Alcohol properties | Alcohols, ethers, epoxides, sulfides | Organic chemistry | Khan Academy
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Alcohol properties | Alcohols, ethers, epoxides, sulfides | Organic chemistry | Khan Academy

Let’s think a little bit
about some of the properties of alcohol. So the general formula for an
alcohol we saw is some type of group or chain of carbons bonded
to an oxygen, bonded to a hydrogen. And of course, the oxygen
will have two lone pairs just like that. Let’s compare this to water. So water just looks like this. You have a hydrogen bonded to
an oxygen, bonded to another hydrogen with two lone pairs. Now in the case of water,
the oxygen is much more electronegative than the
hydrogen, so it hogs the electrons towards it. So you have a partial negative
charge at the oxygen end. Then you have partial positive
charges at the hydrogen ends. That’s what allows oxygen to
kind of– or sorry– that’s what allows water to bond to
itself or to have not a ridiculously low
boiling point. So let me show this. Let me copy and paste this. We’ve seen all this before
in regular chemistry. So copy and paste. So let me draw some more
water molecules here. Let me draw another water
molecule here. So you see water because the
oxygen end has a partial negative charge and the hydrogen
ends have partial positive charges, the oxygen of
one water molecule will be attracted to the hydrogen of
another water molecule. And we’ve seen this before. This we call hydrogen bonding. So that right there is
hydrogen bonding. The exact same thing can happen
with alcohols, although alcohols really only have
the partial positive charge on the hydrogen. We don’t know exactly what’s
going on here. We probably have carbons
bonded to the oxygen. And with the carbons, they’re
reasonably electronegative. They’re not going to have their
electrons hogged as much as a hydrogen would. So in the case of an alcohol–
let me draw. Instead of having this R for
radical there, let me make it a little bit more concrete. Let me draw an actual alcohol. So an actual alcohol. Maybe we have methanol. Maybe we have methanol that
would look like that. It has a hydrogen
right over here. Oxygen is much more
electronegative than the hydrogen, so you have a partial
negative charge there. And then you have a partial
positive charge there. So it too, because of these
hydrogen bonds, it will have a reasonable boiling point. It won’t just turn immediately
into the gaseous state. It would actually try to
bond to each other. Let me copy and paste that. So it can also form the
hydrogen bonds. Although they won’t to be quite
as strong as what you see in water. And that’s why something like
methanol actually has a lower boiling point than water. It’s easy to make it boil. It’s easier to make these bonds
break apart because you don’t have as much of the
hydrogen bonding. So this is an example
of hydrogen bonding with methanol. Now because methanol can have
hydrogen bonding and it has this slight polarity to it and
water obviously has hydrogen bonding, methanol is actually
miscible in water. And all that means is that it’s
soluble in water in any proportion. No matter how much methanol
or how much water you have, it is soluble. So if I were to draw some
methanol molecules– actually, maybe this is the water
right here. So if you draw a methanol
molecule right there, that would have a hydrogen bond
right over there. If I were to draw another
methanol molecule maybe right over here, you would have
another hydrogen bond right over there. And that’s what allows
methanol to be soluble in water. Now, as this chain grows, or
if you have alcohols with longer radical chains, then
they become less and less soluble in water. But their boiling points
actually do go up. And let’s think about
why that is. So if I have something like–
let me do butanol. So butanol’s going to
have 4 carbons. So it’s going to be H3C, H3–
let me just draw it like H3C, CH2, Ch2, CH– let me
do it like this. H2C. Then that carbon, that last
carbon right there is going to be bonded to the oxygen. It’s going to be bonded
to an oxygen, which is bonded to a hydrogen. Now, when you have a situation
like this, the oxygen will have a partial negative
charge. The hydrogen will still have
a partial positive charge. Just like we saw up
here with both the water and the methanol. But now you have this big thing here that has no polarity. So this part of the alcohol is
not going to be soluble in water, and it’s going to make it
harder for this part to be soluble over here. So this right here
is less soluble. This is less soluble. It’d still be a little
bit soluble. So if you have some oxygen here,
you will still have a little bit of the hydrogen
bonding. You still will have a little
bit of the hydrogen bonding going on. But this part is kind of– you
can imagine it’s almost– it doesn’t want to dissolve
with the water. It is non-polar. You could actually, for
example, butanol in particular, it actually
is soluble in water. But not in any proportion. So methanol is miscible. Let me write this. This is a new word. I don’t think I’ve ever used
it before in the context of the organic chemistry videos. So methanol is– let me write
that in a brighter color since it’s a new word. Methanol is miscible, which
just means soluble in any proportion. So I don’t care what percent
is methanol, what percent is water. The methanol will dissolve
into the water in any proportion. If you look at butanol, it
is soluble but not in any proportion. If you had a ton of butanol,
some of it would not dissolve in the water. So this is soluble. So the butanol right here
is soluble, but not miscible in water. If you have too much of the
butanol, all of a sudden, some of it will not actually be
able to be dissolved. If this was a decanol or
something with a really long carbon chain, then of
course, it’s going to be very non soluble. You might be able to get a
couple of molecules in the water, but most of them
will not dissolve. Now the other reason– I
hinted– look, you know the reason why the alcohols have a
reasonable– not too low of a boiling point is that
they’re able to do this hydrogen bonding. But you would say well, look. You know, these longer carbon
chains, these are going to have less of the hydrogen
bonding going on. Maybe these would have
lower boiling points. But actually, the longer the
chain gets, these actually have higher boiling points. And that’s because
these chains can interact with each other. So the longer the chain, so
longer R or the longer R chain, I guess, I could say,
we could say the higher the boiling point in an alcohol. Higher boiling point. It’s harder. You have to put more heat into
the system or the temperature has to be higher for the
things to break apart. And that’s because this is one
decanol molecule here, another decanol molecule might
look like this. Maybe it might look like this. You have an oxygen and a
hydrogen and then you have your carbons. So you have your CH,
your CH2, CH2, H3C. So you have this other
butanol here. And what the interaction between
these two chains are– these are the van
der Waal forces. So even though they have
no [INAUDIBLE], so these guys are going to have
some polar interactions. They’re going to have the
hydrogen bonding. We’ve seen that multiple
times already. But these long chains, they’re
going to have the London dispersion forces, which are a
subset of van der Waal forces. Where even though they’re
neutral, every now and then, one of these might become
slightly negative on one side. So you might have
a very temporary partial negative charge. And that’s just because
of the randomness of how electrons move. On this side of the molecule,
all of a sudden, you might have more electrons
over there. So you have a partial
negative charge. And because of that, you’re
going to have– the electrons over here, they’re not going
to want to be there. So you’re going to want to have
a partial positive charge there and you’re going to have
a very temporary interaction. That’s a very weak force. Much weaker than
hydrogen bonds. But as these chains get longer
and longer, as they possibly even get intertwined with each
other and get close to each other, these London dispersion
forces or van der Waal forces are going to keep propagating. So all of a sudden, maybe these
guys are going to be attracted to each other and
that’s going to disappear. Than these guys are going be
attracted to each other and then that’s going
to disappear. And then these are going to be
attracted to each other and then that’s going
to disappear. And so you can imagine, the
longer the chain, the more of these type of interactions
you’re going to have. The more attracted they’re going
to be to each other. And it’s going to be harder to
break them apart, higher boiling point. So those are just kind of the
two big takeaways on the properties of alcohols. Especially smaller chained
alcohols are soluble in water. The very small ones are
completely miscible. And the longer the chain you
have, the harder it is to dissolve in water. But also, the higher
the boiling point. The harder it is to break them
apart because you have these London dispersion forces.

35 thoughts on “Alcohol properties | Alcohols, ethers, epoxides, sulfides | Organic chemistry | Khan Academy

  1. I love these vids; it is great to wake up and get a chem lesson in the morning. It has been 20 yrs since I took organic. fyi – your butenol is missing a hydrogen.

    Question: do alcohols act as acids/bases or affect the pH in any way?

  2. @minoc2

    Most alcohols have a negligible degree of (de)protonation in water, but phenols, OH groups directly bonded to benzene rings, are acidic in water.

    Acids can protonate alcohols, and bases can deprotonate alcohols, so in this sense they do have an effect on the H^+ concentration in water.

  3. London Dispersion forces were something which my teacher in high school took two or more lessons to tell us about and I still didn't understand them, and you explained them in about 30 seconds, and it made perfect sense. School children should just watch your videos instead of going to class!

  4. Please can you upload a video on haloalkanes and haloarenes!
    please sir, it's a humble request! πŸ™‚

  5. Practically not. Alkoxides are stronger bases than hydroxide, so they'll immediately deprotonate a water molecule. Alkyl oxonium ions similarly are extremely strong acids and usually have only fleeting existence before they either deprotonate again or lose a water molecule forming a carbocation which can either combine with water to reform the alkyl oxonium ion or with another alcohol molecule to form a dialkyl oxonium ion which can deprotonated to an ether.

  6. organic chem has been hard for me all this while,but not now.thankyouu.I couldnt say no more.It's really help to figure all this reactions,

  7. Alcohols absolutely participate in acid/base reactions. Your typical non-cyclic alcohol: R-OH has a pKa of 15-18, and water has a pKa of 15.7, which implies a base such as NaOH is not very favorable. Instead a fairly strong base (such as Hydride or Na metal) is needed to deprotonate an alcohol. In general though, When assessing the strength of any acid, one needs to assess the stability of the conjugate base. the most important factors with alcohols are usually resonance, and induction.

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