So right here I have two potential solvents
that you might see some type of reaction take place in.
The one on the right is a very typical one, probably
the most common of all solvents.
This is water.
And this right over here is diethyl ether.
And we'll talk about naming things
like this in the future dates, but you
can see what's going on right over here.
You have two ethyl groups attached
to this oxygen between diethyl ether.
But the important point here is to realize
where are these two solvents similar?
Where are these two molecules similar
and where are they different?
The places that they are similar is in both cases oxygen,
which is one of the most electronegative of all
of the atoms, that in both cases,
it's bonded to things that are less electronegative.
And so that allows oxygen to hog the electrons more often.
It has a higher electron affinity.
And so you'll have a partial negative charge
for both of these molecules on the oxygen side.
And you'd have a partial positive side
from the thing that's getting to spend
less time with the electrons.
So both of these we could call them polar solvents.
And a general idea throughout chemistry
is that polar solvents are better at dissolving ions.
Because a polar solvent you have kind
of positive negative charges, things are flipping around,
they're partially attracted to the other positive ends
of the other molecule.
You could imagine that it's very easy for an ion,
for something that is positively or negatively charged here,
to kind of get mixed in to the fray.
Now the difference between these two
is obviously what the oxygen is attached to.
Here the oxygen is attached to some carbons.
Here it is attached to hydrogens.
And oxygen is far more electronegative
than the hydrogens are.
It's a much bigger differential than the case
between oxygen and carbon.
So in this situation you will see scenarios
where it is possible where in a mixture--
remember these things get really, really messy.
There's a lot of just random bumping that's
going on over here-- that oxygen is so electronegative that it
can often times take this pair that's
in this bond right over there.
And if it did that you would be left with the oxygen now having
a negative charge.
Before this-- let's see if I can draw
all of its valence electrons.
It was one, two, three, four, five, six.
So now it's going to have one, two, three,
four-- actually maybe I can draw them a little bit more
carefully.
So you still have, you could say half of this bond to hydrogen.
Now you have these two pairs.
So one, two, three, four.
And now it gains another pair.
It got both of these electrons.
It was already kind of sharing half of this bond
but now it has the whole bond.
It has a negative charge.
And then you have the hydrogen.
Well if you strip it of its electron,
hydrogen is just a proton floating around.
So let me draw that.
So this hydrogen is now just a proton floating around.
So when you have water, water is actually
called a protic solvent.
And then maybe another water molecule
might pick this up and take it someplace
forming a hydronium cation.
But this right over here, water is protic.
Which means that you might have these protons floating around,
these hydrogen protons.
While diethyl ether is not protic.
Even in the case that somehow one of the bonds--
you don't just have little hydrogen protons
floating around in this.
And so what I want to now think about,
now that we know about polar solvents, why they're polar.
But we also know examples of protic and nonprotic solvents.
Let's think about which might be better
when we're talking about the types of reactions
we're studying.
So right over here you might recognize-- in fact,
it's a good exercise.
Pause this video and tell me these top three
steps, what type of reaction is this,
and these bottom two what type of reaction is this.
Well, in this top one we see that this leaving group leaves
first, and that this is the rate determining step.
And then a nucleophile comes in here
and attacks the carbocation and attaches.
So this is a substitution reaction,
but the rate that with a nucleophile, but the rate
determining step involves only one of the reactants.
The leaving group's got to leave.
So this right over here is an SN1 reaction, only involving
one of the reactants.
The leaving group leaving is the rate determining step.
This, on the other hand, both the nucleophile attacks
and the leaving group leaves in one step.
So this is a substitution involving a nucleophile,
but both reactants are involved in the rate determining step.
Now since both SN1 and SN2 reactions
need these ions floating around you
can imagine that they both benefit from polar solvents.
So polar solvents are good for both.
Now let's think about whether it's
good to have protic or nonprotic environments
for either one of these reactions.
Well to think about that a little bit more
let's think about the types of nucleophiles
and the types of molecules we're dealing with.
SN2 reactions, in order for this to happen you
have to have a reasonably strong nucleophile.
And the SN1 reaction, well you already
have the carbocation set up here,
the leaving group leaving was actually the hard part.
So this could even work with a weak nucleophile.
The real thing you have to do is to get this leaving group
to leave.
And also if we want to remind ourselves
of some of the other kind of the stereochemistry
of the reactions, in an SN1 reaction,
it's actually nice to have a tertiary carbon.
If these are also carbons, these little circles I drew here,
that helps stabilize the carbocation.
So if this was a tertiary carbon this would favor SN1.
We also know that tertiary carbon would not be good.
If this is a tertiary carbon that would not be good for SN2.
Because if these were carbon chains of some
form-- these three, this purple, this blue,
and this orange thing-- then they
would have steric hindrance.
The nucleophile would have trouble getting in.
So this would be more likely to happen
if you're dealing with a methyl carbon or a primary carbon,
maybe a secondary carbon.
So here the nucleophile is reasonably strong.
Here it can even work with a weak nucleophile.
Polar solvent in either case.
And the SN1 the key is stabilizing the leaving group,
making it OK for the leaving group to leave.
So what's going to help a leaving group to leave?
Especially when a leaving group leaves
it typically has a negative charge.
Well if there were a bunch of protons flying around that
could help stabilize this negative charge,
that would help.
In fact, this could then form a bond with one of those protons.
It could donate electrons to one of those protons.
And so a protic environment is good for the SN1.
While on the SN2 since you have these strong nucleophiles,
if you had protons flying around, instead of trying
to attack the substrate right over here,
the nucleophile would attack the protons.
So you don't want that, then this thing won't happen.
So you want nonprotic for the SN2.
So hopefully that gives a little bit
of the basics of the solvents and also
allows us to compare these two reactions in interesting ways.