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From that long list of pKa values,
we'd like to pull out a systematic series
where we vary only one parameter at a time
so we can see the different effects of hybridization
or atom type
on the stability of the charged species
and how that shows up
in terms of the pKa or the strength of the acid.
Let's begin this process by
looking first at the effect of hybridization.
Let's start with Type 1 acids
where we have cationic protonated forms of the acids,
and we're varying hybridization for the
positively charged nitrogen,
which for the sp case would be the nitrilium ion,
a very strong acid.
The intermediate form,
ah, the sp2 hybridized iminium cation
or the ammonium cation,
sp3 hybridized.
You can see
that the acidity is very strong acids
in the case of the sp hybridized case
to the very weak acid
in the case of the sp3 hybridized ammonium case
and the intermediate pKa value for the sp2 hybridized nitrogen.
Positive charge
being least stable in the sp form
most strongly promotes the dissociation of this proton,
reflecting that high pKa value,
that- that high acidity of the very low, negative pKa value.
In contrast,
the great stability of the sp3 hybridized nitrogen
makes this proton much less reluctant to be dissociating-
dissociated and reflects in this relatively weak acidity
a positive 9.8.
So that's the case for the Type 1 acids.
Let's look at the Type 2 case
where we're going to dissociate
that proton from a neutral species
and make a negative charge.
So we want to compare the charged species,
and in this case, the charged species for the Type 2 acid
would be negative, ah, anions.
We're going to focus on carbon
as the conjugate base of neutral acids.
So we have in the case of acetylene
losing its proton to make the acetylide anion,
a ethylene, ah, vinyl proton being lost to make vinyl anion,
or in the case of ethane,
a proton being lost to make this alkyl anion
that's sp3 hybridized.
We know that the stability of the anion
increases as the- for sp hybridization,
and it's least stable for sp3 hybridization.
And so we see that reflected in the pKa values.
The conjugate acid of this acetylide anion,
a- acetylene loses its proton most easily of the three.
And we can see that at the other end of the spectrum,
making this very unstable sp3 hybridized carbanion
results in a very, ah,
weak acid that is at the very bottom of our pKa scale.
So, again,
if you were to make -
and I encourage you to do so -
a plot of free energy versus species in equilibrium
for these three anions,
putting their neutral conjugate acids at equal level
and varying the stability of the -
putting the positions of the anions at different levels
according to their stability,
I think you'll see why it is that
acetylene is the strongest of the three acids,
and the alkyl anion like ethane is the weakest of the three.
So that's the effects of hybridization.
Now lak- let's take a look at atom type.
First, for Type 1 acids
where the positive charge is on either sulfur,
oxygen,
or nitrogen.
All of these have the same hybridization state,
sp3 hybridized,
and we see that sulfur
which bears, ah, a positive charge
and is the most unstable of these three
gives rise to a very strong acid,
pKa -6.8.
At the other end of the spectrum, nitrogen,
being a second row and least electronegative of- of the, ah,
ele- elements shown here,
is going to be the relatively weak acid of the three.
It's the weakest acid of the three,
reflecting its ability to hold on to this positive charge.
Sulfur being the least stable of the three
wants to give up that proton so it can return
to its neutral form.
Again, make a plot of free energy
versus species in equilibrium,
and you'll see that the
high instability of that sulfonium group
causes it to be the strongest acid of the three.
For anions,
if we're dealing with the Type 2 acids,
we're going to cause
the neutral acid to dissociate into a conjugate base
that's charged.
We'll keep the hybridization state fixed,
but we'll vary the atom type.
The sulfur anion is the most stable.
The carbanion is the least.
And once again,
in their conjugate acids,
their neutral forms,
we see how the thiol group
is the strongest of these four acids.
Whereas, the aliphatic hydrocarbon
is the weakest of these acids.
Oxygen is closer to sulfur.
Nitrogen is closer to carbon.
And this trend here
reflects the hybridiz- ah,
reflects the electronegativity of those atoms.
Whereas, the- the difference between A and B
is reflecting the difference in size of the atoms.
Sulfur being large,
able to stabilize that extra electron
over a much larger volume,
ah, around the, ah, positively charged nucleus.
And so we see that that great stability of the sulfur anion
is what gives rise to an easy loss
of the proton to make that anion.
While in the case of carbon,
losing that proton makes a very unstable anion,
and it's only lost with great difficulty.