Leah here from leah4sci.com and in this video,
we're going to look at aromatic vs anti-aromatic and non-aromatic. In the previous videos we looked at the conditions
for aromaticity and determined that for a molecule to be aromatic, it has to be cyclic,
planar, conjugated, and obey Huckel's rule with a shortcut that we identified as 4n +
2 = e- means they need to have an odd number of electron pairs. What happens to a molecule that doesn't follow
these rules and does not consider aromatic. There are two different classes of molecules
that you have to be able to recognize, the first is anti-aromatic and the second is non
aromatic. I have seen many students master aromaticity
but when a molecule is not aromatic,can confuse these two and lose points on their exam. So what's the difference between them, how
do we tell if it's anti-aromatic or non-aromatic, this depends on the individual’s situation. Let's sidetrack from aromaticity and talk
about baseball for a second. There are three types of new yorkers, there
are Mets fans like myself, there are Yankee fans like some of my unfortunate friends,
Mets fans, we just don't want the Mets to win, we also want the Yankees to lose, Yankee
fans who don't just want the Yankees to win, they want the Mets to lose and then there
are people like my sister who can't tell a foul ball from a home run even if she's sitting
right by the foul ball. Back to the baseball rivalry, it's not enough
for that thing that's important to you to win, you also want anyone who doesn't exactly
agree with you to lose, you're too close, it's not enough to be one or nothing. But people who don't care, they don't care. If you find the hardcore baseball fan, everything
about them will be very similar. They watch baseball games, they know their
players, they know their pitchers, they know their stats, they tend to wear the same type
of baseball gear except that one specific detail will be different. It will be the New York Mets logo or the New
York Yankees logo, they're very similar to each other and outside they might not be able
to recognize them but they are definitely very different. What does this have to do with aromaticity? Look at the first three criteria, if you quickly
glance at a molecule, you can tell if it's cyclic, planar or conjugate, just like a baseball
fan, you can typically tell if somebody supports baseball or not based on what they are wearing
or the fact that they show up to a game, the thing that makes them different and is slightly
harder to understand is Huckel's rule, Think of odd or even as two different baseball teams,
there is team odd which follows Huckel's rule, that gives us aromatic, there is team even
that does not follow Huckel's rule, everything else about it is the same but because the
number of electrons is different because the energy of the molecule is so high as a result,
it's exactly the opposite and there is rivalry between the odd and the even, between Huckel's
rule and non Huckel's rule, so we can say that molecules that are not aromatic despite
everything else being the same they are anti-aromatic, they don't want team aromaticity to win but
then if you have someone who doesn't care about baseball, doesn't care about aromaticity
whatsoever meaning they don't follow the first three obvious criteria then those are considered
non aromatic. Let's bring this to life with a few examples. Say we have a molecule that looks like this,
it looks very much like benzene. It's got six carbons, three pi-bonds, resonating
system, is it aromatic or not? Let go backwards on this but skipping Huckel's
rule because this is one you only want to check if the first three criteria are met
because if not, it doesn't matter if it obeys Huckel's rule or not. Is the molecule conjugated? Absolutely, we have three pi bonds that can
resonate over the entire system back and forth, is it planar? Yes, every carbon has a pi bond, it's SP2
Hybridized and that makes it flat but is it cyclic? No its not because carbon one is not attached
to carbon six and that means this is an aliphatic or linear molecule rather than a cyclic compound,
this molecule doesn't care about baseball, it's a linear compound, it has nothing to
do with aromaticity and so it is non aromatic, this next example looks a little scary at
first, don't let the positive charge scare you, follow the checklist as you would with
anything else. Is the molecule cyclic? Well yes it is, it's a hexagon just like benzene,
is the molecule planar? That's a good question, typically if the molecule
is all carbon, we would look and see if they're SP2 Hybridized as our shortcut and in this
case we have four obvious SP2 but what about the other two carbons? The carbon with the carbocation is SP2 Hybridized,
carbocations like this are SP2 Hybrids because carbon is bound to two carbons, one hydrogen
and it has an empty P Orbital that's where that positive charge sits, the carbocation
is a deficiency with an empty P orbital that makes this carbon SP2. With this last one down here does not have
a charge, not having a charge and not having a pi bond tells us that it's gotta have a
second hydrogen, two bonds to carbon, two bonds to hydrogen tells us it's SP3 Hybridized
which is tetrahedral, this is a 3D carbon, not flat, three dimensional making the molecule
not planar. The second you figure out it’s not planar
you can say it doesn't care about baseball, not an aromatic compound, non-aromatic. But if you weren't sure, you can continue,
you don't have to if you really don't have the answer but just in case. Is this molecule conjugated? Technically yes, we can have resonance between
the pi bonds and into that carbocations empty P orbital but the resonance does not carry
through the entire ring making this anon fully conjugated for aromaticity is gotta be fully
conjugated around the entire ring, once again, not a baseball fan, non-aromatic and we're
done, we don't even have to look at Hackel’s rule. Hackel’s rule only applies if the first
three criteria are met. Here's a fun tricky one that professors love
to put on exams and many students get this wrong, is the molecule cyclic? Well yes it is, I can trace a path from the
first to last carbon, is this molecule planar? It appears to be so, all carbons are pi bound,
is this molecule conjugated? All the carbons have a pi bond that means
they all appear to have the ability to resonate throughout the entire system, yes it appears
conjugated and if we use the trick for Huckel's rule, we have one, two, three, four, five
pi bonds which is an odd number, that means it's team Huckel's rule or team aromaticity,
it appears to be aromatic except for one tiny tiny little detail and that is what's going
on right here. In addition to what we see, there's also an
invisible hydrogen on every carbon atom, when carbon is SP2 hybridized, the ideal bond angle
is 120 degrees and that means for nearly every carbon, the hydrogen atom is facing out. No big deal except for these two, at a hundred
and twenty degrees, the hydrogen should be over and the other hydrogen should be over
here. They appear to want to occupy the same space
which is impossible. Remember that atoms have their personal space
and if two atoms get too close together, it's going to cause strain in the molecule, the
two sides of the molecule are going to push each other away just like if you were talking
to someone and they invaded your personal space, your instinct would be to want to push
them away so you can have a little bit of breathing room. The way this happens in the molecule is that
one of the carbon atoms gets pushed slightly up so the hydrogen points upward, the other
one gets pushed slightly so that the hydrogen points downward which suddenly means that
the molecule is no longer planar. Even though it appears to follow all this
molecule is considered non aromatic, it's a pretend baseball fan but it really doesn't
care about baseball. You can build this with a model kit but to
help you visualize I want you to imagine that this carbon atom is light blue, this carbon
atom is green, and the rest of the ring is blue. If we show the ring from the side we'll have
these four carbon atoms like so and these four carbon atoms would be directly on top
of them because from a side view, this molecule is trying to be flat with the bonds looking
like that, now the light blue carbon atom we said got pushed down, the green carbon
atom got pushed up and this allows their hydrogens to face in a different direction but look
at that, the molecule is not flat, it doesn't sit in one plane because these two carbons
are out of the plane one above and one below. Let's go back to cyclobutadiene which we've
seen in the last video and run through the checklist again. Yes, the molecule is cyclic, yes it's planar,
all SP2 carbon atoms, 2 pi bonds. Yes it's conjugated, we've already seen that
it can resonate up and down around the ring but now let’s look at Huckel's rule from
the perspective of a fan or an anti-fan. We have four pi electrons which is two pi
bonds and even number of electron pairs gives us a fan of the opposite team, of the anti
team making the structure anti aromatic. Now what happens with ions, molecules that
look aromatic but have a + or - charge or heterocyclic compounds that have something
other than carbon in the ring like nitrogen, oxygen, sulfur and so on. How do we determine if they're aromatic, anti-aromatic
or non-aromatic? That's exactly what we'll cover in the next
video, for even more on this topic including the common aromatic compounds cheat sheet
and aromaticity practice quiz, first give this video a thumbs up then visit my website
leah4sci.com/aromaticity