Organic Chemistry Infrared Spectroscopy, Part I

Hi; welcome back to Educator.0000

We are going to talk about infrared spectroscopy today--which is a really important tool for analyzing the structure of organic molecules.0002

Now, all spectroscopies work generally the same way: we are going to take our sample; we are going to irradiate it with some kind of energy (in our case, we are going to be using infrared light--that is why we call it IR spectroscopy).0010

And we are going to take a molecule (let's say we have a molecule like this, a molecule of HCl); we are going to hit it with a photon of light.0022

Now, that photon is going to be described in terms of its energy: its energy can be put in terms of its wavelength.0028

And so, we have this constant in the speed of light; and so we see that, if the wavelength gets smaller, the energy increases; so light of a smaller wavelength, a shorter wavelength, is higher in energy.0038

And we are going to be using a different...so this is wavelength, this λ here...and we are going to be using this: this v with a line over it is short for wavenumber.0052

These are in units called reciprocal centimeters; so it's centimeters to the -1.0067

And you can see they have an inverse relationship (so that is where the -1 comes from), and we will find that energy has a direct relationship with these wave numbers.0073

We are going to be using these numbers, reciprocal centimeters, to describe the photons of light that we are using; and what we need to remember is that, as you increase your wavenumber--increase the number--you are increasing in energy.0084

OK, well, what is going to happen is: when this sample is irradiated, when it's hit with just the right energy of light, that energy can be absorbed.0097

And what happens when infrared light is absorbed by a molecule is: it undergoes vibrations; we say that the molecule becomes vibrationally excited.0109

Now, what does that mean?--well, here we have a hydrogen-chlorine bond, so the only motion that can happen is: this chlorine can move closer and away from the hydrogen.0119

We call that stretching of the bond: it stretches and compresses.0128

That molecular vibration is the result of absorption of IR light.0135

Now, how can this help us analytically--how can this help us analyze a sample and learn something about its structure?0141

Well, here is what we are going to do: we are going to irradiate the sample with IR light, and we are going to record the frequencies that are absorbed; so we are going to pay attention to which frequencies are absorbed by the molecules and which ones are not--which ones are just transmitted.0149

OK, because it turns out that certain functional groups have characteristic absorption: so if you have, for example, an OH group in your molecule, OH groups absorb a certain wavelength of infrared light.0163

And so, if we see an absorption at that wavelength, that tells us that our molecule has an OH group on it; so it's going to be...IR analysis is a way for analyzing for functional groups.0177

Now, it turns out that the intensity of the absorption--how strongly the molecule absorbs that light--is proportional to the change in dipole.0188

So, if we have something that is not polar--a bond that is not polar--then it is not going to be something that will absorb IR light.0197

So, for example, let's take a look at a molecule, a more complicated molecule, like carbon dioxide.0207

If we take CO₂, and we irradiate it with infrared light, what can happen--how can this molecule become vibrationally excited?0213

Well, one thing it can do is: the CO bonds can bend; the oxygens can bend toward one another; so we can maybe draw that motion like this.0223

And we would describe that motion as bending, and that would be...a certain wavelength of light can be absorbed that will correspond to that motion.0234

Now, another thing that carbon dioxide can do is: we can have stretching of bonds, like we saw here; and so, we can have both of these oxygens stretching and getting longer and shorter, and longer and shorter.0249

Because they are going in sync, we call this a symmetric stretch, symmetric stretching motion; or we can have a motion where, as one carbon-oxygen bond gets longer, the other one gets shorter: we call that an asymmetric stretch or an antisymmetric stretch.0263

Now, it turns out that, because carbon dioxide is a nonpolar molecule, not each of these motions would result in a change in the dipole of the molecule.0289

Now, if you bend the oxygens in one direction, that is going to now make the molecule a polar molecule; so that definitely changes the dipole, and that would have a signal in the IR.0301

OK, but if I were to pull these two oxygens in opposite directions with the same force, that would keep the molecule nonpolar; there would be no change in dipole.0312

And therefore, there would be no absorption--meaning there is no signal.0327

There is no signal in the IR for that motion; but if we were to stretch the molecule like this, where both oxygens move in the same direction, then it is going to have a change in dipole, and we get a signal here.0337

OK, so this is just one little note of all of the peaks that we are going to be observing in the IR spectra: we should recognize that all of the motions we are describing do result in a change of dipole.0350

Otherwise, they wouldn't appear in the spectrum, and we wouldn't even be able to observe them.0363

OK, so let's take a look at various functional groups and see what their IR spectra look like.0371

The simplest compound we can have is an alkane: an alkane like pentane has just carbon-carbon bonds, carbon-hydrogen bonds--there is nothing else there; there is no other really significant functional group--no other atoms besides carbon and hydrogen.0376

And let's see what that spectrum looks like: OK, first of all, though, let's take a look at what an IR spectrum...how they are presented.0390

OK, we can see here that our numbers are shown as wavenumbers, as promised; so the unit here is reciprocal centimeters, or inverse centimeters.0399

And our range is about 4,000 to about 400 (or, in this case, 600).0411

So, that is the typical range of IR that we are interested in, that is going to give these characteristic absorptions that are going to be useful to us to analyze.0417

OK, and what we are showing over here is: we are showing percent transmittance.0428

So, what we are asking is: as this sample is being irradiated with light, how much of that IR light is just being transmitted directly through the sample and not absorbed?0432

And so, up here, we have 100% transmittance, meaning we had 0...so there is always a little bit of absorption here, but a straight line going all the way across here means that our IR light comes straight through, and nothing gets absorbed.0442

Every time we have a dip down from that top line, it means we record that as an absorption; so it's those dips--it is these long peaks, as we call them, that we are going to interpret and try to make some sense of.0458

OK, and down here is 0, meaning no light was transmitted; all of the light of that frequency was absorbed.0473

So, a very, very strong absorption would be one where nearly all of the light is absorbed, and almost none of it is transmitted.0480

OK, so let's see what we are going to expect for an alkane.0490

Well, what kind of bonds do we have that could potentially stretch or bend?0493

OK, well, we have a carbon-carbon bond here; and then we have these carbon-hydrogen bonds, and those are the bond that can do some stretching or do some bending.0499

Of course, with pentane, we have a pretty complicated molecule; so can you imagine the motions this can have when it's vibrating?--it can be wiggling all over the place.0510

It seems like there would be an infinite number of vibrations that it can have.0518

But really, there are just some basic, fundamental ones that we are going to be seeing.0522

OK, the way we characterize this carbon of an alkane: it's a tetrahedral carbon, so it's just a plain old sp³ hybridized carbon.0526

And hydrogens that are attached to sp³ hybridized carbons have this characteristic set of peaks, right here, just below 3,000.0538

So, just below 3,000 reciprocal centimeters, we see all of these peaks; and every time we see that, it is going to tell us that our molecule contains sp³ carbons with hydrogens attached--in other words, plain old alkyl hydrogens or alkane groups.0551

Now, why are there so many peaks?--well, we have groups that are like this one (a CH₃); we have groups like this one (that are CH₂s); and for each of these, we can have symmetric stretching (where they are going in the same direction); we can have asymmetric stretching (where they are going in opposite directions).0570

And so, this is something that results in several little peaks, but we don't have to break them all down; we could just say this whole region that is just to the right of 3,000 tells us that we have sp³ CH's.0587

Now, these peaks here represent CH bending, and that is where, rather than stretching, the molecule is just kind of tipping and rocking and that sort of motion; we are bending one bond with respect to another.0604

OK, and we are not going to see an awful lot of these peaks; we are not going to be able to pick out a lot of these peaks, because bending is a very easy motion--very low-energy motion for a molecule to have--it is very easy for a molecule to bend.0624

Imagine taking a rubber band; and if we are trying to bend a rubber band versus stretch a rubber band, which is the harder process to do?0637

It is more difficult to stretch the band, which means that is going to take more energy to put in there; and so that is why we see that to stretch a bond takes about 3,000 reciprocal centimeters, where to bend it, it just takes something like 1,400.0644

Remember, as you decrease your wavenumbers, you decrease your energy.0658

So, because this is such an easy motion to have happen, what we end up with is a very complicated region down here; and this whole region is so complicated, we will often see lots and lots of peaks--see all these little wiggles?0662

This is called the fingerprint region, and it is called the fingerprint region because it is so complex that it ends up being unique for a given molecule.0679

Just like your fingerprint is unique for each individual, the fingerprint region is unique for a molecule; so this is very handy in analysis; let's say we are analyzing a drug sample or something isolated, and we want to see if it's an illicit drug or something.0697

What we could do is: we could take an IR of that sample, and then we could screen it against a library of known compounds; and if we find a match in that fingerprint region with a known compound that we have in our spectra library--in our database--then we would have proof of the identity of the molecule.0714

OK, so in general, IR spectra don't give us a lot of information--they just tell us what kind of functional groups we have in here.0733

However, if we had computer tools to be able to analyze the fingerprint region--we couldn't do this just by looking at it, but it is possible to get absolute identifications of molecules, based on their fingerprint region.0743

OK, but when we look at this overall spectrum, the only thing that is really of interest that we are going to be picking out is this peak down here, below 3,000; and it just tells us that we have alkyl CH bonds in our molecule.0757

It is a pretty boring spectrum; there is not much to it; and if this is a spectrum of pentane, what if we looked at the spectrum of hexane or heptane or decane?0770

It is actually going to look very, very similar to this; so it is not going to tell us exactly which alkane we have--it is just going to tell us that we have an alkane, and we have no other significant functional groups in there.0781

OK, what happens when we have an alkene?--an alkene means we have a carbon-carbon double bond, and so how is this going to affect our spectrum?0795

Well, first of all, we can see that we still have our CH's that are attached to an sp³ hybridized carbon; so where did we find those in our spectrum?--we found those just below 3,000, and it looks like we still have those peaks here.0807

So, we can go ahead and label these as sp³ CH bonds; we don't have to call it a stretch--it is actually the bonds stretching--but we can just indicate that the functional group of interest here is an sp³ CH.0828

Well, what we have new that is in our structure: this is a new peak, right here, just above 3,000; and this comes from having these bonds.0844

We have hydrogens that are not on sp³ hybridized carbons; we have hydrogens that are on sp² hybridized carbons; this is a trigonal planar carbon; this is sp² hybridized; and so, when you have hydrogens attached to those kinds of carbons in your molecule, the IR spectrum is going to show it by having a peak just above 3,000.0854

So, we will label this peak as an sp² CH; so when I look at this spectrum, I know that my molecule must have both hydrogens attached to alkene-type carbons or hydrogens attached to alkane-type carbons.0875

What else is there in this spectrum?--well, again, kind of a noisy fingerprint region: we are not going to try and pick out all of those peaks.0890

The other interesting peak is right here, around 1,600; it is where we find carbon-carbon double bonds that are stretching; so this carbon-carbon double bond--when we hit it with infrared light of 1,640 wavenumbers, that is exactly the energy that is needed to cause that carbon-carbon double bond to stretch.0897

And so, we see a peak there.0922

OK, you notice, though, that this is a very weak signal; and so, we are not always going to be able to find that; that is also not a very polar bond, is it?0925

So, there is not a big change in dipole; and so, a lot of times, it is difficult to find carbon-carbon double bond peaks, and sometimes they disappear altogether; if this is symmetrical, we won't even have that peak.0932

Now, an alkyne is what we call it when we have a carbon-carbon triple bond; let's analyze this structure.0951

We see that we still have this region of the molecule that kind of looks like an alkane; it has sp³ carbons with hydrogens on it.0956

And so, we expect to find that on our spectrum; where is it?0966

We are going to always look at this 3,000 mark; we are going to go to the right of 3,000, and sure enough, we are going to see a variety of peaks there, typically.0970

I'll label that sp³ CH; now I'm labeling this, and it's a good idea to have these spectra printed out, so you can label them, too, and get some practice in that, because that is one of the goals of learning about IR spectroscopy: not only to understand the theory of it, but to be able to interpret a spectrum.0979

And you are expected to mark it up and label peaks and use that to fully interpret or explain what you see in an IR spectrum.0996

OK, so what else is in this spectrum--what other kinds of hydrogens do we have in the structure?1007

The hydrogens are always going to be really important in our spectra; so, we have some hydrogens that are attached to sp³ hybridized carbon, but we also have a hydrogen at the end of the molecule here that is attached to an sp hybridized carbon.1012

And so, that is going be a significant peak in the IR, and that is going to show up right here, even further to the left of 3,000--right here at 3,300, this peak comes from the sp carbon with the hydrogen on it.1027

That bond is stretching, and that is going to show up around 3,300.1042

OK, so we can see a trend here: the sp³ is just below 3,000; sp² is a little higher; and sp is a little higher still.1046

We have sp and then sp² and sp³; we don't have any peaks here for sp², because this molecule has no sp² hybridized carbons--so it can't have any hydrogens on those.1055

OK, the other interesting thing that is in this structure is right here--this peak at 2,200.1064

It comes from the carbon-carbon triple bond: when you have a carbon-carbon triple bond, and we cause that triple bond to stretch--those carbons to get further away from each other and closer together--that signal shows up around 2,100, 2,200, somewhere around there.1070

Even though this is a very small peak, normally this is a pretty empty part of the spectrum, and so it should be easy to pick out when you do have a triple bond.1086

A carbon-carbon triple bond looks like this; a carbon-nitrogen triple bond comes at about the same region.1094

That is what an alkyne looks like, like pentyne.1100

How about if we had an aromatic compound?--now, we use the word "aromatic" a lot when we are talking about IR spectroscopy, and when we use the word "aromatic," we are talking about something like benzene.1106

Benzene is the molecule when we have a 6-membered ring with three π bonds in that--alternating π bonds.1119

That is a very special molecule; it's called benzene; and so, when we have derivatives of benzene (meaning we have groups attached to any of those carbons), we describe those as aromatic compounds, because benzene is an aromatic compound.1127

There are other things besides benzene and benzene derivatives that are aromatic, but for now, in the introductory stages, when we are first learning about IR, we will kind of use these two synonymously.1141

Any time we are talking about "aromatic," we mean benzene or phenyl (we call it a phenyl group when you have a benzene as a substituent--something attached to a benzene--we call it a phenyl ring).1154

So, benzene itself means that you have this compound with 6 carbons and 6 hydrogens; we can have substituted benzene or benzene derivatives, or we could have a benzene phenyl ring attached--we call it the phenyl group.1166

OK, so any time you see the word "aromatic" (sometimes this is abbreviated Ar for aromatic), you need to picture the benzene ring.1184

Here is an example of an aromatic ring: this is called methylbenzene, because it has a CH₃ attached to the benzene.1194

What do we expect to find in its IR spectrum?--well, we saw that C-H bonds are always very important: what kinds of C-H bonds do we have?1200

We have this carbon; it's an sp³ hybridized carbon, so we expect to see these hydrogens stretching symmetrically, asymmetrically.1209

And where do we expect to find those?1220

OK, if we kind of highlight that 3,000 line, just below 3,000 is where we find our sp³ hybridized CH's; so these peaks.1223

And what other kinds of hydrogens do we have?--well, there is a hydrogen at each one of these positions, and so on; and the hybridization of an aromatic ring (like benzene) is sp² hybridized.1235

Where do we expect to find that?--that is going to appear just above 3,000; this is where we find the sp² CH's, and sure enough, we see some peaks here.1248

Now notice: not every peak is labeled; sometimes the computer picks up a peak and labels it with a number; sometimes it doesn't.1257

So, don't worry about whether or not a peak is labeled; just focus on whether or not a peak is present--that is what is important here.1264

So, when we talk about sp² hybridized carbons, that could be an alkene, meaning just a random carbon-carbon double bond; or it could be a double bond within a benzene ring; so it could be an aromatic peak.1272

Now, another interesting thing that we see for aromatic compounds are these peaks down here.1286

Now, you notice this little pattern up here: we have 1, 2, 3, 4 peaks--we have this little ripple.1292

Sometimes, this is described as a little aromatic ripple in this area.1300

And the other interesting thing that we see here are these two peaks, and we are going to get very strong peaks in these regions around 700 and 750 every time we have a benzene ring with this substitution pattern.1305

And the way I would describe this benzene ring is: I have benzene with just one group attached.1324

This is called a monosubstituted aromatic.1328

This is called a monosubstituted aromatic: methylbenzene is an example of that.1337

Every time we see that, we get these two peaks; and these two peaks result in these four peaks--this little ripple up here.1342

This is just something you can kind of look for: normally, this is pretty flat, but when you see this little ripple here, and we can look to this area, if we think we have a benzene ring, this is going to tell us something about the substitution pattern.1352

Now, what is this--why do we have these signals here?1366

Well, these are called out-of-plane bending motions.1369

And I said that bending absorptions are usually not very significant, because they come in the fingerprint region, and that is certainly true.1375

OK, but these aromatic ones are usually so strong that they are very significant signals, and you can kind of pick them out as a needle in the haystack.1383

OK, and what is happening here is: benzene and other aromatic rings are planar, and so, what can happen in one of the motions called an out-of-plane bending is: one of the molecules wobbles like this.1391

And, depending on how many groups are attached to that benzene, that wobbling is going to be happening at slightly different frequencies.1406

OK, so when it's monosubstituted, we get these two peaks at around 700 and 750.1414

Now, all of these numbers that I am rattling off, by the way, as we go along--these are things that are going to be made available to you in tables called correlation charts; and so, there is a sample one of those in the handout; we will look at that a little later.1418

I am just referring to these numbers now, but you will be able to look those up at a later time.1431

OK, so a monosubstituted aromatic looks something like this: let's take a look at some other substitution patterns to see what they might look like.1436

Here is an example of one of these tables.1447

When it's monosubstituted, meaning there is just one group, we are going to get these out-of-plane bending peaks somewhere around 750 and somewhere around 690.1449

If it is disubstituted, there are three different ways you can arrange two groups on a benzene ring.1458

If they are right next to each other (1,2), we describe that as being ortho--having an ortho relationship.1464

And when they are right next to each other, that motion--that out-of-plane bending--changes a little bit, and we now get just a single absorption at 750.1471

And, if they are 1,3 to one another, we call that the meta relationship; a meta disubstituted benzene ends up with three peaks around 880, 750, and 690.1482

And when the two groups are opposite each other, we call that 1,4 disubstituted, or para disubstituted (para substituted ring); then, we get just a single peak at 815.1494

OK, and again, these numbers--you don't have to worry about where they come from, or you don't have to memorize them; they have to do with the relationships of the hydrogens that are on the aromatic ring and the type of out-of-plane bending.1506

Sometimes, we called these oop peaks--out-of-plane bending peaks.1520

OK, so we saw an example of the monosubstituted; let's take a look at some others.1526

This is called dimethylbenzene; if it's 1,2 dimethylbenzene, or ortho dimethylbenzene, we said we are just going to get one peak in this fingerprint region.1531

And notice how, in the fingerprint region, this one peak really does stand out as a very strong peak; this tells us we have ortho disubstituted; we get a peak somewhere around 750.1546

OK, and I forgot to explain where this little aromatic ripple comes from.1556

What happens is: whatever pattern we have down here for the out-of-plane bending--if we absorb twice that amount of energy, we can get that motion to be more intense, and we get a second absorption that is about twice the frequency.1562

So, whatever pattern we have here, we have a concomitant pattern; we have another related pattern that we can pick out.1580

The pattern for an ortho disubstituted aromatic ring looks a little different than the pattern for the monosubstituted; remember, the monosubstituted had a nice 1, 2, 3, 4 look, and the ortho is a little more wiggly.1587

OK, but that is what our ortho looks like; what other peaks do we expect here?1603

Well, we expect the usual: this area is going to look the same, regardless of the substitution pattern; we still have our sp² CH stretches and our sp³ CH stretches.1607

But the way I can tell that this molecule is an aromatic compound, and not just an alkene, is because it has this little aromatic ripple, and I can pick out this out-of-plane bending peak here.1623

When we have a 1,3 disubstituted pattern, that is called the meta relationship.1636

And when they are meta, we now expect to have these two peaks: about 690 and about 700.1644

OK, and then, there is a third one, although it is usually not so easy to pick out: it is this one up here.1654

And so, if you can't see it right away, that is OK, because these two are a little more obvious.1660

And what do we expect to find?--again, we have sp³ hybridized CH's; we have sp² hybridized CH's; so here we have those peaks: an sp³ CH and an sp² CH.1667

OK, be careful with your labeling: you don't want to get lazy here and just label this as an sp³.1682

All right, there is no such thing as an sp³: that is not a functional group.1687

What we are saying here is: by the presence of these peaks, we know that the molecule has sp³ carbons with hydrogens attached to those, and that stretching motion of that C-H bond occurs somewhere very close, around 2,900 reciprocal centimeters--just under 3,000.1690

OK, so it is an sp³ CH--the actual functional group that we are identifying here.1711

This is an sp² CH.1717

And then, finally, the para disubstituted--when it's 1,4, we call that para; and the para has just this single peak somewhere around 800.1721

OK, and again, this is something we can kind of look for to dig out evidence; it is not something that is necessarily going to jump right out at us when we see it.1735

And the para pattern looks something like that: that is also a characteristic that we expect to find for our little aromatic ripple area.1744

We still have our sp² CH and our sp³ CH.1756

OK, so these are called out-of-plane bending peaks that we can use to pick out the substitution patterns of aromatic rings.1762

OK, what other functional groups can we look for?1777

So far, we have really just looked at the C-H bonds, the different kinds of C-H bonds we might have in a molecule, and how they all vary--if we have alkanes or alkenes or aromatic, or maybe an alkyne triple bond.1779

OK, well, if we have an OH group, we call those molecules alcohols, and they are very characteristic in an IR, because what we can look for is that OH bond itself stretching.1790

That comes right here--this big, broad peak is the OH stretch.1806

We call it that because it is literally the O-H bond stretching.1816

It comes right here typically, around 3,300; but notice that it is very broad--and this is characteristic of an OH functional group and an OH peak.1820

It is very broad, meaning it starts all the way here around 3,600, and it doesn't end until right here, around 3,100; so rather than being sharp and narrow, like the peaks we have seen before, it's very broad and spread-out.1829

OK, and that is because this can undergo hydrogen bonding with another molecule of 1, 2, 3 ,4, 5...pentanol.1843

This is a very strong partial minus; the hydrogen on an oxygen is a very strong partial plus; and so, what we have here is hydrogen bonding between molecules, and that affects the spectra, and we end up seeing this broadening of the peak.1853

OK, so that is what we can look for for an alcohol; let's go ahead and see if we can find anything else in this spectrum.1873

What else do we expect to find?--well, we have our alkyl group here, and the way we would see that are these four peaks to the right of 3,000, just below 3,000; those are our sp³ CH's.1879

Notice, we are going to see that in almost every single IR spectrum, because almost every single organic molecule has some kind of alkyl group on it, right?--a methyl, ethyl, propyl, butyl, something like that.1894

And so, nearly every IR spectrum is going to have that peak; and so, you should be used to seeing that, even without a correlation table; you should know to look right at 3,000 and analyze that area very carefully.1906

What else is there?--well, we can also point out: there is another bond that is unique in this molecule, and that is this C-O bond.1920

And C-O bonds, C-O stretches, show up at around 1,050 or so.1927

And again, that is a stretch: that is where the C-O bond is getting longer and shorter, and it usually comes around 1,050.1934

But because this is in the fingerprint region, it is not so easy to pick out; it is not always a strong peak, either.1944

So, in this case, we can see it with pretty good confidence, and we can label it; but sometimes, we are not going to be able to do it quite as easily.1950

Now, if we have an amine--we call it an amine when we have a nitrogen in our compound--here we have NH groups, and again, these can hydrogen bond, so they are going to be somewhat similar to an OH, and they are going to come at the same region, somewhere around 3,300.1961

OK, but we can see that they are going to be typically weaker than an OH: an OH is a nice, strong peak; that means it goes pretty low--there is a very strong absorption, because it's a very polar bond.1982

But the NH is usually a little weaker; and notice that we have two peaks here, and that is because this is an example of a primary amine: it has just one alkyl group attached to the nitrogen.1995

And we call those amines primary amines; and so, a primary amine must have an NH₂, and we can have those hydrogens stretching either symmetrically or asymmetrically.2014

So, if they are stretching symmetrically, we get one absorption; if they are stretching asymmetrically, we get a second absorption; so because of this NH₂, we end up getting this little double set of peaks here.2032

Notice, both of these are going to show up, because this is a polar molecule, and both of these motions in this case result in changes in dipole.2047

So again, we are going to see every N-H bond that is in the molecule; we are going to see it as a stretch.2054

But it is usually a little smaller here.2059

OK, is there anything else in the spectrum we can label?--sure enough, we have our usual sp³ CH; nothing else here of note.2062

The C-N bond...the C-O bond was difficult to find; the C-N bond is not diagnostically useful, so the only thing that is interesting in this amine (besides the sp³ CH) is the NH stretch.2072

Now, depending on the type of amine, this peak is going to look a little different: let's take a look at some examples.2086

OK, so a secondary amine is what we call it when we have two alkyl groups or two R groups; that is why we call it a secondary amine.2092

And so, we know nitrogen likes to have three bonds; so that means there is just one hydrogen attached here, and so our peak looks a little different; there is just one because that NH stretch--there is no other motion that could be happening there.2106

We could have this single NH stretch.2118

And so, this is our NH bond; that is the NH that is stretching, so that looks a little...secondary amines look a little different than our primary amine; and again, we have this whole big mess here as a result of our sp³ hybridized CH's.2121

OK, all of this other stuff in the fingerprint region, we are not going to be worrying about.2142

How about if we had a tertiary amine?--a tertiary amine has three alkyl groups; so what will we look for in its spectrum?2149

Do we expect to find an NH signal?--it looks pretty empty here--it looks like we have just a total transmission; there is no absorption at 3,300.2158

Why does this amine have no absorption at 3,300?--because that absorption was the result of the NH bond in the amine stretching; this has no NH bond, and therefore, there is no absorption there.2168

So, this amine looks very much like any other alkane; all we have here are sp³ CH's and nothing else.2183

So, an amine is only interesting in the IR if it's a primary or secondary amine, because it is the NH bond that we can look for and pick out.2192

OK, how about a ketone?--a ketone is what we call it when we have a carbonyl (a C-O double bond is called a carbonyl), and when we have a carbon attached to either side (like we do here--we have a methyl here and an ethyl here), we call it a ketone.2203

And the C-O double bond is very polar; and so, when it stretches, there is a big change in dipole, which means a carbonyl group has a really strong absorption of IR light.2227

That means we are going to get a really huge signal.2240

And where does it show up?--it shows up right here; somewhere around 1,700 is where we are going to see our C, double bond, O.2242

You could just label it as a C, double bond, O; and that is where we see our carbonyl, and I want you to notice that it is incredibly strong.2249

The carbonyl peak is going to be your strongest peak in the entire spectrum.2257

OK, you can't miss it; if you are ever looking at a peak somewhere around 1,700, and you are looking and you are saying, "That might be a carbonyl--do you think that's a carbonyl?"--it is not a carbonyl.2263

OK, unless it is so big that it almost touches the bottom line, and you have a complete absorption, then it is not a carbonyl; so a carbonyl is something that you should never mistake in an IR spectrum.2272

And it is another one that is kind of handy--as you work with IR's, you will get to know this number very readily; it always comes around 1,700.2285

Somewhere around 1,700...now, it can shift a little to the left or the right, depending on whether it is a ketone or an aldehyde, or exactly what groups are attached to either side.2296

If you have a double bond attached on one side, so that the carbonyl is conjugated with other π bonds, that shifts it to a lower number.2306

So, it can move somewhere around 1,700, but it's always in that range.2314

OK, so that is what a ketone looks like; what else did we have in this ketone?--well, we have our usual sp³ CH; so there is nothing else too interesting to label.2319

Another thing I want to point out is: take a look at this little peak.2327

Now, you might look at that and say, "Oh, is that maybe an NH, or is that an OH, because it's coming around 3,300?"2330

OK, well, definitely not an OH, because an OH is another thing you are not going to miss: it's a nice, big, broad peak.2337

OK, an NH is usually smaller; but what this peak actually results from is: this carbonyl is so strong--and once again, if this were to absorb twice the amount of energy for that carbonyl to stretch even further, you would see a second signal at about twice this.2344

OK, so this is at 1,700; this is right around 3,400; and so, this we describe as an overtone for the carbonyl.2367

And so, we can ignore that; it just means...it is the same functional group we already identified; it's the carbonyl.2378

I just want to call attention to it, though, because sometimes (because carbonyls are so strong)--sometimes this peak can look pretty big, and might give you a false idea that you have an OH or an NH.2384

But there is a little warning: if you have a carbonyl here, you want to be skeptical of a peak up here--that it could just be the overtone of the carbonyl, like it is in this case--there is no other functional group present, so that must be just the overtone.2395

Now, if you have an aldehyde, how is that going to differ from a ketone?2411

Well, what makes an aldehyde an aldehyde is that, attached to the carbonyl, we have a hydrogen.2415

OK, so that means this carbon-hydrogen bond is unique: remember, all the hydrogens in our structure are going to be important for IR.2423

And now, it is not just attached to an ordinary sp² hybridized carbon, like we have in an alkene or a benzene ring; it is attached to a carbonyl--that is a very unique type of carbon.2430

We are going to call this a carbonyl...I'm sorry, let's call it an aldehyde CH; it's a very special kind of bond.2443

And, when you have an aldehyde, that shows up right here: we are going to get two peaks (one of them was labeled, and one of them was not).2456

It comes at about 2,850, and around 2,750, we get these two peaks that kind of look like vampire fangs--I always see them as vampire fangs.2465

And they are usually weak like this--they are usually pretty small--so it is very easy to overlook these at first.2476

OK, this peak I see very clearly; but this one is almost totally obscured by this larger peak; so you have to look very carefully.2482

OK, but both of those peaks combined, we are going to label as an aldehyde CH: that tells us that we have an aldehyde.2491

Now, what else should we find in this spectrum?2499

If you have an aldehyde, that means you must have a carbonyl; where do carbonyls show up?--right around 1,700--strongest peak, biggest peak in the whole spectrum.2502

So, no missing that: there is our carbonyl.2513

And what else does this aldehyde have?--well, the rest of these peaks up here, just below 3,000--these are sp³ CH's.2516

OK, so that is what an aldehyde looks like.2525

Here is another case where this looks like a pretty significant peak up here--this looks pretty decent in size; but because we have a carbonyl, we are going to look up at that and recognize that just as an overtone--the carbonyl overtone.2529

Even though that is pretty big here, it is still no other additional functional group--it is just that carbonyl absorbing twice the energy to cause that increase in energy.2544

OK, one other type of carbonyl compound we will take a look at--a couple of others, actually: we call it an ester when the carbonyl has an O-R attached to the carbonyl carbon; so we have just a carbon group over here, and an oxygen with an alkyl group.2560

All right, so we could describe this as an O-R group.2577

And so, what does that look like?--well, we still have a carbonyl; OK, that carbonyl is going to be somewhere around 1,700; we see it shifting up a little higher now, compared to a ketone or an aldehyde.2582

Esters come a little different: you can get a table of just carbonyl peaks, on where they come, depending on the exact type of groups attached to that carbonyl--what functional group you are looking at.2597

OK, but somewhere around 1,700--really, really strong peak, clearly the carbonyl.2609

OK, we also expect--these are all sp³ hybridized carbons--all the hydrogens in this molecule are attached to sp³ hybridized carbons.2614

We expect to find those right here, just below 3,000.2622

And the only other thing that is interesting in an ester is: we have a C-O bond here, and we have a C-O bond here, and these are different C-Os.2627

And so, sometimes, we could pick those out, or we should try and find those.2639

Remember, a C-O comes somewhere around 1,050, so if we look around 1,050, we have a nice, strong one here: 1,200 and 1,080; so those are probably our two C-O single bonds.2642

A C-O double bond is called a carbonyl that comes around 1,700; a C-O single bond is somewhere around 1,050.2657

OK, and a carboxylic acid is the last functional group we are going to look at, and what makes a carboxylic acid functional group?2668

Well, you have an OH group; OK, but it is not an alcohol, because that OH is attached to the carbonyl; so this whole functional group together is described as a carboxylic acid.2677

OK, now what is it going to have in common with what we have seen before?--well, any carbonyl-containing compound is going to have a carbonyl stretch, and that is going to come somewhere around 1,700, so here it is; there is our carbonyl.2689

OK, and it is also going to have an OH stretch; but this is going to be a little different from an alcohol OH, because this is extremely polar, and it can very effectively hydrogen bond--in fact, so much so that two carboxylic acids can come together an dimerize with very strong hydrogen bonding.2705

And so, what happens is: remember, the OH was a very broad peak: well, a carboxylic acid gets even broader: in fact, it turns out that it is this whole peak here--we could abbreviate a carboxylic acid as RCO₂H, so we could call it a carboxylic acid OH, or would label it as...we could give it the name "carboxylic acid OH."2726

I don't want to just label it as an OH; it is the OH bond that is stretching, but there is more information in the spectrum, so I want to make sure I label it completely.2752

I can tell from the shape of this OH that it is a carboxylic acid OH; so I want to make sure I note that in my labeling here.2761

And remember, a normal alcohol was somewhere around 3,300, but a normal alcohol, just so we can compare it, looks something like that.2771

This would be an ROH, or an alcohol OH; so if that OH was attached to an ordinary alkyl group, then we expect to have this nice, broad peak, but it starts and then it stops, and then we have our CH's.2781

Here, the OH starts around 3,500 and goes all the way down to 2,400--really, really broad.2798

That is what a carboxylic acid does; in fact, carboxylic acids can get really, really messy and ugly; so if you ever find a spectrum where it is just a total disaster in this area, and really weird, and you don't know what has happened, look for a carbonyl; think about whether or not you maybe have a carboxylic acid that could answer some of those strange questions.2807

OK, now what do we see peaking out below this whole carboxylic acid OH?2828

Well, sure enough, we can pick out these little jagged peaks that are partially obscured, but we can see them at the bottom here; those, of course, would be our sp³ CH's, because we have an alkyl group attached to our carboxylic acid part.2833

OK, so just to give you an idea of the significant functional groups that we are going to be seeing, we have gone through lots of examples.2849

OK, and as promised, you are going to be given some kind of correlation chart whenever you go to work with IR problems.2857

Now, as you gain experience, a lot of these numbers and different areas are going to kind of become more second nature to you.2866

But, in the meantime, you will always have a chart like this...you will almost always have a chart like this to refer to.2875

So, make sure you use it as you are working on IR problems.2880

OK, so as we are looking from the highest numbers of things we saw in the very far left of the spectrum (where our OH and our NH--remember those? came), around 3,300--somewhere in that region, and it was very broad--that is characteristic of the shape of that peak.2884

An S, the letter S, in one of these correlation charts, means that it's strong; M stands for medium; and W stands for weak; or sometimes, you see a V--that means variable intensity.2906

Sometimes it is strong; sometimes it is not.2922

OK, so you might see that--rather than just "where is it?" it tells you something about the shape of the peak, typically.2925

And all of these are going to be sharp peaks (they go up and down), except for the OH and the NH; because of that hydrogen bonding, they spread out; those are broad peaks.2931

OK, if we look at the CH's (sp, sp², sp³), notice that 3,000 is the very important cutoff between the sp²s just to the left of it and the sp³s just to the right of it.2940

And the sp's show up around 3,300.2957

Now, if the sp CH shows up at 3,300, and the alcohol shows up at 3,300, how do we know which one we have?2960

Well, this, again, comes to the shape of the peak: the OH is going to be very broad, where the sp CH is going to be very sharp--it is going to be very narrow.2968

Those actually do look quite different from one another.2977

OK, the other type of CH that is interesting--so CH's are the only one that we describe by the type of carbon that they are attached to.2980

Everything else...an OH is an OH; an NH is an NH; but if it's a CH, we have to describe exactly what kind of CH it is, because then it appears at different places on the spectrum.2991

So, it could be sp or sp², like in an alkene or in a benzene ring, an aromatic compound; sp³ means it's just a plain old tetrahedral carbon, alkane.3001

And then, an aldehyde CH means that the hydrogen is attached to a carbonyl carbon--directly attached to the C-O double bond; we call that the aldehyde functional group.3011

OK, as we move down around 2,200--somewhere around 2,200 is where have our triple bonds (carbon-carbon triple bond, carbon-nitrogen triple bond).3021

Around 1,600 is where we have our carbon-carbon double bond; again, this is variable.3030

It can be strong, or it can be very weak; and especially these carbon-carbon double bonds--these will disappear entirely if it is a symmetrical molecule.3034

OK, so if we are looking at a carbon-carbon triple bond, and you have the same group on either side of that triple bond, then when that stretches, there is no change in dipole; we get no absorption of IR light.3057

So, sometimes you might be looking for a peak at 2,200, and there is nothing there.3066

That doesn't necessarily mean that you have no triple bond; you might just have a symmetrically substituted triple bond--the same can be true for a carbon-carbon double bond.3071

OK, carbonyl stretch--this is a big one, because this is our biggest, strongest, most obvious peak there is; that comes around 1,700.3081

And then, our CO single bond stretches come around 1,050; they're typically pretty strong, but sometimes they are just hard to pick out, because now we are in that fingerprint region, where we have all sorts of bending and wobbling going on in our molecule, so it's just a very messy part of the spectrum.3090

OK, so we will use a chart like this when we go to interpret IR spectra; and before we get to interpreting IR spectra and breaking down a spectrum if you are given one, first let's take a look at some sample molecules to see if we can predict what we would find in the IR spectrum.3105

You might want to pause this and try it on your own: see if you can pick out the significant functional groups--and not only pick out the significant functional groups, but then use your correlation chart to predict approximately where you might find that peak in the IR spectrum.3129

OK, so how about our first molecule?3146

Definitely, the C, double bond, O, jumps out at me; so I know that is going to be in my spectrum.3150

I have a carbonyl somewhere around 1,700 reciprocal centimeters, or inverse centimeters--somewhere around 1,700.3156

What else is in this molecule?--there is not much else; we just have these alkyl groups.3165

And so, what do we see in the IR spectrum of an alkyl group?3172

The carbon-carbon single bonds--not diagnostically useful: there is nothing we would see for that.3177

OK, but the hydrogens in the molecule are always important; so how would you describe these hydrogens that are attached here and here and here?3182

They are attached to sp³ carbons, so we would call that an sp³ CH; that is how you would label that peak, when you saw the spectrum: this is an sp³ CH, and where do they show up?--just below 3,000.3190

That is what we would expect to find for a ketone.3208

How about this next one?--this next one is an alkyne; we have an alkyne...let's start with our CH's--what kind of CH's do we have?3210

We have our usual sp³ hybridized CH's, and how about this one?--that is now attached to an sp hybridized carbon CH, so we have an sp CH.3219

The sp³ occurs just below 3,000; and the sp--remember, the sp³ CH is just below 3,000, and then we have just above 3,000 as sp², and then even further up, a higher number is sp.3235

This comes around 3,300.3252

OK, so those are all the CH's in our molecule: what other functional group do we have that we might find in the spectrum?3258

Well, we have this triple bond; and when that carbon-carbon triple bond stretches, that gives rise to an absorption; so we could just label that as a C-C triple bond; and that comes somewhere around 2,200.3264

OK, how about #3 here?--we have...let's start with the CH's again; let's start with the CH's.3281

We have our sp³ CH's right here; remember, we have to be able to interpret our line drawing to envision all of the significant bonds there.3288

So, we have sp³ CH just below 3,000; and how about this benzene ring--this aromatic ring?--what would we see?3300

We would see sp² CH's just above 3,000; any other kinds of CH's that are unique?3311

Is this an sp² CH, or is this something different?3321

No, this is actually an aldehyde; and an aldehyde CH is a unique type of hydrogen; and those are our little vampire fangs--that comes at around approximately 2,850 and 2,750; we get two peaks--two small peaks--when we have an aldehyde CH.3326

OK, so those are all the CH's; what other peaks might we find?3349

There is one that would be really obvious--that is the carbonyl: the carbonyl is always a significant peak.3354

Somewhere around 1,700 is where we find our carbonyl.3360

Those are going to be our most obvious peaks; anything else...well, remember, any time we have a benzene ring, you also might have carbon-carbon double bonds.3365

Those show up somewhere around 1,600; but those are variable intensity--sometimes we don't see them very easily.3375

Anything else?--what about the benzene ring--what about its substitution pattern?--how would you describe that benzene ring?3387

It has just one group attached, so we would call that a monosubstituted aromatic.3393

A monosubstituted aromatic, remember, has those out-of-plane bending motions that give rise to very strong peaks in the fingerprint region, and those come (let me just check my chart and see which numbers I gave you) somewhere around 750 and about 690.3401

Somewhere around 700 or 750, we expect two strong peaks for that monosubstituted.3422

OK, how about #4?--let's start with our CH's; do we have sp³ CH?--we do, right here--an sp³ CH just below 3,000 for our alkyl group--this ethyl group here.3429

And the aromatic ring has sp² CH's; those come just above 3,000; those are all of our CH's.3445

We have a carbonyl again; that comes around 1,700; and the same thing we have up here--we have maybe the carbon-carbon double bond at around 1,600.3456

And we have our monosubstituted aromatic at around 750 and 690.3479

Plus, we will have that little aromatic ripple: that monosubstituted has a nice 1, 2, 3, 4-peak somewhere around 2,200; we can probably pick that out of the spectrum, as well.3486

OK, so it's about looking at a structure, now, and picking out what are the peaks--what are the functional groups that we know give rise to peaks when we put in an IR spectrometer and take a spectrum?3496

OK, let's look at four more.3512

How about our first spectrum here, compound 5: what functional groups?--we really don't have any functional groups--just a plain old alkane.3514

So, how would that look in the IR?3523

We would have sp³ CH's just below 3,000; and that is pretty much it.3526

I know we would have some of those C-H bends; those might be kind of a little more obvious, because the spectrum doesn't have much else going on; but it's nothing significant; it's not something we really have to pick out.3533

OK, so that would be a very simple IR spectrum.3546

How about 6--how about compound 6?3548

We have sp³ CH...what kinds of CH's do we have?--we have sp³ CH and sp² CH; this is just below 3,000; this is just above 3,000.3553

And the obvious functional group here that is new is an OH; so we were getting that OH stretch for sure--that is a nice broad peak that we expect somewhere around 3,300.3574

We can make a little note here that it's going to be broad, as a reminder of what it will look like when we see that spectrum.3585

Anything else that is interesting?--well, we might find our carbon-carbon double bonds, and any time we have an oxygen in our structure, we could look for our C-O single bond (that is around 1,050).3593

All of these are approximate, of course; it depends on the exact structure.3610

So, around 1,600 for the double bonds...so somewhere around 1,050 for a C-O single bond; and then finally, we can have our monosubstituted aromatic; that is kind of the most commonly-encountered aromatic compound.3615

It is just a benzene ring with one substituent--so just a phenyl group.3630

That is a pattern that we might come across pretty frequently; again, that is about 750 and about 690...plus our little aromatic ripple.3635

So, a lot of stuff that is interesting in that spectrum, we can look for.3647

And how about #7--what kinds of hydrogens do we have here?3653

Once again, we have sp³ CH's and sp² CH's.3657

It is always that cutoff just to the right; that is where we have our sp³s; just to the left is where we have our sp²s.3667

And we have a carbonyl; that is a big one that we can't miss--somewhere around 1,700, we expect to see a peak for a carbonyl (C, double bond, O) stretching.3677

And, of course, we have an OH; but what kind of OH is this?3693

It is not an ordinary alcohol; it is a carboxylic acid OH; so let's put that in our notes: carboxylic acid OH, because that means it goes from 3,500 down to 2,400--a huge, huge, huge, very broad signal that kind of obscures a lot of these C-H peaks.3697

And then, finally, we can come back to our monosubstituted aromatic; we can look for a carbon-carbon double bond, and we can look for our monosubstituted out-of-plane bending--that's what it's called; so 750 and 690.3719

OK, and finally, our last one here: what do we have?3741

A kind of boring structure: we just have carbons and hydrogens; so what do we see in the IR?3744

What is it we look for in the IR?--always, always, always the CH's are going to be incredibly important; so what kinds of CH's do we have here?3749

This part of the molecule has sp³ CH's, meaning tetrahedral carbons; that comes just below 3,000.3757

And this part of the molecule--the aromatic part--has sp² CH's just above 3,000.3769

We can also, once again, look for our aromatic; our carbon-carbon double bonds might be somewhere around 1,600, and we can look for those two peaks at 750 and 690.3777

OK, so hopefully, after this lesson, you will have some comfort in looking at a structure and being able to predict what IR peaks you can expect.3793

If you flip through any textbook, at sample IR's you will see: it's a good idea to just start getting familiar with "here are some sample ketones," "some sample aldehydes," "some sample alcohols," so that you can confirm what these look like, because the next lesson is going to be: if you are given an IR spectrum with all of those wiggles and waggles, how can you come up with a structure--or more likely, match it to a set of structures that are given?3805

Because that is going to be the final test that we have in understanding IR spectroscopy.3829

I'll see you soon; thanks for coming to Educator.3838