Hydrogen bonding can be so confusing, so I want to talk about some common mistakes people
make so you won't ever make them.
Okay, so hydrogen bonds are a type of intermolecular bond.
Intermolecular bonds form between molecules, that's why we call them intermolecular.
Now this "between" part is super important.
Between two different molecules, I'm underlining it twice in red.
Okay, let's look at hydrogen bonds in some ethanol molecules.
So, ethanol's an alcohol and if we could take a drop of ethanol and zoom in zillions and
zillions of times in it this is what we'd see.
We'd see a bunch of different molecules all twisting around to different orientations
and here they are.
So, where are the hydrogen bonds in these molecules?
Well as I said earlier, intermolecular bonds, hydrogen bonds, form between molecules, okay?
So this can't be a hydrogen bond because it's not between molecules, okay?
The CH, that can't be a hydrogen bond either because it's in the same molecule.
It's not between molecules.
In the CC, it can't be a hydrogen bond either because it's in the same molecule, not between
different molecules.
So that's the first important thing to keep in mind; if a bond is in just one molecule
it's not an intermolecular bond and it's certainly not a hydrogen bond.
Okay, so that means that the hydrogen bond has to be between different molecules.
Here's how it works.
In hydrogen bonding, in this case, an H connected to an O is bonded to an O on another molecule.
Okay, so this H is attached to an O and so it will form a hydrogen bond with this O over
here.
I'll indicate this hydrogen bond with a couple red dots so there is a hydrogen bond here.
This H connected to the O here can form a hydrogen bond with this O over here on this
molecule.
So again, a hydrogen bond I'll indicate with a couple of these red dots here.
So there's a hydrogen bond here.
Okay now, a common mistake that some people make is they think anytime there is an H,
it can form a hydrogen bond between molecules.
They say there's an H here connected to the C, there's an H here connected to the C, so
this can form a hydrogen bond.
Right?
No.
It can't.
It has to be an H connected to an O bonding with another O in this case.
So it can't be this H with the C and this H with the C, that's not a hydrogen bond.
Another mistake is taking a H that's connected with the C and connecting it with an O. That's
kind of half right because in this case O's are forming hydrogen bonds but if the H is
connected with a C it cannot form a hydrogen bond either.
It has to be an H connected with an O making a bond with an O on another molecule.
This is wrong, this is wrong, but these two hydrogen bonds between the molecules is the
way to go.
Okay, so now that you know how to find and draw hydrogen bonds,
let's talk about how hydrogen bonds form in the first place.
Here I have two ethanol molecules; they're going to form a hydrogen bond between them.
In this case, it's all about the O and the H. So the O and the H in this molecule are
connected to each other with this line which indicates that it's a covalent bond that's
holding them together.
They're sharing electrons to form this covalent bond so they're stuck together because they're
sharing electrons.
Here's the thing though with the sharing, they are not sharing those electrons equally,
okay?
The oxygen has what we call a higher electronegativity.
All that means is that when it shares with hydrogen, it hogs the electrons.
So it's maybe supposed to be sharing equally but instead it's hogging the electrons.
Because it has all these extra electrons that it's hogging, it has a little bit of a minus
charge because of those electrons that are spending more time with it.
So we can write this minus sign here in this squiggle that's actual a lower case Greek
Delta letter D.
We have this lower case Greek Delta with a negative sign meaning the O is a little bit
negative because it's hogging those electrons.
On the other hand, hydrogen here is getting the electrons hogged away from it.
So it's missing those electrons so it has lower case Delta plus, it's a little bit positive.
Now the same thing is happening over on this molecule.
The oxygen is hogging an electron so it has a Delta minus sign because it has a little
bit of a negative charge; those extra electrons that it's hogging away.
An H that is getting thieved away from here, it has a little bit of a positive charge.
So now you can see what's going on here.
The positive hydrogen here is going to be attracted to the negative oxygen here.
So this attraction, it's sort of like magnets.
You know how magnets attract together?
It's like this attractive force between the positive hydrogen and the negative oxygen.
So we can say that the hydrogen bonds formed because O is hogging electrons from H. It's
making O a little bit negative, it makes H a little positive so this positive H is going
to be attracted to a negative O on another molecule.
Now here's the thing, earlier I said that if an H is bonded to a C it will not form
a hydrogen bond.
Here's why.
It's because C and H share their electrons evenly in this spot.
Because they're sharing them evenly, it's not possible for the C to become negative
and the H to become positive because the electrons spend equal time with both of them.
So you don't get these negative and positive charges when H is bonded to a C since there
are no charges.
H's bonded to C's can't form hydrogen bonds.
So it turns out that O and H aren't the only atoms that can form hydrogen bonds.
We saw just a minute ago that O when it's bonded to H, O hogs those electrons away from
H. It's not the only atom that can do that.
Fluorine also has a high electron negativity which means it likes to hog so fluorine when
it's bonded to an H also has a little bit of a negative charge making the hydrogen get
a little bit of a positive charge.
Nitrogen(II) is able to hog electrons away from hydrogen so it gets a negative charge
getting hydrogen a positive charge.
So there are three atoms that can participate in hydrogen bonding besides hydrogen and they
are F, O, and N. If a hydrogen is bonded to a F, O, or an N that hydrogen can form a hydrogen
bond.
You can think about it because it spells FON, not correctly, but F-O-N.
So here's the thing, these guys all have to attach, they have to be attracted to an atom
on another molecule, okay?
So for example, this positive H here can be attracted to an O on another molecule because
that O will be a little bit negative.
But this O could also bind with an H attached to an F. Or an O because it's a little bit
negative could bond to an H if it's attached to an N. Okay?
So that's possible.
Or you can have an F, a fluorine, bonding with any of these H's if H is attached to
it.
F or an O or an N this slightly negative fluorine will be able to hydrogen bond with a positive
hydrogen.
Similarly, nitrogen(II) when it's on another molecule can form a hydrogen bond with any
of these three combinations; the H to the N, the H connected to the O, or the H connected
to the F. So whenever you see a hydrogen connected to a fluorine, oxygen, or nitrogen that means
that it can form a hydrogen bond.
Any of the hydrogens can form a hydrogen bond with a nitrogen, fluorine, or oxygen on another
molecule.
So that is hydrogen bonding.