Properties of ethers and crown ethers | Organic chemistry | Khan Academy
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Properties of ethers and crown ethers | Organic chemistry | Khan Academy

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.


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