So, that’s basically for
So, let’s move on to biologically relevant
examples of such benzene or aromatic-containing
compounds. So, natural amino acids are tyrosine
and phenylalanine and also, if you look carefully
tryptophan also contains a benzene ring system.
This is part of an overall indol ring which
is beyond the terms of reference for this
lecture, but you may wish to investigate it
Note how we have monosubstitution in the case
of phenylalanine and 1,4 di-substitution
in the case of the tyrosine ring. Bearing
in mind, however, these are, of course, the
accepted names for these amino acids. The
proper IUPAC name would be a lot more complicated,
but thankfully, when you are either purchasing
these for experiments or when you see these
described, they will always use these names
given for the amino acids.
A little bit more complicated and something
which has considerable biological relevance
is folic acid. This is this large molecule
shown here. Vitamin Bc or pteroyl L-glutamic
acid is the other name and aside from being
essential for the production and maintenance
of new cells, not least because it’s responsible
for the conversion of, for example, uracil
into thiamine, which is very important via
methylation reaction, which is again beyond
the terms, is also a very, very important
molecule when you consider, for example, the
role of sulfa drugs in the treatment of certain
bacterial infections because, indeed, pteroyl
synthase, which is the enzyme responsible
for the conversion of para-aminobenzoic acid
by bacteria into this folic acid, is inhibited
by those sulfa drug. And so, an appreciation
and understanding of what you can make that
mimics a natural substrate is important in
the biological sense.
So, what do we mean by aromatic? The problem
is, it is used by organic chemists in a rather
different way to that which is normally applied.
The origins of it are effectively from folklore
where, for example, cinnamon bark, wintergreen
leaves… wintergreen leaves, vanilla beans
and anise seeds contain these fragrant compounds
that had common, but also unexpected properties.
Such compounds, as we have seen, have a very
low carbon to hydrogen ratio by virtue of
the double bonds. It was, therefore, hypothesised
that they may indeed contain more than one.
Nevertheless, they were much more stable than
the correlating alkenes and did not give some
of the double bond typical reactions.
You may recall when we were talking about
alkenes in one of the previous lectures within
this modules that we talked about the idea
of decolourising bromine water, a way of actually
qualitatively identifying an alkene is effectively
to add some bromine water to it or vice versa
and observe the decolourisation as the addition
reaction takes place to give rise to a halohydrin,
or in the case of just pure bromine, a dibromoalkane
which is colourless.
But, benzene does not give this reaction.
This suggests that the double bonds themselves
are actually not as prone to addition and
therefore, cannot be considered to be as nucleophilic
as conventional alkenes.
One of the first identified aromatic compounds,
the structure of which I have already shown
you, is benzene.