Leah here from leah4sci.com and in this video we're going to look at HOMO and LUMO molecular
orbitals specifically in conjugated systems as we'll see them in Organic Chemistry reactions.
Organic Chemistry is all about the reaction mechanisms. You have a molecule that attacks
another one and gives you a product. But to truly understand why the reactions happen and even more
specifically where on the molecule they happen. We need to understand the concept of HOMO and LUMO
where HOMO stands for Highest Occupied Molecular Orbital and LUMO is Lowest Unoccupied Molecular
Orbital. Don't worry, we'll keep it simple but let's focus on the Math and Physics
that you learned back in General Chemistry and more of an overview that actually makes sense
so that you can apply it to reactions. In the last video, we look at molecular orbitals
for sigma and pi bonds, specifically molecules that have just one pi bond.
You can review that using the link below or go into my website leah4sci.com/MOtheory.
When you have a system with more than 2 electrons in the molecular orbital system
as you'll see in conjugate molecules like 1-3m butadiene or 1-3-5, hexatriene and so on. How do
you figure out? how many bonding and antibonding orbitals you have and what do they even mean?
As a reminder, the number of atomic orbitals has equal the number of molecular orbitals because
each one gets converted to the other which means that in the butadiene, we have four electrons
involved in the conjugated system, that's 4 atomic orbitals to give us 4 molecular orbitals.
And in the hexatriene where we have 6 electrons in the conjugated system,
we will have a total of 6 molecular orbitals. When we look at a bond of 2 electrons, we compare it
to a relationship. A couple made of two people. Whether we are looking at 4 or 6 let's just think
of them as a group of friends where sometimes they all get along, sometimes there are groups
within the group that get along and sometimes none of them get along. When it comes to drawing
the interactions in molecular orbitals, we draw the orbitals with the most number getting along
and we cut it in half and then cut each piece in half so what on earth am I talking about?
This right here represents a group of four friends and the 4 different molecular orbitals.
In the lowest energy where they're all getting along, they are all very happy. If a rift forms
so that we cut the group in half, meaning we put a node in between them. We have these
two getting along and these 2 getting along but the two groups don't get along with each other,
they're still considered stable because for the most part the group is still getting along but
it's slightly higher energy because there's that rift going on between them. But if we take it a
little bit higher, now we have a situation, where we only have two in the middle that get along with
each other but there's a fight between the two in the middle and that one on the right as well as a
fight with that one on the left. Right now we're starting to get into trouble with some territory
because they're not really getting along, it's only these two that are getting along
with two nodes in the system. And in the final drawing, we have the most unstable, most unhappy
highest energy system where none of the friends are getting along, they're all fighting with each
other putting a node between each and every person making it the most unstable situation.
Let's take this and apply it to how you will draw a system for the 1,3 butadiene that this is based
on. In each situation, we have a molecule, 4 carbons that has 4P Orbitals, 1 on each carbon.
In the lowest energy system where everyone is getting along, all of the orbitals are perfectly
align, they're all overlapping and the way we're going to represent that is showing the dark colors
altogether and the lighter colors altogether, showing that electrons are able to flow between
them because the orbitals, the way they're drawn tells us the probability density of where that
electron could be found. That means the electrons are free to move over the entire molecule.
But now if we add a rift in the system like a fight between the two groups, we're going to
see that node in the middle so that the two in the left have their orbitals parallel but the
two in the right even though they're parallel to each other or anti parallel to the other group,
and since the colors don't overlap, the electrons can only flow between the right two and the left
two but they can't pass the middle because we have a node. Same thing through the third,
now we have two in the middle getting along that means they're orbitals aligned the same way
and the two on the outside are antiparallel so there is a node sitting over here and over here
not allowing the electrons to flow between them. And last but not the least, we have our highest
energy system where none of the atoms are getting along and that means none of the orbitals are
aligned in the same direction, and the electrons are stuck on the atom that they are a part of,
they can't go anywhere. As you can see, we have three nodes in the system making it very unstable.
This molecule, the 1,3 butadiene has 4 pi electrons per two pi bond
and that means the 4 atomic orbitals turned into formed molecular orbitals as we see here
but there's only 4 electrons. So outside an extreme situation that forces the electrons
to go at a higher energy, we expect to find two of the electrons at the lowest energy
molecular orbital and the next two electrons had the next lowest energy molecular orbital.
If there's energy added to the system, they can temporarily jump up here or up
here. When that energy is removed, they prefer to sit in the lowest and most stable position.
Since the electrons are sitting in the lower two molecular orbitals we can draw a line between the
top and the bottom and recognize that the lower two drawings represent the occupied orbitals
and the upper two represent the unoccupied and this is where the fun comes in. Of the two
occupied systems, notice that this one is higher in energy that this one which made sense because
it's higher up and that means this drawing right here is considered the highest occupied molecular
orbital and when we look at the two unoccupied we recognize that this right here is the lower
energy of the two because it's lowered down and that means this right here is lowest unoccupied
molecular orbital or LUMO. And what I used to find confusing is that lower is actually higher on the
graph and higher is lower on the graph until it understood what it meant. HOMO represents the
molecular orbital sitting directly under that line that separates them or smack them in the middle
of the graph, LUMO represents the one sitting right above it and in reactions you'll have the
electrons that occupy the HOMO or LUMO as the one that react which you'll see in your Diels-Alder
reaction, your Pericyclic reactions and so much more and I want to show you one more thing which
is NOT how you want to think of it and if you draw this line in your exam you'll probably lose points
but this is what finally made it quick for me so I want to show you just to have in the back of your
mind for understanding. To understand what all of this means, what I actually picture in my mind
when looking at problems that have the orbitals written out like this, here's what I picture,
I have the skeleton for my 1 3 butadiene and when they're more stable at the lowest
molecular orbital, I just imagine complete resonance between the system, it's the most
conjugated the electrons are just all over, it's very stable. If we take it up a notch,
I imagine that there are 2 pi bonds here and even though the pi bonds are stable as is, there's
no interaction between them one stays to the right one, stays to the left without resonance,
remember that resonance equals stability, it helps stabilize molecule and that's why I see
this as slightly less stable. Now If I were to draw the resonant structure something like this,
I would get a pi bond here, the electrons as a lone pair on the right side with a negative
charge and an empty or emptied carbon atom with nothing to replace that 4th bond, giving you the
positive charge on the left side, this is how I imagine this molecule or this molecular orbital to
be the first of the unoccupied unstable because now we've introduced a separate enough charge,
this molecule isn't very heavy except for right here in the middle and it really doesn't wanna
sit in that position. And last but not least I want you to imagine we'd take this pi bond and
we break it with the resonance and this pi bond breaking it with resonance giving me a lone pair
of electrons here with the negative and this one with the positive charge, another lone pair with
a negative, another positive and this is just a complete and total disaster, this molecule has
extreme separation of charge, it is so unhappy, it is so unstable and it never wants to be in
this position unless it absolutely has to be because it's getting zapped with so much energy
where its group of friends are just all out fighting, rolling on the floor, kicking,
screaming, fighting, I mean this is chaos. Are you with me so far? Keep this in the back of your mind
knowing that THIS is what we actually care about, please make sure to give this video a thumbs up
and remember, we skipped over all the math and physics because that is not what you want to
focus on when you're studying this for organic chemistry, instead you want to ask yourself,
"do you really understand this enough to make sense?", "Can I draw out the orbitals for a
butadiene or a hexatriene?", "Can I quickly and easily identify that line of demarcation in the
middle and realize the one right under is HOMO and the one right over is LUMO?" without going crazy,
without worrying about wave functions and other crazy physics calculations? If yes, that's
all what you'll really need for organic chemistry. For even more on this, see the link below or visit
my website leah4sci.com/MOtheory , again that's leah4sci.com/MOtheory and to see how this works in
actual reactions such as Diels-Alder or Pericyclic reactions, come join me in the organic chemistry
study hall where I go through those reactions step by step and so much more. Full details on
my website link below or go to leah4sci.com/Join , again that's leah4sci.com/Join