- [Voiceover] Solvation can have a stabilizing effect on a conjugate base. So if we look at water, here's the water molecule, if water donates this proton then our electrons in magenta are left behind on the oxygen, which gives the oxygen a negative one formal charge, and we form the conjugate base to water, which is the hydroxide anion. The hydroxide anion can be
stabilized by our solvent, so if water is our solvent... let me go ahead and draw in
a few water molecules here. So, here is one water molecule. Let me draw another one over here. We know that water is a polar solvent. It's a polar molecule. The oxygen has a partial negative charge, and the hydrogens have
partial positive charges. That's because the oxygen is more electro-negative than hydrogen. And now we have a situation where opposite charges attract, the partially positive hydrogen on water is attracted to the negatively
charged oxygen on hydroxide, so there is an attractive force between these opposite charges, and that attractive force stabilizes or helps to stabilize the negative charge on the hydroxide anion. So over here, we have
another water molecule, so partial negative, partial positive, we have another stabilizing force, another force to help stabilize that negative charge
on the hydroxide anion. So, the more water molecules you have, the more solvent molecules
you have solvating your anion, the more stable the conjugate base. So, if we look at our
four compounds over here, so water, this proton on water has a pKa value of approximately 15.7. For ethanol, this proton
has a pKa value of about 16. For isopropanol, this proton
has a pKa value of 17, and for tert-Butanol, this proton has a pKa value of about 18. The lower the pKa value,
the stronger the acid. So, water is the strongest
acid out of these four because water has the
lowest value for the pKa. If water is the strongest acid, that means the conjugate
base must be the most stable. So the hydroxide anion is the most stable out of our conjugate bases, and that's because we can fit more water molecules
around our hydroxide anion. The hydroxide anion is the
best at being stabilized, the best at being solvated by our solvent, and that's because of something
called steric hindrance. So, attached to this
negatively charged oxygen, we have a really small, little hydrogen. We look at the other conjugate bases. So, this is the ethoxide anion, which is the conjugate base to ethanol, we have a CH2 here,
and then we have a CH3. CH2 and CH3 take up more
space in this hydrogen. As we go down, we see even more stuff, even more steric hindrance. This carbon right here is
attached to a hydrogen. It's also attached to a CH3 and a CH3, so we have even more
stuff to get in the way. And finally, for our last conjugate base, this is called the tert-butoxide anion. We have a carbon attached
to a CH3, a CH3, and a CH3, so even more stuff to get in the way, and that steric hindrance,
those bulky groups prevent the interaction
of solvent molecules with our conjugate base. So, if I draw in a water molecule here, so, there will be some
stabilization, right? We know oxygen is partially negative. We know that hydrogens
are partially positive. So there is an attractive force between our opposite charges, and that would help us
stabilize the conjugate base. The problem is these bulky CH3 groups prevent a lot of water molecules from being able to stabilize
this conjugate base, and that means that
this tert-butoxide anion is the least stable conjugate base. As we move up in this direction, we're decreasing in steric hindrance. We go from three bulky methyl groups to two bulky methyl groups, to one right here, and then finally to this hydrogen. So, we're decreasing in
the steric hindrance, we're increasing in the
ability of our solvent to solvate our anion
and stabilize our anion, and, therefore, the hydroxide anion is the most stable in our solvent. If this anion is the most stable, that means that water is the most likely to donate its proton, and that's why we see the
lowest value for the pKa.