Physical properties of aldehydes and ketones | Organic chemistry | Khan Academy
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Physical properties of aldehydes and ketones | Organic chemistry | Khan Academy

Voiceover: Before we get
into the physical properties of aldehydes and ketones,
I just wanted to cover where the names for those
functional groups come from. So, one way to make aldehydes and ketones is to oxidize alcohol. So if we start over here on the left and we have methanol,
we can oxidize that to methonal over here on the right. Also called formaldehyde. And if we analyze the atoms here, one carbon on the left and
one carbon on the right, one oxygen on the left and
one oxygen on the right, four hydrogens on the left
and only two on the right, so a loss of two hydrogens
can convert methonol to methonal, and so the name of aldehyde comes from these words here. So if I write alcohol
and then dehydrogenatum, which refers to the fact
that we are losing hydrogens. If you look closely you can
see the name for aldehyde. If you take the name al from alcohol, and then this portion of this word, and then add an e on, you get the name aldehyde. So that’s the idea. You can also make ketones. So if I oxidize this alcohol
on the left to propanol, also called isopropanol
or isopropyl alcohol and then finally rubbing alcohol. If you oxidize this molecule, then you get this molecule
over here on the right. So there are three carbons … So a three carbon ketone
is called a propanone and of course no one usually
calls this propanone. This is a famous molecule. This is acetone. And the old German word for acetone … If you spell out the old
German word for acetone, it’s easy to see where the
word ketone comes from, right? ‘Cause if I take this
right here and add an e on, I get ketone. So just a little bit of
insight into those names which I think is pretty interesting. In terms of physical properties, let’s use these last two molecules here to describe boiling points
of aldehydes and ketones. Let’s take two propanol
over here on the left, and let’s compare the boiling point of of two propanol to acetone. So when you are talking
about boiling point, you need to think about
intermolecular forces, so the forces between molecules. So let’s draw out two
molecules of isopropanol here. Let’s go ahead and draw
one, so we have our oxygen, we have our hydrogen right here. Now we know that oxygen
is more electronegative than hydrogen, so the
electrons in this bond are going to be pulled
closer to the oxygen giving the oxygen a
partial negative charge and giving this hydrogen
a partial positive charge. If another molecule of
isopropanol comes along, let’s go ahead and show that, it has the same situation, right? The oxygen is partially negative and the hydrogen is partially positive. We know that opposite charges attract. Right, so this partial positive charge is attracted to this
partial negative charge, and this intermolecular force
is called hydrogen bonding. So this is an example of hydrogen bonding, which we know is between hydrogen and a very electronegative atom like fluorine, oxygen, or nitrogen, and also this hydrogen has to be bonded to another electronegative atom, so here we have oxygen. So this is an example of hydrogen bonding. The strongest type of
intermolecular force. It takes a lot of energy to
pull these molecules apart, so it takes a lot of heat. And so the boiling point of
isopropanol is relatively high. The boiling point is
approximately 83 degrees Celsius. So let’s compare that
situation with acetone. So let’s go ahead and
draw out acetone here. And so here is one molecule of acetone. If we think about oxygen compared to this carbonyl carbon here, oxygen
is more electronegative, and so there is going to
be a polarization, right? So the oxygen is going to
withdraw electron density making the oxygen partially negative. It is taking electron density
away from this carbon, so this carbonyl carbon
is partially positive and so we have a dipole situation. So this molecule has a dipole moment. And if we think about
another molecule of acetone, right so another one has the
exact same situation, right? The oxygen is partially negative, this carbonyl carbon
is partially positive, and so we have an attraction between this partially negative oxygen and this partially positive carbon. So there is an attraction
between these two dipoles. So we call this dipole-dipole interaction, which is another type
of intermolecular force. Actually hydrogen bonding
is just an example of a very strong
dipole-dipole interaction. So dipole-dipole interactions
are not as strong as hydrogen bonding,
so molecules of acetone aren’t attracted to each other as much as molecules of isopropanol,
so it doesn’t take as much energy to pull
apart molecules of acetone, and therefore the boiling point is lower. The boiling point of acetone is approximately 56 degrees Celsius. Both of these temperatures
are above room temperature, but both of these boiling points
are above room temperature so at room temperature and pressure, two propanol and acetone are both liquids. Let’s look at some other molecules and let’s compare them here. So we have all molecules
with three carbons. So over here on the left, this is propane. And the boiling point for
propane is approximately negative 42 degrees Celsius, so that’s well below room temperature. Room temperature is approximately between 20 and 25 degrees Celsius, and so since the boiling point for propane is well below room temp, the
propane is already a gas. So this state of matter
of propane is a gas here. It terms of intermolecular forces, the only intermolecular
forces holding together alkanes are London dispersion forces. So let’s go ahead and write that up here. Next, let’s analyze an aldehyde. Right so a three carbon aldehyde, one, two, three, so this must be proponal. The boiling point for proponal is approximately 50 degrees Celsius. Once again, higher than room temperature, so proponal is a liquid. We have just analyzed acetone. Our next boiling point is
approximately 56 degrees, and for both proponal and for acetone, you have the dipole-dipole
interaction between molecules. So we already covered acetone. The same situation
exists for this aldehyde. So we have a partial negative here and a partial positive right here, and so there is going to be
dipole-dipole interaction between molecules of proponal. So we have once dispersion for our alkane, and then for our aldehyde and ketone we have dipole-dipole interaction. And then finally we have another alcohol. So instead of two propanol,
this is one propanol, which has a boiling point
of approximately 97 degrees. And one proponal also has
of course hydrogen bonding. So we can see that the
boiling points reflect the type of intermolecular force. Hydrogen bonding is stronger
than dipole-dipole interaction, and so therefore the
boiling points for alcohols are higher than the boiling
points for aldehydes or ketones, but aldehydes and ketones
have a higher boiling point than alkanes because
dipole-dipole interactions are stronger than London
dispersion forces. So let’s look at solubility next. I just did boiling point, now lets think about solubility in water. So let’s go ahead and write that. And once again, let’s think
about acetone as our example. And so if we draw this
out, here’s one molecule of acetone and I can go
ahead and put my lone pairs of electrons in there on my oxygen, once again, the oxygen gets
a partial negative charge so the oxygen withdraws
some electron density so it gets a little more negative and this carbonyl carbon gets
a little bit positive, and so we have this polarized situation in our acetone molecule. The thing about solubility and water … I’ll go ahead and draw the
dot structure for water. We know that water is also polarized here, so these electrons and this bond are pulled closer to the oxygen. So these electrons are
pulled closer to the oxygen giving the oxygen a partial negative, and giving this hydrogen
a partial positive. So we can see there is going
to be an attractive force between this partial negative
and this partial positive. In terms of intermolecular forces, we should recognize that
as hydrogen bonding. So this is hydrogen bonding right here. Because of that, we know that acetone is going to be soluble in water. So we have hydrogen bonding. Now one quick point that
I have forgot to mention in the previous example. Some people are confused
as to why molecules of acetone can’t hydrogen
bond with themselves, so let’s go back up here and look at those two molecules again. So if I think about the possibility of hydrogen bonding here, there is a hydrogen
connected to this carbon, but that’s the point. This hydrogen is connected to a carbon. It is not connected to
something like oxygen, which is what we had over here. So hydrogen bonding between
molecules of acetone is not possible because
the hydrogen is bonded to a carbon and not to
something like an oxygen. So even though hydrogen bonding between molecules of acetone is not possible, hydrogen bonding between
acetone and water is possible, and so acetone is going
to be soluble in water. So same idea for other
small aldehydes and ketones. Small aldehydes are
ketones are going to be relatively soluble in water. However, as you increase the chain length, so if you think about the alkyl groups attached to either a
ketone or an aldehyde, so let’s just look at
the alkyl groups here. As you increase the number of carbons that are bonded to an aldehyde or ketone, that increases the non-polar
character of the molecule. So as you increase the chain length, you make the molecule more non-polar, and therefore you are going to decrease the solubility in water.


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