and the again what I want to continue with is the idea

of being able to recognize some patterns,

pick out important classes of compounds

and probably get mastery

of maybe two dozen different categories of compounds.

We'll also take a look,

at the end at a few more specialized examples

and see some things that you can tell with IR

and just a few neat things and ways it can talk to you.

But let's start with some basic nitrogen containing compounds,

so amides continue our discussion of carbonyl groups.

We talked about the amide carbonyl band and we said

of all the common functional groups

of all the common carbonyl compounds.

Amides have the lowest carbonyl stretching frequency at 1650

to 1690 and sometimes this is referred to as the amide 1 band

and we'll see in a moment what the amide 2 band is.

Now what's interesting is you often end up looking for things

in pairs, looking for sets of data that point

in a particular direction with IR spectroscopy

so the other thing for primary and secondary amides,

that is amides where you have either two hydrogens

or one hydrogen on the nitrogen is you're going

to have NH stretching frequencies.

So if primary amides, I'll write it as primary amide compounds

where you have NH2 groups have two NH stretches

and by now you should all now what are these NH stretches

in terms of of their type?

>> Symmetric.

>> Symmetric, NH symmetric and their frequency is going

to depend obviously it's going to be everyone by now knows

that that region above 3000 is where you get OHs and NHs

and so forth and it's going to fall in that general region.

Amides have just like carboxylic acids they're very prone

to hydrogen bonding and so often you'll see different behavior

with different states.

So for a primary amide in solution and I'll say dilute,

in other words not super-concentrated

so you don't have a lot

of hydrogen bonding those bands show up at

about 3520 and about 3400.

Remember my emphasis really has been on reading spectra,

in other words looking at a spectrum

and seeing patterns here and so when I look I sort of look

at that region below 3000 and I'd be seeing a band

and these bands tend to be kind of broadish, not as broad

as your typical alcohol bands but certainly not

as sharp as the CH band.

Hydrogen bonding weakens the NH stretch and so

in the solid state they're certainly hydrogen bonded even

when you grind up your molecules with KBr or grind them

with [inaudible] or grind with mineral oil

to make a mole the molecules are still in particles micron

in size and so forth, so they're all hydrogen bonded together.

So in solid, and I'll put parentheses hydrogen bonded,

the band shift to lower frequencies

and they typically broaden out I'll say about 3550, about 3350

and 3180 and the pattern changes a little bit.

I'll try to graph things on the same scale here

so now we fall below 3500.

The bands typically become a little bit broader

so you'll see this kind of like an alcohol stretch

over here becoming broader.

So just like we talked about carboxylic acids, amides,

particularly primary amides

and lactams can form these hydrogen bonded ring structures

and are quite prone to doing so.

And the reason, so primary amides are more prone

to doing this and as are lactams and the reason is

that by the time you get to a secondary amide,

that is where you have one R group on the nitrogen,

typically the hydrogen prefers to be transed [phonetic]

to the carbonyl group so it doesn't set itself

up as nicely performing ring structures.

You can still form chains but they don't tend

to form as extensively.

Secondary amides also have an N H stretch, so again what I drew

over on the other blackboard is the secondary amide

and typically you're talking about an NH stretch

from about 3400 to about 3500 and if you're hydrogen bonded

or about 3000 about 3300 if it's hydrogen bonded, so in addition

to these bands which are called the NH stretch bands,

as I said you also have for primary

and secondary amides what's called the amide 2 band.

The amide 2 band is an NH bend and that typically is

on the order of about 1500 to about 1650 and it may fall

on top of the amide 1 band in which case you wouldn't see it.

All right, I'd like to help us get in the habit

of the pattern recognition on these and start

to see the differences in these classes of compounds just

so you get good at seeing what's out there.

So let me pass out-- I've got some hand-outs here.

Let me send these down and we'll look at a few spectra.

You might need one more.

Send the extras back there.

All right, so this might be, might also be nice,

a nice time for a little bit of a question here.

So we're on the subject of amides

so we know we have an amide here.

The problem here, this was taken from a book.

The problem says, we have a compound

with a molecular formula, C6H13NO

and it contains the tert butyl group.

See if we can figure out the structure of this compound.

[ Silence ]

>> Anyone have an idea?

>> Just a guess.

There's a methylene group off [inaudible]

>> So a methylene group that's attached

to the tert-butyl group.

So we've used up our-- so what do we do with this?

So we have two spectra here.

We have one spectra in chloroform solution

and I'll tell you it's a concentrated spectrum

so this is-- you notice these numbers here

so we have one number at 3400 and another number at 3300,

so clearly, clearly we have at least one hydrogen bonded

at least something in our NH is hydrogen bonded

and they don't look quite like that

so is it primary or secondary?

It's got to be a secondary amide

so what do we do with the structure?

:6

[ Multiple responses ]

So it's CH3 and a tert-butyl and like so or the other way

and with this level

of information we can't tell the difference between it

and in fact this structure is the first one here.

Now there's one-- question.

>> Yes, so why are there two [inaudible]

>> Ah, okay.

So we have, so this is tert-butyl acetamide.

It's a secondary amide.

This is our free NH.

This is hydrogen bonded, so typically you'll be working

with like a 5 or 10 percent solution so many

of the molecules are monomeric and you see a band

for the NH stretch at about 3400.

You're also going to have some cases in which the molecules

or hydrogen bonded together like so either to form two

or more molecules and so the hydrogen bonded NH stretch is

going to be the one at about 3300 wave numbers.

[ Inaudible Student Question ]

For a primary amide?

>> Yeah.

>> They're a little bit different in size.

Usually the one that's the higher wave number is slightly

longer in length.

>> So how will we know if you have a high rate [inaudible]

and if it's concentrated?

>> Ah, okay so great question.

So what experiment could we do with this sample in chloroform

to determine whether the band at 3300 was associated

with hydrogen bonded dimer formation diluted

and what would you expect to see?

[Multiple speakers] The decrease in the 3300

or more specifically both bands would of course decrease

because it was diluted but the relative intensity

of the band at 3300.

[ Inaudible student question ]

Based on the spectrum alone if I looked

at this pattern here I'd say well this clearly doesn't look

like a monomeric primary amide because I said you'd expect it

at about 3,520 and about 3400 and it probably doesn't look

like a hydrogen bonded primary amide.

First of all this band at 3300 is a little bit high

and it's a little bit narrow.

Remember, I said typically 3180

but if you saw my steps you would sort of have broad

like an alcohol where it basically spanned

about 300 wave numbers.

So you might or might not be able to do this

with pattern recognition.

I mean, as you see more of these spectra you'll get better

and better.

Now what's happening here

in the solid state is you're only having the hydrogen bonded

bands so you're seeing just the hydrogen bonded NH.

Other thoughts and questions?

These are good ones.

>> Can you explain that little shoulder that's on the--

>> That little shoulder, right here.

>> Like you have a little shoulder that's

on the peak at 3268?

>> The peak, which spectrum?

>> On the second spectrum.

>> The peak at, the shoulder that's on there?

Probably different states of hydrogen bonding, probably some

with more hydrogen bonds for example,

you can have a bifurcated hydrogen bond

so it may be something like that.

It's something clearly that's characteristic

of the solid state.

Now we're dealing with and again this is what I was saying

for this sort of pedagogy of spectroscopy.

We're dealing with a relatively small molecule here.

It's only got six carbon atoms.

By the time you get to bigger molecules the relative intensity

of your NHs is relative to for example, to your CHs is going

to be less and this is where I said KBr is really bad

because you typically, it's hard to get out every last bit

of water and it doesn't matter so much for a molecule this size

but if you're dealing with a molecule say the size

of strychnine you're going to have almost ten times,

let's say five times the carbon to nitrogen ratio or carbon

to amide ratio so that peak's going to be a lot smaller.

You're really going to have trouble seeing it.

Okay the one other thing I wanted to bring up was just

about problem solving strategies on this

and I presume everyone's seen this but it's worth bringing

up the idea, so this molecule was C6H13NO

and the first thing you want to think

about when you're asking what's this compound is the

unsaturation number and you can do that mathematically.

In other words, how many double bonds and rings are in there?

That's an incredibly useful piece of information,

in fact molecular formula is probably the most useful

information that you can get for starting

with a compound followed by what functional groups are in it

which is why we're starting the course with IR and mass spec.

So there are a couple of ways you can do it.

Honestly, I never do it the way I'm about to write it.

The unsaturation number is the number

of carbons minus the number of hydrogens

over 2 minus the number of halogens over 2.

I hate formulas.

I don't even keep formulas in my head, plus the number

of nitrogens over 2 plus one and if you plug

in right that's 6 minus 13 over 2 plus 1 over 2 plus 1 is equal

to 1 which is one double bond of carbonyl group.

I never do things this way.

Basically, the way I think about it is

if a molecule has 6 carbons in it,

if it were an alkane it would be C6H14.

If it has halogens in it they just count as hydrogen

so in other words, C6H14 is saturated C6H13Cl is saturated.

Oxygens don't count toward unsaturation numbers.

Sulphurs don't count toward unsaturation number.