for today's practice problem I've drawn seven molecules on the board and we want to
identify which ones are aromatic so if this is confusing go ahead and check out
my tutorial on aromaticity and see if you can figure out which ones are
aromatic
ok so when we're thinking about aromaticity let's recall that there are
three criteria that must be satisfied for a molecule to be aromatic. number one:
it must be completely planar. number two: must be fully conjugated, the pi electrons
have to be fully conjugated around the entire ring or set of rings and lastly it
must satisfy Huckel's rule, that is there must be a number of pi electrons in
conjugation that are equal to 4n + 2, where n can be any integer so
if n is 0 you get 2, so 2 is an acceptable number. if n is 1, you get 6, so 6 is an
acceptable number, and so forth. so let's look at all of these, now this is
completely planar and this is fully conjugated meaning we can draw resonance
structures that push these pi bonds all around the ring so there is delocalized
pi electron density all along the perimeter of that molecule so those two
criteria are satisfied. now the third one, Huckel's rule, let's count of the number of pi electrons
so each pi bond or actually any covalent bond we know has two electrons
so in terms of the pi bonds we've got 2, 4, 6, 8, 10 so we've got 10 pi electrons, now
we need to know if that satisfies 4n + 2, well if n is 2
(4 x 2) + 2 = 10 so that means that this one is good to go, 10 is
a number of electrons that satisfies Huckel's rule so this will be an aromatic
compound. so let's go to the next one here, now this is fully planar, those
are all sp2 carbons, however we are not fully conjugated inside the ring
remember that the pi electrons all have to be inside the ring, so the fact that there
are two pi electrons outside of the ring, it doesn't matter that six pi electrons
does satisfy Huckel's rule because these are not all part of the ring system so
it is disqualified, we don't even have to look at Huckel's rule, it is not
going to count as aromatic. ok so let's look at the next one, here once again
this is planar, those are all sp2 carbons and it is fully conjugated
because the presence, if this carbocation was not there then it would not
be fully conjugated, because we'd have an sp3 carbon where the conjugation
doesn't get to, but because of this carbocation, we can push these double
bonds and consequently that formal positive charge all around, so we do have
a delocalized positive charge all the way around the ring so it is fully
conjugated, however let's look at Huckel's rule, we've got 2, 4, we've got 4
pi electrons, and 4 does not satisfy Huckel's rule because the allowed
values are going to be 2, 6, 10, 14 etc. we add multiples of four like that so
the fact that we have 4 pi electrons, that does not satisfy Huckel's rule so
even though it's fully planar and fully conjugated it's not gonna be aromatic
it's actually going to be antiaromatic let's even write that down
antiaromatic is a molecule that is fully planar, is fully
conjugated, but has a number of pi electrons that does not satisfy Huckel's
rule so 4, 8, 12, 16 etc.
those are going to be antiaromatic. ok let's look at this one over here, now once
again this is fully planar and fully conjugated, these are all sp2 carbons
and these pi bonds do resonate around the entire ring however when we add them up
2, 4, 6, 8, that is 8 pi electrons, that does not satisfy Huckel's rule so once again
that is going to be antiaromatic, this one does not work
ok looking at this one now this is also fully planar and fully conjugated because
we can push these pi electrons all the way around this ring system but again
2, 4, 6, 8 that's 8 pi electrons, that is not gonna
be aromatic because it does not satisfy Huckel's rule. ok let's look at these
last two now the interesting thing about heterocycles, a heterocycle being a
cyclic compound that has some element other than carbon within the ring system
itself, these atoms have lone pairs which are also pi electrons, lone pairs are pi electrons
and they can either participate in the conjugation if they need to do so in
order to generate aromaticity or they can not participate if by not
participating the ring will be aromatic so in other words we can look at
something like this and we realize there's some lone pairs that may or may
not be incorporated in the ring system in terms of conjugation and right
here we're understanding that if this lone pair, if one of the lone pairs on
the sulfur atom is participating then we can have pi electron density here push that
one there, push that one there, and then we do in fact have a fully planar
fully conjugated pi system and so if we have 2, 4, 6, if we have those six
participating but not these and not these then that is going to satisfy
Huckel's rule and that is going to be aromatic. that's the interesting thing
about heterocycles, the lone pairs may or may not participate in conjugation if
they need to, or must not in order for aromaticity to be the case.
looking at this last one we've got something that's fully planar, fully
conjugated, we can push these pi bonds all the way around, in this case the
nitrogen lone pair is not needed because we have 2, 4, 6, 8, 10 pi electrons even without
that lone pair, that does satisfy Huckel's rule so there we go that one is
aromatic. so we've got three aromatic, four that are not and we must recall that we
need the ring system to be fully planar, fully conjugated, and satisfy Huckel's
rule in order to be aromatic
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