Right. Okay. So, let’s talk about aromatic
ring systems. Now, ostensibly they may look
like alkanes; they all contain carbon and
hydrogen. But, if we take a look, for example,
at the first molecule, benzene, C6H6, you
can see that we actually have three double
bonds inside. And what we are going to cover
in this lecture is what that means in terms
of the reactivity of the species and also
some of its other properties.
Benzene derivatives are often shown as the
second drawing in the board slide section
that you can see here where we have an R group
substituting on a benzene ring, where R can
either be alkyl or indeed, it can be X, a
I have also drawn naphthalene and antracene
which contain two aromatic rings and three
aromatic rings respectively. I have only restricted
myself to these three as relatively simple
examples. But, you can increase the number
of rings and then you start using… looking
at large polyaromatic compounds like pi rings.
These aromatic hydrocarbons, as opposed to
the aliphatic ones that we have covered before
,such as cyclohexyl, cyclopentyl groups, are
also known as arenes and are all derivatives
of that single benzene ring system. In general,
aromatic groups can be called aryl groups
and are often abbreviated with the letter
Ar standing for aryl which means benzene or
a related family member.
So, let’s have a quick look at nomenclature
before we actually start looking at the molecular
orbitals and how they distinctly influence
the reactivity of the benzene ring and make
it quite so different from a conventional
alkene or indeed, an alkane. Here we have
a look at the first example which is ethylbenzene.
This is where we have monosubstitution. So,
we have a single substituent on the benzene
ring. In this case, the benzene will go last
and whatever our substituent is will go first.
We have chosen a simple example here where
CH2CH3, if you recall, correlates to ethyl
derived from ethane. This is ethyl benzene,
but equally if it was another carbon in length
would be propylbenzene and so on and so forth.
Indeed, if you have heteroatom on there, so,
for example, a chlorine, it would be chlorobenzene.
That’s relatively straight forward in the
case of monosubstituted benzenes. But, in
biology and also in medicine, these are relatively
rare as drug treatments. More often than not,
so, for example, in the case of paracetamol
or as known in American, Acetaminophen, you
will find a dye substituted system.
And this is where we move on to the nomenclature
associated with the second example compound.
Here, we have a benzene ring in the center,
as you can clearly identify, with a methyl
group in one position and then an amine group,
that is a nitrogen bound two hydrogens in
a position which is two carbons away. This
is known as 3-methyl-aminobenzene. Note in
this particular case, as we saw before, when
you attach a heteroatom to a carbon and when
that heteroatom has a large atomic mass, okay,
bearing in mind, nitrogen has an atomic mass
of 14 whereas carbon only has an atomic mass
of 12, it means that the carbon to which the
heteroatom was attached takes priority.
Therefore, the numbering system for the benzene
ring becomes automatic. It travels in a clockwise
direction from the amine ring. Obviously,
of course, you could flip it on its side and
this may effectively mean that the CH3 group
would actually be in the five position. But,
conventions dictates that you use the shortest
distance to get to the other substituent.
So, in this scenario, we have an aminobenzene
with a methyl group in a what is the 3 position
with respect to the amine and H2. So, this,
therefore, becomes 3-methyl-aminobenzene.
Now, that’s relatively straight forward
when we actually are treating the substituents
as the… as those parts which are attached
to the benzene ring. However, if more substituents
are present in their second example here,
their relative position has to be indicated
by numbers. If we actually look, however,
as benzene as a substituent it is sometimes
referred to as a phenyl group.
This group is sometimes abbreviated Ph, for
phenyl, and this is where the name for the
third compound here, phenylcyclopentane, comes
into place. In this particular case, the core
structure is the cyclopentane and we refer
to the phenyl group being substituted on that
cyclopentane ring, this cycloaliphatic ring
that we see here.