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We often want to compare the pKa's of two species
or even two acidic protons within the same molecule.
To do this,
we can really break the cases down
into four examples
and classify those examples into one of two types.
In the first type,
we're dealing with a neutral acid
that dissociates into an anionic conjugate base, A-,
and a proton.
The unique charged species in this case
is the anion, A-, and so that's really
the species that's going to be the key
when we're looking at the stability difference
between two neutral acids,
HX and HY.
So what you're looking at here
are two energy diagrams
that illustrate two possible situations.
In the situation on the left, case #1,
the dissociated components, X- and Y-,
are more stable
than the initial neutral acids.
Going downhill in energy
from the starting materials to the products,
we see that from HX to X- is a larger free energy change
than from HY to Y-
and because that energy change is downhill,
we would predict that HX would be more acidic than HY.
Thinking about things in the opposite direction,
we would expect Y- to be more basic than X-
because the energy from Y- to HY is less,
and this is an uphill climb than the uphill climb
from X- back up to HX.
So as a result
the more stable anion, just looking at the energy diagram
is clearly X-,
the strongest acid is HX,
because of the downhill fall from HX to X-,
the weakest acid is HY
but the strongest base is Y-,
because from Y- up to HY is the smaller uphill climb,
and the weakest base would thus be X-.
Now think about the case
where the dissociated species are higher in energy
than the starting neutral compounds.
We place the neutral compounds at the same energy,
because they really are similar in energy
and it's the energy difference between the charged species
that really dictates
why one molecule is more acidic than the other.
So here we see
that in going from the associated form
to the dissociated form
it's HY that has the smaller energy climb,
HX has to climb a larger hill
in order to get to its dissociated forms.
As a result,
we would expect HY to be the strongest acid.
Y is the more stable anion
and that's working in our favor in this case
because it's a smaller uphill climb
to go from HY to Y-
than it is to go from HX to X-.
Thus the weakest acid would be HX
and the strongest base would be X-.
And this is because
the downhill fall -
the exothermic reaction from X- to HX,
is more favorable
than the downhill reaction from Y- to HY.
As a result of that
X- is the strongest base,
Y-, the weakest base.
The other case we can think about
in terms of this picture right here
in which the starting acid is charged
and dissociation occurs to give a neutral conjugate base,
this is what's called the Type II acid,
the last case Type I,
and within the Type II acid manifold
we can have the same two situations
where the dissociated components are either more stable
or less stable
than the associated and in this case charged components.
I won't spend too much time on cases 3 and 4
because the analysis is very similar
to the analysis of cases 1 and 2,
but what you should recognize
is that the analysis here
really hinges on the stability difference
between the charged species.
Understanding why charged species differ in energy
is thus a huge part of understanding
why one molecule is more acidic than another,
or even why one position is more acidic than another.
In the next webcast we'll take a look
at the factors that control the stability of charges.
This way
you'll be able to think about these energy differences
on structural terms
so that you can look at a molecule
and decide whether a proton within it
is likely to be acidic or not.