Physical Properties of Alcohol: Hydrogen Bonding, Solubility and Boiling Point

Leah here from 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 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

22 thoughts on “Physical Properties of Alcohol: Hydrogen Bonding, Solubility and Boiling Point

  1. Thank you, mam! Followed you for almost 1 year and i can't speak much about the improvement in organic chemistry. Mam, one more help is needed on chapter AMINES!

  2. Ms. Leah, I wanted to inform you of my recent progress in Organic 2. Our first exam was this week on NMR, ethers, epoxides, grignards, and cuprates. I own some gratitude to you for helping me study. You were the enzymatic catalyst that led to my victory with a score of 98%, well above the curve average of 70%. You are an amazing person. Sincerely, Jonathan.

  3. As an organic chemist myself, this is a good video, but a warning to the viewers: Melting and boiling points are not always correlated! In the case of the isomers of butanol, n-butanol melts at -89C, sec-butanol -115C, isobutanol -108C. You'd think that t-butanol would have the lowest melting point of all based on this trend, but it's actually +24C.
    A general video on this topic might be a good idea?

  4. Your video on 'chair '& 'ring flip' conformations of cyclohexane was outstanding . It really makes Chemistry easy to understand using 3 -D models.

  5. At 2:12 The partial charge on hydrogen should have positive charge and that on oxygen should have partial negative charge….. Its a correction… 😊😊

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