Alcohol Properties Alcohol Properties
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- 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
- 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.
- 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
- So I don't care what percent is methanol,
- what percent is water.
- The methanol will dissolve into the water in any
- If you look at butanol, it is soluble but not in any
- 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.
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