Leah here from Leah4sci.com and in this video we're going to look at Molecular Orbital Theory
for Sigma and Pi Bonds. If you look up what is Molecular Orbital Theory,
you'll find some complicated Math and Physics explanation of Quantum wave function that overlap
and all really nice except that in Organic Chemistry, we're looking to understand the
simple take away rather than the complicated over your head don't need to know this information. And
that's what I wanna look at today. So let's back up and talk about Atomic Orbitals. Atomic Orbitals
as you remember tells us where an electron is located or specifically in a type of orbital
that an electron is located around an atom. And if you go back to my Orgo Basics video series,
you'll see how we have the sp3, sp2, sp and even p orbitals that house different electrons. If
you need a refresher, visit the link below or go to my website, leah4sci.com/OrgoBasics.
For example, in a molecule CH4, we have a carbon atom bound to 4 hydrogen atoms. The
carbon is sp3 hybridized and each of the hydrogen atoms have an s-orbital only so no hybridization.
The bond between carbon and hydrogen is an overlap between the sp3 hybrid orbital of carbon
and the s-orbital of hydrogen. But when that bond forms we're no longer looking at the individual
atomic orbital because now, these two orbitals have fused together to make a molecular orbital
to show that the atoms are bound together. The difference between atomic and molecular orbitals
is that the atomic orbitals refers to just the atom where the molecular orbital now shows us
the electrons on the entire molecule. The energy between an atomic and molecular orbital is very
different and depends on the specific situation. So let's take a look at a simple molecule, H2 gas.
This is made up of 2 hydrogen atoms that each have a lone electron. If we plot this on an
energy diagram, then each of the hydrogen atoms has its energy somewhere in the middle. They're
not happy but they're not unhappy, this is how they are and they're okay with it.
This one electron refers to the atomic orbital for each hydrogen sitting in the 1S orbital. When they
come together to form a bond, we no longer have the atomic orbitals because now the electrons
are sitting in a new molecular orbital. Another way to visualize this is the two hydrogen atoms,
we have the one atomic orbital, the second atomic orbital. Now they're overlapping and you get one
giant molecular orbital. When the two atoms come together to form that molecular orbital,
what's happening is they get combined mathematically using the LCAO, the Linear
Combination of Atomic Orbitals. No, no, no! This is too much mathematical mambo jumbo that we don't
care about. What we do care about, the takeaway is that we get two different options. The first
is constructive interference or should we say low energy bonding molecular orbital.
And the second is destructive interference or should I say High Energy Antibonding
Molecular Orbital. You'll often see an asterisk at the Antibonding Molecular Orbital. So where
does this go on the energy diagram? We have the low energy bonding molecular orbital
and we have the high energy antibonding molecular orbital where the bonding is a sigma bond and the
antibonding is a sigma star. Remember that we only had two electrons and that means we have
the options of putting them into the low energy bonding molecular orbital where they're happy
or bump them up into the antibond. But they're not going to occupy both molecular orbitals at
the same time. So how does this make sense? I like to think of the electrons as people
in or out of the relationship. We start with the atomic orbital where we have a single electron.
Here we have a single person and another single person,
they don't know each other, they're just flipping life and happy enough.
And then one day they meet and fall in love. Together, they are so happy and so stable even
more stable than their single days giving them the lowest possible energy because remember, happy,
stable, unreactive. And if energy represents anger and temper, there's not a lot going on there.
But as with most couples, every now and then they get into a fight. They're still in a relationship,
they're still together, but right now, they're so mad at each other, they have opposing views,
differing opinions, there's yelling and screaming and very very high energy, that together,
in a fight, there's much higher energy meaning much less stable than their single days.
As a reminder, the two electrons are sitting in a bonding molecular orbital, if they get excited
by something upsetting, these electrons can temporarily come up to the antibonding
but if that source of extra energy goes away, they prefer to sit here in the comfortable,
happy, stable, bonding molecular orbital. This temporary rift between them, this fight, is called
the antibonding node, the thing that separates them even though they're technically together.
Turning our stick figure back into hydrogen, we have almost a single bubble where the
two electrons exist together in the bonding molecular orbital and this right here for the
antibonding molecular orbital where there's a clear node between them showing that the
atoms while still together are definitely not holding on to each other as well.
This is a topic I use to struggle with and my TA told me, oh don't worry, this is just mathematical
physics beyond what you need to understand, which made sense that I didn't need to understand it but
I was so confused. So even though I'm telling you about the same thing, don't worry about the
math and physics behind all this, I still want you to have a simple enough takeaway of what
the heck is going on here. So if you're with me so far, make sure to give this video a thumbs up
and let me know what's your biggest takeaway in the comments below
and then let move on to pi bonds. Molecular Orbital for pi bonds start
out very similar to sigma but can get complicated, so let's take a step back.
A pi bond or a double bond doesn't refer to two bonds, it's actually the second in a double bond.
For example if we look at the molecule Ethyne or Ethylene which is CH2 CH2, the simple drawing for
this is a carbon double bound to another carbon atom and each carbon has two additional carbon
atom sigma bound to the side. The double bond between them has two lines that don't
differentiate so it makes it look like it's two of the same when in actuality we have one sigma bond
and one pi bond. Remember that a sigma bond like we saw at the hydrogen atoms is a simple overlap,
and don't forget the carbon to hydrogen bonds are also all sigma. The bond would be shown
with the Sp2 hybrid from carbon overlapping with the S non-hybrid from hydrogen to form a sigma
bond. But since our interest is the pi bond, let's simplify this as follows. We have carbon single
bound to carbon and almost imagine that this is drawn just tilted off the plane on the page
so that for each carbon we have one hydrogen coming forward out of the page and one hydrogen
going back into the page. That lets us put our eye right here and looks straight on to see
about the pi bond. The pi bond is made with a non hybridized P-orbital that individually sets just
above and below the plane of the molecule, in our case the plane of the page. When those p-orbitals
come together and overlap, the pi bonds formed. Now, let's bring them a bit closer together.
What this shows us is the electrons are free to go up and down, around and around between
the two atoms with a node in the middle so they're sitting either at the top half or
bottom half of that pi bond. The energy diagram for this will look exactly the same as the energy
diagram for hydrogen. We start out with some in-between neutral energy for the 2p orbital.
Each of which has a single electron. The 2p orbital comes from the hybridization
of carbon which as we remember is 1s², 2s², 2p². When the 2s and 2p electrons hybridize
we get three 2 sp², so let me tell you what the numbers mean. We have three, 1,2,3
sp2 hybrids in row 2, let's put all the numbers, so we can take away that too. And then on the side
we also have that single p but because it's coming from the second row, it's coming from a 2p, that's
what this 2p stands for. When the electrons come together, we get a pi bonding molecular orbital,
but if they get excited, they jump up into the pi star antibonding molecular orbital
which we can imagine something like this. As you can see, it looks very
much like our initial ethylene molecule but with a very obvious node between them
because right now, the carbons in this relationship are not getting along very well.
If you're following up to this point, but you're trying to think how to make sense of this in the
grand scheme of molecules and reactions, here is how I like to think of it. We have our ethylene
molecule, CH2 CH2 which if we try it out, we have a pi bond between the two carbon atoms.
I like to think of the bonding molecular orbital as a pi bond. Remember on resonance structures
one type of unstable resonance is to take the two electrons and collapse it onto one of the atoms.
Show the double headed arrow and this new less stable contributing structure.
We still have the sigma bond between them, but now we have one lone pair on the carbon on the right,
that means it's got extra electrons and a negative charge. The carbon on the left, loss the carbon,
got nothing in return is now deficient and therefore gets a positive formal charge.
Remember, this is not the mathematical and physics explanation of bonding and
antibonding but instead, this is how I like to think of it, in bonding, they're together,
they're happy. In antibonding, they're separated, there's the burden of charge,
one is negative, one is positive, they don't like that burden, that's what makes them so unstable.
As students, you take away that extra energy, the electrons were able to come back, able to
resonate back to the comfortable structure, back to the happy low energy bonding molecular orbital.
And to be honest, once I started thinking of it this way. that's when the topic finally clicked.
But this is just one pi bond with two electrons. What about a more complex system with 4,6,
or more electrons and lots of resonance involved? Remembering that not all of the molecular orbitals
will be occupied brings us to the concept of Highest Occupied Molecular Orbital or HOMO
and Lowest Unoccupied Molecular Orbitals or LOMO. And this is exactly what we'll discuss
in the next video which you can find on my website by clicking the link
below or going to leah4sci.com/MOtheory . The link again, leah4sci.com/MOtheory.