Properties of ethers and crown ethers - Chemistry, Class 12 Video Lecture

If we look at the boiling points
of ethanol and dimethyl ether,
we can see there's a large
difference between them.
Ethanol has a much higher
boiling, 78 degrees Celsius.
Whereas dimethyl ether
is negative 25 degrees.
And this explains the state
of matter of these molecules.
Ethanol, since its
boiling point is
higher than room
temperature, is, of course,
a liquid at room
temperature and pressure.
Whereas dimethyl ether, with
a much lower boiling point,
has already turned into a gas.
And so we can explain
the states of matter
by looking at the
intermolecular forces that
are present in these molecules.
So if I think about one
molecule of ethanol,
I know that the bonds
between oxygen and hydrogen
is polarized.
I know that oxygen is
more electronegative.
So it will be
partially negative.
And the hydrogen is
partially positive
as it loses some
electron density.
If that molecule of ethanol
interacts with another molecule
of ethanol, the second molecule
of ethanol is also polarized.
The oxygen is
partially negative.
And the hydrogen is
partially positive.
And we know that
opposite charges attract.
So the partially
positively-charged hydrogen
is attracted to the partially
negatively-charged oxygen
like that.
And there's going to be
attraction between those two
molecules.
And we call this intermolecular
force hydrogen bonding,
the strongest type of
intermolecular force.
So hydrogen bonding is present
between molecules of ethanol.
And this accounts for
its large boiling point.
Let's look at more details
about hydrogen bonding here.
So hydrogen bonding exists
when you have hydrogen bonded
to an electronegative atom
like that to this oxygen.
But students forget
that you also
need another electronegative
atom over here to give you
more of a difference
in charge and to make
that hydrogen more
partially positive.
So it's really three atoms
involved in hydrogen bonding
there.
Let's look at
dimethyl ether and see
why it does not exhibit
hydrogen bonding.
So if I were to draw one
molecule of dimethyl ether
here.
And think about the
polarization between the oxygen
and this carbon right here.
Oxygen is more electronegative.
So it will be
partially negative.
This carbon will be
partially positive like that.
If I think about the
interaction of that molecule
of dimethyl ether with another
molecule of dimethyl ether
like that, you might
be tempted to say,
well there could be
some hydrogen bonding
because I know that
this carbon right here
has some hydrogens
attached to it.
And so some students
will say, oh there
must be hydrogen bonding
between this oxygen down here
and this hydrogen.
But that is not the case
because this hydrogen right
here, while it is
interacting with an oxygen,
this hydrogen is
bonded to a carbon
which is not very
electronegative.
And so there's no
large differences
in electronegativity in the bond
between carbon and hydrogen.
Even the carbon's a little
bit more electronegative.
There's not enough to make
this a true hydrogen bond.
And so really there's
only a small amount
of dipole-dipole interaction
between two molecules
of dimethyl ether.
So somewhere on this
second molecule,
there is a partial
negative, partial positive.
And so there will
be a little bit
of dipole-dipole interaction.
But it's not very strong.
And certainly nowhere near as
strong as the hydrogen bonding
exhibited on the left.
Hydrogen bonding being
just the super strong form
of dipole-dipole interaction.
And so dimethyl
ether does not have
as high of a boiling
point as ethanol.
Again, the answer
is hydrogen bonding.
Let's see what happens to
the boiling point of ethers
as we increase the number of
carbons in the alkyl groups.
So if we're going to look
at that dimethyl ether
again, and let's compare
that to an ether that
has more carbons than the
alkyl group, so diethyl ether.
We've already seen the boiling
point of dimethyl ether
as approximately negative
25 degrees Celsius.
Whereas diethyl ether is
about 35 degrees Celsius.
And so there's a large
difference in boiling points
diethyl ethers boiling point
is just higher than room
temperature.
So it is still a liquid at
room temperature and pressure.
So let's see if we can look at
why diethyl ether has a higher
boiling point.
We know that ether molecules
can't hydrogen bond
with each other.
So that cannot be the
intermolecular force
responsible for this
increase in boiling point.
So if we look at two molecules
of diethyl ether interacting,
one of the other
intermolecular forces
that we discussed was
London dispersion forces.
So London dispersion forces,
you watched earlier video
for more details.
But when you have these
large alkyl groups,
provides more surface
area for a form
of attraction called
London dispersion.
And so that increased
attraction between alkyl groups
means that it's harder to
pull those molecules apart.
It requires more energy to
pull those molecules apart,
requires more heat
in order to do so.
And so that's the reason for
the increase in boiling point
that we see for diethyl ether,
up to 35 degrees Celsius.
And even though London
dispersion forces
are the weakest intermolecular
forces, they're additive.
So the effect is
added when you have
lots and lots of molecules.
And that's the reason
for the large difference
between dimethyl ether
and for diethyl ether.
And so the increase
of the number
of carbons in the alkyl groups
increases the boiling point
just above room
temperature but not much
above room temperature.
So this makes diethyl
ether an excellent solvent
for extraction.
The other thing the
alkyl groups do,
is they increase the nonpolar
part of the molecule.
So it's a little bit
more nonpolar due
to these alkyl groups
right here which
means that diethyl ether
is very good for dissolving
a lot of nonpolar
organic compounds.
And so if you can dissolve a lot
of nonpolar organic compounds
and the boiling point is
just above room temperature,
it's an excellent
solvent for extraction
because you can dissolved are
nonpolar organic molecules.
And then you can just
boil off the ether.
And you're left with
your organic product.
So you'll use diethyl ether
a lot for extractions.
Let's look at
another type of ether
which is a kind of
an interesting one.
And we call these
ethers, crown ethers.
So if we look at that
gigantic either there,
it's called a crown ether.
This was discovered by a guy
named Charles Peterson who
won the Nobel Prize for this.
And the system of
nomenclature for crown ethers
would be to first count
up how many atoms comprise
your a ring here, your crown.
So if we go 1, 2, 3, 4, 5, 6,
7, 8, 9, 10, 11, 12, 13, 14, 15,
16, 17, and 18.
So there are 18
parts of this crown.
So we would write
an 18 right here
like that, followed
by the name crown,
followed by the number
of oxygens in here.
So we have 1 oxygen,
2, 3, 4, 5, and 6.
So the nomenclature would
be, 18-crown-6 ether.
And that just tells you
what sort of crown ether
that you are dealing with.
So why is it called
a crown ether?
Well, the interesting
thing about crown ethers
are that they can interact
with different ions.
For example, the size of
the potassium ion, so K+,
happens to fit right
in the center of this.
So the spacing is just
right for a potassium ion
to fit in there.
And since all of these oxygens
have lone pairs of electrons
on them, so negatively
charged, there's
an attraction between the
positively-charged potassium
ion and the
negatively-charged electrons
or the partially negative
charge oxygen atoms.
So there's attraction.
Opposite charges attract.
And those negative
charges are going
to hold that potassium
ion in here like that.
So it looks like a crown.
If you think about
the potassium ion
as being someone's
head and then that's
wearing this ether crown on
someone's head like that.
And crown ethers have proved
to be very useful, very
useful things.
For example, if you had
some potassium fluoride,
so some K+ F-.
Well, normally
potassium fluoride
would not dissolve in a
nonpolar organic solvents.
But if you use a crown
ether, the oxygens
can take care of the potassium.
And the outside of the
crown either is nonpolar.
So this portion
and this portion,
the outside of
the crown ether is
nonpolar which will dissolve
in an organic solvent,
in a nonpolar organic solvent
like benzene like that.
So like dissolves like.
So this portion would
dissolve in benzene.
And then what that
would do is, that
would free up your
fluoride anion.
That would increase the
nucleophilic strength
of your fluoride
anion which could
participate in an SN2 reaction.
So that's one of the
uses of crown ethers
is to go ahead and take the
cation, leaving the anion
to function as a
better nucleophile
because the potassium ion is
solvated by the crown ether.
And of course, since different
kinds of different sizes,
you can get different
sized crown ethers
to take care of those ions.
So crown ethers I just think
are very interesting molecules.
And if you could look at a
three dimensional representation
of a crown ether,
it's much easier
to see that the outside
is very nonpolar.
So interesting,
interesting molecules.