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