules as the reaction progresses which is why we
show energy in the Y-axes and the X-axes known as the reaction coordinate is something that
I simply think of as time.
What happens to the energy as time progresses? as the reaction goes from step 1 to step 2
and so on.
The reaction begins with our starting molecule 2-bromo-2-methylpropane just sitting there
minding its own business at a relatively low energy.
It's a stable molecule.
But when this molecule is given an input of energy which we saw as heat, the energy of
the system begins to go up and up and up and up until there's so much energy that the bromine,
the leaving group, starts to break away.
At the very top of this energy diagram we have what is called a transition state that
looks like this.
We have the carbon chain and we have the bromine atom.
They're not quite attached to each other but they're not quite broken away.
This is that high energy partially formed, partially broken bond and therefore it's not
something we can isolate.
The transition state is that in between before the next molecule is about to fully form.
The starting molecule was neutral, but when bromine breaks away taking the electrons with
it, it will have a negative charge.
Therefore in the transition state when it's nearly broken away, it has a partial negative
charge.
Carbon, when abandoned by bromine when those electrons are taken away from it will have
a positive charge.
And so the transition state shows a partial positive charge because bromine is in the
process of stealing its electrons.
The transition state is fleeting and not something we can isolate until we get to a valley.
When bromine has finally broken away and we're left with the carbocation in solution.
This carbocation is stable, it is definitely something we can isolate.
But notice the level of the energy.
It is much higher than the starting molecule because it is a lot less favorable.
A tertiary carbocation is stable compared to other carbocations but it's not a very
happy molecule.
And so, when the water approaches and looks to attack, with just a little bit of energy,
we form another transition state where water has partially grabbed and partially bound
to that hydrogen atom.
Hydrogen is still partially bound to the carbon atom and that brand new pi bond is partially
formed between the alpha and beta carbon.
The more water pulls on hydrogen, the more the bond between carbon and hydrogen will
break, the more that pi bond will form until finally water and hydrogen have completely
broken away resulting in a release of energy to give us our final alkene product.
Notice that this right here is our second transition state once again not something
we can isolate but our final product is very very stable.
There's a couple more things we want to recognize in this diagram.
Notice the big jump in energy between the starting molecule and the top of that first
transition state.
This is the activation energy, the input of energy required for this reaction and it's
very very high.
The higher the required activation energy, the less likely the reaction is to occur because
you have to find that energy, the slower the reaction itself.
This is why both Sn1 and E1 have a high activation energy for a slow first step, a rate determining
step.
Because once we have that intermediate, it's already at a high energy and yes it requires
a little bit of an input of energy, we need that second activation energy.
But look at how small it is compared to the initial one.
That means that once the system is already at a high energy, it's not unfeasible to imagine
finding a little bit of energy to accomplish that second step.
And then when we compare the energy between the reactant and the product, you'll notice
that the reactant begins at a much higher level and the product is at a much lower level.
That means the product is very very stable and has lower energy than the reactant.
This gives us a negative delta G for a spontaneous reaction.
We've talked through this reaction from left to right showing that the reaction proceeds
in this direction.
However, this is a reversible reaction and that means with the right input of energy,
we can actually take this reaction backwards which you may recognize as the alkene halogenation
reaction that I teach in the video linked below.
With everything written out, let's review the reaction one more time to make sure you
follow what's going on.
We started with the relatively stable molecule, a carbon holding a leaving group and with
an input of energy, the leaving group decides to break away on its own so that we're left
with the carbocation and the leaving group floating somewhere in solution.
Another slight input of energy in the presence of a water molecule showed us that water grabbed
the beta hydrogen breaking the bond between itself and carbon, collapsing those electrons
towards the former carbocation resulting in a massive release of energy to give us a very
stable alkene final product.
For even more on E1 reactions, first, make sure to give this video a thumbs up and subscribe
to my channel if you haven't already so that you don't miss out on any of the new orgo
videos, then visit my website leah4sci.com/substitution-elimination or see the link below for my entire series
on substitution elimination reactions including videos, practice quiz, cheat sheet, and so
much more.
The link again is leah4sci.com/substitution-elimination