Now, I want to briefly touch upon ester formation.
So, it isn’t just a question that we usually
want to convert alcohols into haloalkanes,
there are a number of different important
steps which proceed via the formation of esters.
Alcohols react with carboxylic acids and carboxylic
acid derivatives, which you will see a little
later on, to give esters as products.
If, for example, you take an alcohol, I’ve
shown here ethyl alcohol or ethanol, it’s
possible to react it with acetic acid or ethanoic
acid to give rise to ethyl ethanoate plus
Note the importance, in this case, of the
arrows between the two reactions. This shows
that the reaction itself isn’t particularly
spontaneous and indeed the reaction between
a carboxylic acid and the alcohol takes place
as a dynamic equilibrium. And often a number
of steps are used in order to facilitate the
formation of more product than starting material
such the removal of water.
The equilibrium, in this particular case,
is forced by the ester formation using an
excess of one reagent, usually the alcohol,
which often acts as the solvent, in this particular
case, and removing the water as it is formed.
Now, let’s have a quick look at other things
you can do with alcohols.
So, alcohol oxidation is very important in
organic synthesis because it affords us carbonyl
groups such as the aldehyde groups shown here
in green. It also ultimately forms the carboxylic
acid group, which is shown at the other end
of this particular diagram, RCOOH.
And this is important not just from a synthetic
perspective because it is this particular
function, this oxidation function, which so
many enzymes in the body are responsible for.
For example, if you’re talking about the
oxidation of alcohol, ethanol, it is first
converted into acetaldehyde by alcohol dehydrogenase.
After that, it is then converted into acetic
acid where it is passed.
These are the synthetic steps that correlate
to what happens from a biological perspective.
That is to say you have the same result, but
you don’t necessarily use a biological system.
So, what are the oxidation steps?
Primary alcohols can be converted either directly
to carboxylic acids in the presence of a strong
oxidising agent. Those strong oxidising agents
are things like potassium dichromate, remember,
of course, the ammonium dichromate from the
lecture on ionic chemistry, and also, potassium
permanganate, shown there as KMnO4.
These will actually bypass the formation of
the carbonyl and go directly to forming a
carboxylic acid. However, that is not always
desirable. In fact, sometimes it’s very
undesirable. And so, therefore, a more delicate
oxidising agent can be used. In this case,
I’ve used the example of pyridinium chlorochromate.
However, there is a huge range of different
oxidising agents and protocols and processes
that can actually be used to selectively oxidise
a primary alcohol up to the corresponding
aldehyde, which is shown in green there.
So, if we look, however, at secondary alcohols,
we’ll see that it’s only actually possible
to oxidise it straight up to a particular
type of carbonyl compound called a ketone.
If you think about what that means, it means,
in fairness, you only actually have two alcohol
groups on either side. So, it’s not possible
to lose two protons in the formation of the
aldehyde. So, you’re left purely with a
ketone. And indeed, if you were to react your
ketone any further, all you’d end up doing
is breaking it apart and combusting it.
And finally, tertiary alcohols, this is where
we have a carbon to which an OH group is bound
with three alcohol substituents, cannot possibly
be oxidised up using potassium permanganate
or potassium dichromate. This, of course,
is because that carbon bears no hydrogens
that it can lose in the process of oxidation.
And so, therefore, this will give you no reaction.
As I said before, it is possible to further
react these tertiary alcohols like the secondary
ones, but would result in combustion products
rather than anything remotely useful.
In acidic conditions, dehydration may happen
instead and this is what we’re going to
come onto in a moment.