to the base 10 of Ka, we obtain pKa.
If you go back to what we were talking about
in terms of bond polarisation, this has an
effect of in terms of the delta positive or
delta negative, dipolar or charge separation
we observe in a particular bond. The acidity
of a carboxylic acid can be increased if electron
withdrawing groups are attached to it or decreased,
if electron donating groups are attached to
So, let’s have a quick look at the pKa for
some of these commonly occurring carboxylic
acids. Pay particular attention to not just
the nature of the substituent, but also its
location on the alkanoic acid chain itself.
Let us take, for example, the pKa in the top
set on the board where in this scenario we
are looking at a 1, 2, 3, 4 butanoic acid,
okay, derivative, count your carbons.
So, in the case of the unadorned butanoic
acid derivative, we have a pKa there of 4.8.
If we add an electron withdrawing chlorine
group to the alpha position, we decrease the
pH… pKa to 2, rendering this carboxylic
acid, this 2 chlorobutanoic acid far more
acidic than the starting unadorned carboxylic
acid. If, on the other hand, we take that
chloro group and we move it to the beta position
or the 3 position on this butanoic acid chain,
we see that it has a substantially less pronounced
effect on the acidity of this carboxylic acid.
And finally, if we move the electron withdrawing
group all the way back to the 4 position on
the butanoic acid, we see that it has almost
a marginal effect on the acidity of this carboxylic
So, it’s not just the nature of the substituent,
but also its position which is important when
you are trying to modify the acidity of a
carboxylic acid. Let us take, for example,
however, the difference between the groups.
I have used halogens, just for the case of
ease, but there are a number of other electron
withdrawing or electron donating groups.
Right. Okay. So, let’s have a look at methanoic
acid which is shown in the bottom left of
the board. This has a pKa of 3.75. If we compare
that to acetic acid, shown there with a pKa
of 4, we can see that it’s the actual
electron donating effect of the CH3 group
on the ethanoic acid which actually decreases
the stability of the carboxylate. In the earlier
equilibrium we showed the formation of the
hydronium ion and the carboxylate, thus methanoic
acid is actually more acidic, is a stronger
acid than ethanoic acid.
Again, we talked earlier a little bit about
the positioning of the different electron
withdrawing groups, but I just want to touch
upon the different types of electron withdrawing
group as shown here in the bottom right of
the board. We have substituted in the 2 or
the beta position… alpha position - fluorine,
chlorine and bromine. As you can see fluorine,
which is substantially more electronegative
than either chlorine or bromine, results in
a stronger carboxylic acid in terms of its
acidity. Chlorine is the next one along in
terms of acidity and finally, the bromine
substituted carboxylic acid is the weakest
acid out of the group.
So, hopefully, you have been paying attention.
So, now, what that means is you should be
able to put the following structures in the
correct order of decreasing acidity starting
with the strongest acid and moving down to
the weakest acid. I will give you a couple
of minutes to attempt that, after which time,
I will provide the answers.
Right. Okay. So, preparation of carboxylic
As you’ll see a lot of the organic chemistry
that we have covered is largely intertwined
and all of the functional groups are related
to one another or are interconvertible in
between each other in one way, shape or form,
but it is actually important to actually separate
these out into core functional groups for
the sake of clarity amongst other things.
But, the reality is that if we take an alcohol,
primary alcohol or an aldehyde, if we oxidise
it up with potassium permanganate, shown at
the top, or potassium… sorry, potassium
dichromate, shown at the top, or potassium
permanganate, shown at the bottom, we can
generate a carboxylic acid. In fact, in exhaustive
oxidation, a carboxylic acid is usually what
is generated any way. So, they are all relatively
easy to form.
Carboxylic acid derivatives.
This is where the real synthetic interest
lies. Carboxylic acids, most acidic, are not
particularly in of themselves very reactive,
as we all find out. The general formula for
a carboxylic acid derivative is given here
and comprises a number of different possible
compounds. Note, where we have an R, which
can be aliphatic or an aryl group such as
a benzene ring attached directly to a carbonyl,
double bound to the oxygen and then with a
group, denoted Z.
We touched upon this in the previous lecture,
where Z can either be an alkoxy group, an
amine group or it can be a carboxyl group
or it can be a halide group such as chloride
or bromide. Note also, what I said before
in the previous lecture slide about the fact
that this polarisability or polarisation of
that carbonyl double bond. So, you have a
delta positive charge on the carbon, a delta
negative charge on the oxygen is one of the
things that actually adds to the reactivity
of these carboxylic acid derivatives as we