So, how do we know if the compound
we are looking at which has more
than one double bond or two or three is actually
And here we turn to the Huckel rule which
is shown in the middle of the board, 4n+2
pi electrons. So, we have to look at the number
of pi electrons in our polyalkene structure.
Let’s have a look, for example, at benzene,
where we take n to be the ring size and we
see that we have one ring, 4 x 1 is 4 + 2
is 6. Do we have 6 pi electrons? Yes, we do.
So, we can say that this is aromatic.
Let’s apply the same for naphthalene. Two
ring systems. 4 x 2 is 8 + 2 is 10. Do we
have 10 pi electrons? Count the bonds. Yes,
we do. Therefore, this is aromatic and so
on and so forth. And sometimes it’s actually
difficult to see when you are dealing with
a very large molecule. But, as long as within
the ring system you can see that 4n +2 and
they are adjacent to each other, you can usually
be confident that they are aromatic.
There are some other examples of aromaticity
that you see in, for example, a deproteinated
pyran ring. This is beyond the terms of reference
of this lecture, but it’s something that
should be borne in mind that aromaticity is
not just restricted to carbon only containing
Right. Okay. So, characteristics of aromatic
compounds that have a high degree of unsaturation,
we have already said this, they give substitution
rather than addition reactions. They don’t
react with bromine water. They are highly
stable in comparison and they have a planar
Let’s have a look and see what that actually
means. All the four bases contain aromatic
rings of one description or another. And therefore,
all of them are planar. Because of this not
only they do form specific hydrogen bonds
between the pairs that hold those individual
bases together, but they also contribute to
the double helix structure, as you can see
in the rotating diagram on the board.
You can see here how this effectively shows
a Watson-Crick or duplex based pairing
in a stacked planar array. And indeed, what’s
just as important, is this stacking of the
aromatic groups is not just a feature of duplex
DNA, but also of quadruplex DNA. And it’s
worthy of note that quadruplex DNA is proving
highly interesting target in anti-cancer treatment
and you should be looking that out as well.
Let’s have a quick look at what I mean by
the aromaticity of the individual bases. Here
we have guanine bonding via hydrogen bonds
to cytosine, adenine bonding via hydrogen
bonds to thymine. Note what I said before.
You may think, “Well, hang on a second.
It’s got nitrogen groups in it.” But,
the reality is this is still an aromatic.
If you look, for example, at the cytosine,
it is possible for that to tautomerase and
for the double bond to form between a carbon
and nitrogen, thus creating your 6 pi electrons
in a cyclic state. Adenine and guanine are
known as the purine bases. This is because
of that structure as you can see with the
two nitrogens on what appears to be a cyclopentene
ring whereas cytosine and thymine are known
as the pyrimidines because the thing is they
do of a benzene ring with carbons in two places,
one free to each other, have been substituted