So last time I tried to give an overview of the course and also an overview to IR spectroscopy.

And I guess one of the take home messages I was trying to give was that IR spectroscopy

really should be something that you [inaudible]

And you want to make it easy to do the experiments and I suggested techniques like placing a

drop on a thin film or [inaudible] form a solution in forming a calcium fluoride cell;

anything to make it easy to do.

There are things—and I hope the example that I started class with really brings it

home—there are things that IR just excels at that, I’ll add, NMR doesn’t excel at.

NMR is going to talk to you about some things; if you’ve got an aromatic ring or an alkene in your molecule

with a hydrogen on it and it’ll probably talk to you pretty well.

But if you have an unusual ring size like we saw in the beta lactone or a 5 membered

ring with a carbonyl—IR can talk to you in a way that NMR just can’t talk to you,

that mass spec isn’t going to talk to you, that UV, which we’re not going to cover

in this course, isn’t going to talk to you.

Other things like a terminal alkyne—we’ll have a problem later on, I guarantee, everyone

is going to look at this and say oh wow, afterward, you know, only with NMR data and not realize,

and then something really talks—there are probably about 2 dozen pieces of information

that easily IR can talk to you and this is what I’m going to put in.

So today I want to talk about CH containing functional groups; in other words I want us

to be able to know how to identify alkane groups and alkene groups, or fragments within

molecules, arene, in other words aromatic, alkyne.

And one of the pieces of philosophy that I’m going to try to take particularly as we get

to certain functional groups is to start to look for patterns here.

Because it’s not so much about tabulating peaks—it’s really about being able to

read spectra.

I want to talk today about oxygen containing functional groups, and we’ll go through:

alcohols, aldehydes, ketones, esters, acids—carboxylic acids of course—acid chlorides, and let’s

say acid anhydrides.

obviously there are other functional groups and I’m just picking ones that I think are

particularly important.

Some of the features I hope we get a chance to talk about today, if not today tomorrow,

more about ring size which I think is really cool in the case of cyclic ketones and esters,

conjugation and understanding the principles involved, inductive effects, and—probably

not this time, as a matter of fact, certainly not this time—we’ll talk about nitrogen

containing functional groups.

we’ll talk about amides, amines, ammonium salts, maybe nitriles—nitriles are another

one that IR I think really shines for identifying—and maybe nitro compounds.

So one of the things I’m going to urge you to do when you look at an IR spectra is really

to learn how to read the IR spectra.

So let me give you my take because part of reading is not just sitting and tabulating

every peak, it’s knowing what’s important.

So when I look at an IR spectrum, generally in my mind’s eye I think from about 4000

to about 600 and probably—and of course these are centimeters^(-1) , wavenumbers,

[inaudible] waves per centimeter—and probably also in my mind’s eye I end up drawing a

line at 3000 wavenumbers.

And I think this really ends up dividing the spectrum into some important regions. So if

you have an IR spectrum maybe you see something like this. You’re reading in—I’m going

to label my axis, of course you’re going to have %T, and of course your spectrum is

going to look something like this.

So you see some peaks, and some stuff over here. Now, the region over here from 1600

to about 3600 is the functional group region. And that’s generally what I look at.

There’s information to be had in this region from 1400 to about 600 wavenumbers; that’s

the fingerprint region.

If we have a chance maybe on Wednesday we’ll take a look at how you can find aromatic substitution

patterns from here; I think in one of the homework problems there’s a—the textbook,

by the way, take the time to flip through it. It’s got very nice appendices that are

extremely useful.

One of the appendices, for example, will give you some correlating peaks to be able to figure

out whether an alkene is—what the substitution pattern is on an alkene; whether it’s terminal,

whether it’s cis, or whether it’s trans.

Anyway—so generally my eye will sort of be drawn to this region; above 1600 I’ll

look for carbonyls, I’ll generally look about this region, just above 2000, because

normally you don’t expect anything there.

If I see anything there it’s going to send up a red flag that there may be something

interesting; we already talked about alkynes.

Generally I won’t pay too much attention to this region right below 3000. But there

are a few things that can show up here at about 2820 and 2720 can be indicative of an

aldehyde if you have the right sort of carbonyl peak.

Things down from 3000 of course might clue you in to alkenes and aromatics. And then

over here, and particularly over here, you get clued in to carboxylic acids.

So it’s really this issue of being able to read the spectrum that I think is really

so key in being able to get some useful information out of IR.

One of the things—it’s not just the position of peaks that counts it’s their relative

intensities, and that can clue you in a lot.

So, for example carbonyl and alkenes, C double bond C, both show up in the same region but

they have different appearances to them. So for example, a carbonyl C=O is generally strong,

and that’s going to mean usually strong relative to other peaks, but again part of

that is what fraction of the molecule is the carbonyl occupying.

If you have a ketone with 6 carbons in it, you’ve got a lot more carbonyl groups in

that path length than if you had a ketone with 60 carbons. So your ketone peak, your

carbonyl peak, is going to be stronger in smaller molecules than in bigger molecules.

Carbon-oxygen single bonds, anything with a big dipole moment in general—not always,

but in general—has strong peaks. So anything where you have substantial differences in

electronegativity between 2 groups.

Bonds that are often weak or moderate are compounds like alkynes, carbon-carbon triple

bond stretches, alkenes, carbon-carbon double bond stretches of various sorts, say, terminal

alkenes and internal alkenes—and we already talked about if there’s no change in dipole

moments, so if you have, say, an alkyne with two alkyl groups on the end, you’re probably

not going to see-or most certainly won’t see—the carbon-carbon triple bond stretch.

If you have an alkene that’s tetra-substituted you’re probably not going to see the carbon-carbon

double bond. So let’s say usually not seen—internal alkenes, internal alkynes.

We talked a little bit when I was talking about methylene groups and I talked about

the asymmetric stretch and symmetric stretch, we talked a little bit about CH groups.

CH stretches are generally very sharp; so things that might clue you in—as I said,

alkyl generally not that informative, but usually they fall between about 3000 and about

2840 wavenumbers. And if you’re good, you really should be able to draw a line at about

2840 in the spectrum because, as I was saying, aldehydes can have little CH stretches and

a Fermi resonance that can clue you in.

Alkenes, arenes—aromatics if you prefer—generally we’re talking about 3100 to about 3000.

And again you’re going to get corroboratory peaks—first of all, NMR will also be useful

but you’re going to see patterns.

For example, aromatics will have a series of bands from about 1650 to about 2000 wavenumbers

that can clue you in.

Alkenes, generally you can see the carbon-carbon double bond stretch. And that’s generally

pretty sharp but not so strong at about 1640 to 1670 wavenumbers.

Alkynes, if they are terminal alkynes of course you have a CH group, that tends to stand out.

So it’ll be at about 3300 and as I was saying they’re generally very sharp, so alcohols

also show up at about 3300, but the pattern recognition is going to be completely different

because an alcohol is going to be a broad band and an alkyne is going to be the sharp

band.

And, as I said, if you are able to see the C – C triple bond stretch it’s somewhere

just below 2000 depending on the exact substitution, about 2100 to 2260.

And again, if you just see something in that general region of about 2000 to 2500 it should

clue you in that there’s something unusual.

Aldehydes, as I’ve said, the CH stretch of aldehydes shows up and you get 2 bands.

What happens is there’s an inactive band, one of these 2 bands I believe it’s the

2820 is the CH and the other one is an inactive band that essentially gets pumped by it’s

called a Fermi resonance. And then that’ll go along with your carbonyls so you’ll be

looking in that region of around 1700 is going to clue you in; let’s say about 1740 to

1720 would be typical for an aldehyde carbonyl, it could a little bit lower if there is some

conjugation there.

Let me put up some examples here just so we see what these guys look like.

Let me get this handed out. [inaudible] with this particular class the best way is to get

these handed out, but we will figure—there’ll be lots of hand outs in the class.

My recommendation, by the way, is to get a loose-leaf binder or else get good at sort

of organizing the handouts.

Let’s see, I have a few extras, let me send some more down here.

We’ll be using transparencies a lot for discussion sections, where there’s really

no substitute for looking at stuff which of course you can do with an LCD projector but

also marking on stuff.

So when you’re up here—and everyone, I hope, is going to have a chance to be up here

in our discussion section—it takes only minor skill but you get in the habit of not

standing there with your shoulder in the beam.

Alright, let’s just take a look at the first one. What is this first one? we’ll just

look at the first 2 right now, what is—what class of compounds is the first one?

The first one here. Alkene—ok great, actually I’m hearing exactly the sort of stuff that

I like to be hearing here. So, I’ve heard carbonyl, aromatic, and alkene. And we can

actually take a very, very good guess.

This peak here obviously stands out, remember I said draw a line in your mind at 1600

So it doesn’t look like a carbonyl; typical carbonyls are going to be a little stronger

and a little bit fatter. It looks like an alkene. This should clue us in, so I’ll

just pick some numbers here. I’ll just write 1642 and here we’re at about 3080.

Now, if I had to guess, and fortunately you’ll probably get other data but if I had to guess

I’d say what we’re seeing here—usually with an aromatic you’ll have a series of

small, very weak bands.

For, like, a phenyl group there will be like 4 of them between 2000 and 1600. And this

doesn’t look anything like that. You’ll usually see more CH’s for a phenyl, below—from

about 3000 to 3100.

So this is indeed an alkene. Now, one of the reasons I’m not putting as much emphasis

on the fingerprint regions—so here, for example, you get some CH bands—is as you

get to bigger molecules the fingerprint region is going to become more and more complicated

and less and less easy to read.

And, part of the problem of the pedagogy of IR spectroscopy is it’s a relatively mature

area; it’s been around largely since the 1950s and used as a tool by chemists but the

molecules that we study have gotten bigger and more complex and people for pedagogical

purposes still tend to look at small molecule examples.

This is a small alkene, maybe octene or something. But as you get to larger and more multi-functional

region molecules, this region gets harder to read.

Alright, so what else do I want to do right now?

What about the second example here? Alkyne, yeah. Terminal alkyne.

And so, some of the things that we see here is this band at about 3310. There’s another

band at about 2119 and the band at 3310 is quite sharp and so that’s very characteristic

of an alkyne.

And so indeed we have an alkyne. This region at 2000 jumps out at me and says at about

2000-2100 you have something jumping out.

In general alcohols will be broad; if you have an alcohol that’s not hydrogen bonded—in

pure form alcohols tend to hydrogen bond to each other, if you have them very dilute they

tend not to hydrogen bond but then they tend to be about here over at about 3400-3500.

Silanols could even be a little bit further.

If the alcohol is very sterically hindered, like a tertiary alcohol, you’ll have less

hydrogen bonding than if it’s more sterically hindered, like a primary alcohol and so you

may see a [inaudible] band at about 3400-3500.

Alright I want to go back to scratching out more examples on the blackboard.

Alright, so alcohols, as I said your OH if you’re neat—neat is just another way of

saying not dissolved in a solvent for example on a film or a solid plate or a KBr pellet

because on a KBr pellet if you are dealing with a solid or a [inaudible] you’re going

to have particles of your alcohol that are hydrogen bonded together and [inaudible] molecules

that hydrogen bond together.

But usually you’ll see a band at about 3300, it’ll be broad; and on the pattern recognition,

again, if I’m drawing these two points in my mind’s eye say the end of the spectrum

at about 4000—I guess the examples I have here may run to 4600—what you’re going

to be seeing is a band that sort of picks up at about 3600 and comes down maybe somewhere

around 3000, and then of course you’ll have some alkyl stuff over here.