- The shape of the signal
can also be important, especially when you're
talking about alcohols. So here, I have a generic alcohol. And we're concerned about the bond between the oxygen and the hydrogen. We know oxygen is more electronegative, so this oxygen here
gets a partial negative, and this hydrogen here
gets a partial positive. Same thing for this alcohol over here. That sets up the opportunity
for hydrogen bonding. So this partially positive
hydrogen can be attracted to this partially negative oxygen here. And this attractive
force-- if this hydrogen is being attracted, that's going to weaken this oxygen-hydrogen bond. So hydrogen bonding weakens
the oxygen-hydrogen bond, and we know if we're weakening
the strength of a bond, that's like decreasing the force constant, or decreasing the spring constant. We saw in an earlier
video, if you decrease k, you're going to decrease the frequency, or decrease the wavenumber, and so the signal is going to
change on your IR spectrum. At any moment in time,
different alcohol molecules are going to have different
amounts of hydrogen bonding. Some molecules might have a
little bit of hydrogen bonding, so k decreases a little bit, and the wavenumber decreases a little bit. But other molecules might have
a lot of hydrogen bonding, and so we can decrease k even more, therefore we are going to
decrease the wavenumber even more. You get a range of wavenumbers, and since you get a range of
wavenumbers for the OH bond, when hydrogen bonding is present, you get a very broad
signal on your IR spectrum. So, if we go over here in this region, so we're talking about the
IR spectrum for 1-hexanol, this is the region for bonds to hydrogen. So we draw a line at 3000, and
we know that just below 3000, we're talking about a
carbon-hydrogen bond stretch, where the carbon is sp3-hybridized. But, this over here, this
very broad signal right here, this is due to the OH. So let me go ahead and highlight that. This bond right here,
this oxygen-hydrogen bond, gives us a very broad
signal on our IR spectrum because of hydrogen bonding. So we get this very broad signal because of the different wavenumbers. And usually you're going to see this somewhere around 3500 to 2900. So if I find this is 31, 32, 33, 34, 35... so usually in this range, maybe
even a little bit higher than that, you're going to find
this very broad signal. In this case, the oxygen-hydrogen bond. And so you know immediately
to think about the possibility of an alcohol functional
group in your molecule. Also, we can draw a line at 1500 here, and this signal actually, so somewhere around 1100 wavenumbers, this is actually the
carbon-oxygen single bond. Let me go ahead and highlight that. So we have a carbon-oxygen single bond, and this is the single
bond region in here. And that's where-- that's
this stretch right here. Not always going to be
super useful to you, but it's just thinking about what we talked about in the earlier video, I think we calculated the
approximate wavenumber for a carbon-oxygen single bond. So that's what the typical
spectrum for an alcohol is going to look like. Look for that broad signal there. Alright, let's compare this
alcohol to another one here. So, this molecule is butylated
hydroxytoluene, or BHT, and I drew two BHT molecules
in there for a reason. Let's think about why. So, you might think at first, "OK, I have another opportunity for hydrogen bonding." So, here's an opportunity
for hydrogen bonding, so we're going to get a broad
signal for this OH bond. So I'm going to highlight it here. I might expect, since I
have hydrogen bonding, that weakens this OH bond. So I might get another
broad signal for my OH. But, in this case, we have
so much steric hindrance from these tert-butyl groups, so there's tons of steric hindrance here. And then we have these
big ones over here too, and that's going to prevent the
hydrogen bonding from taking place. And so, because of steric hindrance, these molecules can't get
close enough to each other for hydrogen bonding to occur. So, we don't get any hydrogen bonding. If we go over here to
the IR spectrum for BHT, as usual, it helps to draw
a line around 3000 or so, and then, we don't see this broad signal. So this broad signal up
here is missing down here. But what we do have is a sharp signal. So let me go ahead and
highlight that here. So we have a sharp signal
right about-- let's drop down and see where we are for wavenumbers, so this would be 31, 32, 33, 34, 35, 36. And so somewhere around 3600,
we see this sharp signal, and this is that OH bond. So this is that OH bond, and
we don't see a broad signal because we don't have hydrogen
bonding to worry about. And so we don't see that broad shape. We see a sharp signal for the OH. So this allows you to think about where you would find this signal here. So if you have an oxygen-hydrogen bond with no hydrogen bonding, you expect to see it around 3600. If you have an
oxygen-hydrogen bond stretch, and there is hydrogen bonding, look for this broad signal
here over this large area. Alright, let's do one more molecule where we're thinking
about hydrogen bonding. And, this time we're talking
about a carboxylic acid. So here's our carboxylic acid over here. And let's analyze the IR spectrum
for this carboxylic acid. So we see this OH here,
so we think to ourselves, "Ah, hydrogen bonding can occur." So, where would that signal be? Before, I said it would be
somewhere around 3500 to 2900, somewhere in that range, so if we look, we see an
ever broader signal here, even broader than the range
we talked about before. And that's because carboxylic
acids have more hydrogen bonding. So, if I go down here, let me show you-- here's some hydrogen bonding
for a carboxylic acid. So, we have opportunity
for a hydrogen bond here, and we have an opportunity for hydrogen bonding here as well. So, a large amount of hydrogen bonding makes the signal even broader, when you're talking about
the OH on a carboxylic acid. So this very broad signal is talking about this
oxygen-hydrogen bond stretch. And once again, if we draw a line at 3000, so if we draw a line right here, we can see this little
signal right in here, and that's actually the
carbon-hydrogen stretch. With the carbon, we're talking
about an sp3-hybridized carbon here. So, the broad signal is
often centred around 3000, so that partially obscures that
carbon-hydrogen bond stretch that we've talked about before. So, there's another
hint that you're talking about a carboxylic acid. But of course, the biggest
hint is when you also see the very strong signal for the carbonyl, occurring somewhere around 1700. So, let's go ahead and identify that. So, we draw a line right here, and then in the double bond region, we see this really
intense signal right here, approximately around 1700. So, usually, I think it's a
tiny bit higher than that. But, this is due to our
carbon-oxygen double bond stretch. So, we're talking about our carbonyl here. So if you see this really broad signal, which tells you OH, and then this really strong signal, which tells you carbonyl, put an OH and a carbonyl together, and you get a carboxylic acid. And so it's pretty easy to
identify the carboxylic acid using an IR spectrum here. And just a quick note
about hydrogen bonding, we talked about hydrogen bonding weakening a carbon-oxygen bond above, a similar thing happens here. So let me go ahead and highlight that. So, if we have hydrogen
bonding right in here, that's going to weaken our carbonyl, so that's going to decrease the double bond character a little bit. So that's actually going to
change where we find the signal. So it actually changes the signal, if it's weakening the bond a little bit, you're going to decrease the wavenumber, and so this carbonyl is a tiny bit lower, in terms of wavenumber,
where you find the signal, than what you would expect. I'll briefly mention
that in a later video, when we talk about carbonyl chemistry and more about IR spectra. So, for the shape of the signal, remember broad, think hydrogen
bonding if it's a broad signal.