Leah here from leah4sci.com and in this video we're going to look at the physical properties
of alcohol from hydrogen bonding to solubility and boiling point.
In the last video we introduced the Alcohol Functional Group as an OH with a Polar Covalent
bond having a partial negative on Oxygen and partial positive Hydrogen.
You can it find it along with the entire Alcohol video series, practice quiz, and cheat sheet
by visiting my website leah4sci.com/alcohol.
This polar bond or dipole allows the alcohol to have dipole, dipole interactions.
But even more so, it allows the alcohol group to do hydrogen bonding.
Remember that hydrogen just like you, likes to have fon, spelled F-O-N where a hydrogen
atom bound to fluorine, oxygen, or nitrogen will be so partially positive that it will
be attracted to another fluorine, oxygen, or nitrogen.
In case of the alcohol, we're just looking at the O in fon, but they're oxygen is partially
negative, hydrogen partially positive and will be very strongly attracted to another
alcohol molecule.
If you're asked to draw it out, you can't simply put two alcohols face to face and draw
an interaction between them.
The reason this won't happen is both of the hydrogen atoms are partially positive and
like charges repel but opposite charges attract.
Instead you have to line them up so that the partially negative oxygen is close to the
partially positive hydrogen.
This allows the partially negative Oxygen and partially positive hydrogen on a left
interact with each other and the hydrogen on the right interact with the oxygen on the
right.
What you see here are actually two sets of hydrogen bonds because each of these scenarios
where a partially negative oxygen attracts a partially positive hydrogen is considered
1 hydrogen bond and this is considered a very strong intermolecular force, much stronger
than things like van der waals or London dispersion.
But it's not just hydrogen bonding between the alcohol functional groups that we care
about, it's also the hydrogen bonds between alcohol and water.
For example, if we take methanol which is the smallest alcohol and throw it into a whole
lot of water molecules, what happens?
The Oxygen is partially negative, Hydrogen partially positive, but water has a very similar
structure.
Instead of ROH, water is HOH.
Each of the bonds between hydrogen and oxygen will be polar covalent giving us a partially
negative oxygen and 2 partially positive hydrogen atoms.
Water like the alcohol is capable of hydrogen bonding but more exciting it's capable of
hydrogen bonding with alcohol in solution.
We can show various water molecules lining up around the alcohol group so that the partially
negative oxygen of alcohol always lines up with the partially positive hydrogen and the
partially positive hydrogen always lines up with the partially negative oxygen of water.
If water gets along so well with alcohol, it's not surprising to see how well it can
dissolve within that solution.
Which brings up the next question.
Are alcohol molecules soluble in water?
And are they missable in the water?
Soluble and miscible, sounds like the same thing, but they're really not.
When we say something is soluble on water, we can say that it dissolves in water.
And we're not referring to any specific amount, it's more of a generic term.
When we say something is missable, we're defining the parameters.
We're saying, that it will dissolve at any ratio.
Say I want to create a solution with methanol and water.
To say that methanol is miscible with water, that means it would dissolve if I have a 1
to 10 ratio, a 1 to 50 ratio, or even a 50 to 1 ratio.
It means no matter how much of each I use, they're going to mix together to form a homogeneous
solution.
Why does this matter so much?
We saw that methanol is very happy when dissolved with water.
Because methanol is a tiny molecule that has a methyl group and the OH.
Because water gets along so well with alcohol, we can say that alcohol is hydrophilic.
Hydro meaning water, and philic from phile, meaning loving.
This is the opposite of hydrophobic which is water fearing as we'll see shortly.
When we compare the size, the carbon portion of the molecule is very close in size to the
OH portion of the molecule.
Carbon has nonpolar bonds.
The electronegativity difference between carbon and hydrogen is so close that there is no
polarity on that portion of the molecule.
That means that if water tries to surround this portion of the methanol it doesn't get
along, it doesn't know how to interact.
I want you to imagine that you're hanging out with your friend and you really like your
friend but you really don't like your friend's significant other, so there's gonna be some
tension because you like your friend but then there is that other person that you just can't
get along with.
Now if that person doesn't bother you, if the carbon group is small, it doesn't really
make a difference.
But the bigger the carbon group, the more it starts to matter.
If we look at something like methanol which is grain alcohol or drinking alcohol, it's
still pretty small.
We have a carbon group, a carbon group and and OH group.
And we can estimate that the alcohol is about a third of a molecule.
It's still close enough to a majority.
Water can still play a nice role in helping dissolve , and so ethanol is going to be soluble
in water.
But the more carbons you add to the chain, the more and more that nonpolar portion takes
over, the less and less that OH group matters.
Say we have butanol, the alcohol is now about a fifth of the total molecule, that means
about 80% of this molecule, the four carbon groups are water fearing or hydrophobic.
Now it's not just hanging out with your friends and they're significant other, but your friend's
significant other's three other friends are also there.
Say you have the one person you get along with, the four other people that you don't,
now we're getting into iffy territory.
A small amount of butanol will still be soluble but butanol isn't considered missable with
water.
Because if I start throwing a lot of butanol into water they're going to start separating
because the water is getting crowded out, it's not able to mix with that hydrophobic
tale which brings us to the general solubility rule for Alcohol.
Small alcohols are soluble in water but as you increase the number of carbon atoms on
the alcohol molecule you decrease the solubility of that alcohol in water.
Let's forget about water and look at what happens when you have just alcohol.
More importantly, what happens to the boiling point of this alcohol solution?
Boiling point simply tells us the temperature of which a solution will boil or the temperature
required to go from a liquid to a gas.
But these are just the facts.
If you can understand what happens in boiling point, you'll have a much better understanding
of the trends for alcohol boiling points.
I want you to imagine that we have a solution of pure alcohol.
Every single molecule in here is an alcohol molecule and they're constantly moving around
in solution.
Remember, a liquid has a definitive volume but it's always mobile, it's always moving
around.
What allows it to stay in that container are the small interactions that happen between
the alcohol molecules.
These are your intermolecular forces.
The intermolecular forces include the hydrogen bonding between the alcohol portion of the
molecule and the London Dispersion forces between the carbon portions of the molecule.
Boiling point we said is temperature and temperature is a measure of heat.
What we're really looking at is the energy required to overcome all of these intermolecular
forces to break the interactions between the molecules so that if you have a molecule that
is no longer holding on to the other liquid molecules, there is nothing to keep it in
the container, nothing to keep it in solution and so it floats away having evaporated from
the liquid to the gas phase.
In other words the boiling point is the amount of heat required as measured by temperature
to overcome the intermolecular forces which then allows the molecules to escape as a gas.
Based on this new definition, what would you expect for an alcohol boiling point?
Let's take a look at two isomers of C2H6O.
We can draw it as a two carbon alcohol ethanol, or as dimethyl ether with a methyl group on
either side of an oxygen atom.
They have the same molecular formula, therefore the same molecular weight.
But if you look at their boiling points, they're going to be extremely different.
The boiling point of ethanol is about 78 degrees celsius but the boiling point of dimethyl
ether is about negative 24 degree Celsius.
How can two molecules with the same structure have such a different boiling point?
Well ask yourself, why does this structure requires so much energy to separate the liquid
molecules compared to this one?
And the answer is, the greater intermolecular force is, specifically the hydrogen bonding.
Remember, alcohol can do hydrogen bonding which is such as strong intermolecular force
that it's gonna require so much more energy to break apart.
This is when we're looking at two same size molecules.
But unlike the trend with solubility where increasing the size of the alcohol decrease
solubility, it'll be the exact opposite with boiling point.
Let's compare a few common alcohols.
If I look at methanol which has just one carbon, the boiling point is about 65 degree Celsius.
If I add just one carbon to give me ethanol, the boiling point goes up slightly to 78 degree
Celsius.
If I add 2 more carbon atoms for butanol, the boiling point jumps up even more to about
118 degree celsius, and the question is why?
If the alcohol group is the same, we're not increasing any hydrogen bonding.
What happens with the larger molecules that a solution of long chain alcohol requires
so much more energy to separate?
Simple, we have 2 types of intermolecular sources.
We have to account for the energy to break the hydrogen bonding between alcohol groups
but we also have to account for the London Dispersion forces required to break the carbon
to carbon interactions.
But it's not just the number of carbon atoms that we look at, it's also the way that they're
connected.
If we look at three versions of C4H10O or the isomers of butanol, we get the regular
linear butanol, we have the sec-butyl alcohol where the OH is on the sec or secondary carbon,
and we have the tert-butyl alcohol where the OH is on the tertiary carbon.
The boiling point for butanol is about 118 degrees.
If we add one branch sec-butanol has a boiling point of just a hundred degree celsius.
Another branch for tert-butanol drops it down to 83 degree celsius.
So what happened here?
Didn't we say that increase with the number of carbons increases the boiling point?
Yes, I know.
We said that increasing the intermolecular forces will increase the boiling point because
we require more energy to separate it.
The reason butanol has such a high boiling point is because the molecules are able to
stack up.
They're able to get so close at every atom so every carbon is able to hold on to what
portion of the other molecule.
With the sec-butyl alcohol, having that branch in there, having that OH stand in the way,
we can get as tight stacking so yes, these two can very nicely line up with each other
but I can't stack a third one on there.
If I try to stack a third alcohol, I have to squeeze it into the side and it can't get
as close.
It can't hold on as tightly so I don't need as much energy to break it.
The tert-butyl alcohol has so much more branching that even two molecules have a hard time getting
close enough to each other and the energy to break them apart is going to be so much
less which drops the boiling point down by almost 20 degrees from the sec-butyl alcohol.
Be sure to join me in the next video where we look at the acid base properties of alcohols.
You can find that video along with the entire video series, practice quiz, and cheat sheet
by visiting my website leah4sci.com/alcohol.