Placeholder Image

字幕表 動画を再生する

  • >> Today's lecture is basically going to show you that a lot

  • of the rules that you learned in sophomore organic chemistry

  • like the N plus 1 rule are simplifications

  • and we'll be seeing a lot of examples that look really weird

  • because I want to show you where these simplifications break down

  • and then next time we'll get to more sort

  • of the typical rules of splitting.

  • So as I said, I wanted to begin with this notion

  • of magnetic equivalents and let me begin with a definition.

  • We'll say magnetic equivalents and let's say that 2 protons

  • or nuclei, in other words, we're normally talking

  • about proton NMR but, of course, it could be fluorines

  • or phosphorescence or whatever are magnetically equivalent.

  • [ Writing on board ]

  • If they are chemically equivalent.

  • [ Writing on board ]

  • And remember 2 nuclei that were chemically equivalent were

  • nuclei that were interchangeable by a symmetry operation

  • or a rapid process like rotation about a bond.

  • So we saw lots of examples of chemically equivalent nuclei

  • by symmetry and then we saw how when you have, for example,

  • a chiral center, methylene group is no longer

  • chemically equivalent.

  • So chemically equivalent.

  • So there is a subset of chemically equivalent

  • and they have the same geometrical relationship

  • to all other nuclei in the spin system.

  • [ Writing on board ]

  • So this brings up one other concept and that's the question

  • of what is the spin system

  • and the spin system is just a complete set

  • and I'll underline complete meaning all of the nuclei.

  • So complete set of nuclei in which members are coupled.

  • [ Writing on board ]

  • All right this is, let's start with the easy idea,

  • the spin system, and let's just do this by example.

  • So, for example, if we have ethylproplether, CH3CH20, CH2,

  • CH2, CH3, we have in the molecule 2 spin systems.

  • We have 1 spin sustain comprising the ethyl group

  • and another spin system comprising the propyl group.

  • So in other words, the ethyl group is a set of nuclei.

  • Obviously we're talking about the protons

  • since for all intents

  • and purposes there are no C13s in this fragment.

  • So we have these hydrogens and these hydrogens

  • and at least one member is coupled to every other member.

  • They make up a set together.

  • In the propyl group, we have the methyl group,

  • the methylene group and the other methlyene group

  • and there's coupling among them.

  • In other words, the CH3 is coupled to this CH2,

  • the hydrogens of the CH2 are coupled to each other,

  • the hydrogens or the CH3 are coupled but that doesn't count.

  • The hydrogens of the CH2 are coupled

  • to the next CH2 and what's important.

  • So each of these is complete meaning it takes

  • in all the coupled nuclei.

  • What's important is we don't have coupling

  • between the ethyl spin system and the propyl spin system.

  • So they kind of have 2 separate sets

  • so we can consider the ethyl, we can consider the propyl

  • and there's no interaction between them.

  • Let's try another example.

  • Let's take a acetyl phenylalanine

  • and we'll take the methyl, the mid.

  • [ Writing on board ]

  • So what are the spin systems in this molecule?

  • [ Writing on board ]

  • So you have the 2 methyls on the end and the benzo group.

  • [ Pause ]

  • For all intents and purposes the benzylic protons are not coupled

  • to the phenyl so what would you do for the spin system here?

  • [ Inaudible question ]

  • Separately so I'm going to revise this, okay,

  • so we have the phenyl that's going to be 1 spin system.

  • What do we do here in the middle?

  • >> Couple those 2 together.

  • >> Alpha and beta.

  • What about the NH?

  • [ Inaudible question ]

  • We saw an example where I said you have an ND20

  • and you exchange so it's deuterium there,

  • which although it has a spin for all intents

  • and purposes you can discount but what about this NH?

  • Is that going to be J coupled?

  • Remember, amides are different than alcohols.

  • Alcohols exchange amides and, remember,

  • I said alcohols can exchange or cannot amides

  • on the laboratory timescale if I throw them in D20 will exchange

  • but on the NMR timescale that NH stays there

  • and we're not doing this in D20 so what should I do

  • with this middle part of the molecule?

  • So that all becomes a spin system and what

  • about the very end of the molecule?

  • [ Inaudible question ]

  • Okay, good.

  • So these guys interact.

  • Now, I'll tell you right now so we have 4 spin systems

  • in the molecule and we have the methyl group, we have the NH,

  • the alpha and the beta protons and we have the phenyl group

  • and then we have the methyl and mid group.

  • So forced in systems in a molecule and we can look

  • at each of these separately.

  • I'll tell you there's a miniscule

  • like undetectably small and I'll show you how to see it

  • if you squint right later on coupling between these hydrogens

  • and the benzogroup, but for all intents and purposes you can say

  • that the phenyl group is not coupled over here.

  • So for all intents and purposes we have this is an isolated spin

  • system, a phenyl group, the alpha, beta and NH,

  • the methyl and methyl NH.

  • Other thoughts?

  • [ Pause ]

  • [ Inaudible question ]

  • Yeah, yeah, and so in chloroform solution, in D20,

  • these would eventually wash out

  • but in chloroform solution what we'd see

  • for this NH is a doublet.

  • It would be a little bit broad.

  • This one is going to have 3 coupling partners

  • in chloroform solution where this has an exchange or in DMSO.

  • So we'd see either a doublet of doublets of doublets

  • or a triplet of doublets or doublet of triplets

  • and we'll talk more about that

  • if you're not familiar with those terms.

  • This NH is going to have 3 coupling partners

  • in chloroform solution

  • where this hasn't exchanged or in DSMO.

  • So we see either a doublet of doublet of doublets or triplet

  • of doublets or doublet of triplets and we'll talk more

  • about that if you're not familiar with those terms.

  • This NH would appear as a quartet

  • and chances are it would be broadened out a little bit.

  • Remember, I mentioned this nitrogen quadrapolar coupling?

  • So couple of ways this can appear so it can appear as a 1

  • to 3, 3 to 1 quartet slightly broadened.

  • It can appear just due to this nitrogen quadrapolar broadening

  • as an envelope that encompasses the whole thing.

  • Or it can appear as something where if you don't see the wings

  • of the quartet and you just see a little dip you might say, oh,

  • it looks like a doublet to me.

  • So depending on the quadrapolar broadening

  • from the nitrogen this methyl group in turn is not going

  • to have significant quadrapolar effects;

  • it's going to be split into a nice doublet.

  • So this will be a quartet or broad quartet or something

  • that looks like a single.

  • If it's very broad, the methyl group will be a doublet.

  • [ Pause ]

  • All right let's now tack all this notion

  • of the same geometrical relationship.

  • So, let's look at this molecule.

  • Let's take 2, 6 dichloro 1 tert butyl benzene.

  • So as far as chemical equivalents goes,

  • we have 2 types of protons.

  • We have the proton that's powered to the tert butyl group

  • and the proton that's meta to the tert butyl group.

  • So these 3 constitute a spin system.

  • Chlorines don't count; they're quadrapolar nuclei

  • and essentially not spin active.

  • The tert butyl group is magnetically isolated;

  • it's its own spin system.

  • So we look at this and we say, all right, we have 2 protons

  • that are the same as far as chemical equivalents;

  • they're interchangeable by a symmetry operation.

  • Now we ask this geometrical question.

  • Do they have the same relationship

  • to all other nuclei in the spin system?

  • This hydrogen says, oh, look, I'm ortho to this hydrogen

  • and this hydrogen says, oh, look, I'm ortho to it also.

  • So, these 2 are magnetically equivalent as well

  • as chemically equivalent.

  • [ Pause ]

  • Now there's a way of naming systems

  • where you have different types of protons

  • and we'll give a different letter to each type

  • of non-chemically equivalent proton.

  • So, for example, we'll use letters like A and B and C and M

  • and X and Y if you need to.

  • The general idea is if the protons are close

  • in chemical shift we'll use letters that are right next

  • to each other in alphabet, As and Bs and Cs.

  • If they're far apart in chemical shift, we'll use letters

  • that are far apart in the alphabet.

  • Letters like A and X or A and M and X. So depending

  • on whether these protons are close in chemical shift

  • to the center proton or whether they're fall in chemical shift,

  • we'll either call this an A to B spin system

  • or an A to X spin system.

  • Now, technically only ones

  • where they're far apart are truly first order,

  • but even if they're close there's some very regular

  • patterns that you can see.

  • If they're far apart in chemical shift

  • and by far apart what I mean is the separation

  • of the peak centers in hertz is many,

  • many times the coupling constant.

  • So like a typical ortho coupling constant is about 7 hertz,

  • so if the peaks are far apart like 10 times as far apart

  • like 70 hertz or 100 hertz or 200 hertz apart,

  • then they will end up being As and Xs.

  • Now, remember, at 500 PPM, 1 PPM is 500 hertz.

  • So, in other words, if these guys are about two-tenths

  • of a PPM apart, you know, three-tenths of a PPM apart,

  • we would call this an A to X spin system.

  • What we'd expect would be to see a doublet for the 2

  • on the outside because they're being split by the 1

  • in the middle and a triplet

  • and so I'll just draw a little squiggly

  • to indicate these are far apart in the spectrum and a triplet

  • like so for the center hydrogen.

  • I guess technically the triplet would be shorter

  • than the doublet so I'll make the doublet a little bigger.

  • If they're close together, and I'm going to actually start

  • in just a moment with the archetypical AB system,

  • if they're closer together, what you will see so if it is,

  • indeed, A to B, what you'll see is a slight tenting inward

  • depending on how far.

  • In other words, the lines of the doublet instead of being equal

  • in height will become unequal in height

  • with the bigger line toward its J coupling partner and the lines

  • of the triplet will be similarly distorted

  • so that the inner line is a little bigger

  • than the outer line.

  • I always like to think of these as sort of tenting

  • in toward each other and that would be what it would

  • like as an A to B system.

  • [ Pause ]

  • Let's try another example and I'll take difluoromethane.

  • [ Writing on board ]

  • Remember, fluorine is spin active, spin of a half,

  • it's magnetic gyro ratio [phonetic] is

  • about 90% of that of a proton.

  • So it shows up a million miles away whereas your protons are

  • resonating at 500 megahertz, your fluorine is resonating

  • at 470 or 460 megahertz.

  • So they are far, far, far away from each other,

  • but they're J coupled to each other

  • so collectively the hydrogens

  • and the fluorines constitute a spin set, a spin system.

  • If I want to remember my geometry because we're going

  • to ask what type of spin system it is,

  • it's a tetrahedral molecule.

  • So the geometrical relationship of this hydrogen

  • to the fluorine is the same as the geometrical relationship

  • of this hydrogen to the fluorine.

  • In other words, we would call this spin system an A2,

  • X2 spin system.

  • We have 2 hydrogens that are chemically equivalent,

  • they're interchangeable by a symmetry operation,

  • and 2 fluorines that are chemically equivalent,

  • they're interchangeable by a symmetry operation reflection,

  • and the hydrogens if you test everyone has the same

  • geometrical relationship to all other nuclei in the spin system.

  • So this is an A2, X2 spin system.

  • [ Inaudible question ]

  • No. Just that they're farer away.

  • As I said, if these guys are more than a few tenths

  • of a PPM away, we would call this X in this case

  • and if there were 2 of them

  • in some circumstances we'd call them X2.

  • So, if you look at the H1 NMR spectrum or the F1 NMR,

  • you would expect the H1 NMR spectrum

  • to show up as a triplet.

  • [ Pause ]

  • Now, I want to contrast this example

  • with another example difluoroethylene.

  • [ Pause ]

  • How would we characterize that spin system?

  • [ Pause ]

  • What?

  • [ Inaudible question ]

  • ABXY, okay, let's start with this issue

  • of chemical equivalence here.

  • So are the 2 hydrogens chemically equivalent

  • to each other?

  • Okay so these guys are chemically equivalent

  • and the 2 fluorines?

  • Yeah, okay, chemically equivalent.

  • So we're going to use the same letter

  • but we have a problem now they're not

  • magnetically equivalent.

  • So we need to introduce another term.

  • When we have hydrogens that are chemically equivalent

  • but not magnetically equivalent, we'll use --

  • or nuclei in general -- we use primes.

  • So what we'll do is we'll call this an A, A prime,

  • X, X prime spin system.

  • The big difference is that while a system that's an A2,

  • X2 system is first order and even these types

  • of systems I'll call pseudo first order.

  • This is a spin system that is distinctly not first order.

  • I want to show you the difference between them.

  • [ Pause ]

  • Okay, so on the top I have, and these are very old spectra.

  • These are spectra from a book that were taken at 60 megahertz

  • and they're probably from the 1960s.

  • They're taken on the CW instrument.

  • CW instruments are non-fourier transform instruments not used

  • anymore, but these little wiggles are just artifacts

  • of it being a CW instrument.

  • So don't worry about that.

  • But the main thing here is the difluoromethane,

  • these are proton spectra, is exactly what you would expect.

  • It's a triplet.

  • The difluoroethylene you'll look at and you say what's going on.

  • There was no simple description of this pattern.

  • [ Pause ]

  • This type of pattern can be calculated, indeed,

  • our NMR spectrometers have software that will calculate it

  • and it can be calculated by computer program,

  • by back of the envelope calculations for simple systems,

  • but basically defies a simple description.

  • [ Inaudible question ]

  • Why are the hydrogens, which one?

  • [ Inaudible question ]

  • Well, here but it's not the hydrogens here,

  • it's the fluorines are splitting the hydrogens.

  • So the difluoromethane is a triplet

  • because the 2 hydrogens are split by the 2 fluorines.

  • In the case of the difluoroethylene,

  • the problem is the adage that we use that hydrogens

  • that are the same don't split each other really replies most

  • rigorously to hydrogens that are magnetically equivalent,

  • but hydrogens that are chemically equivalent

  • but not magnetically equivalent kind of sort of do.

  • There are many circumstances where for all intents

  • and purposes you don't see any effect and these are cases

  • that I'll call pseudo first order cases, and what I want

  • to show you today is how all the stuff that we learned

  • in sophomore chemistry

  • for coupling really doesn't rigorously apply to lots

  • and lots of common systems.

  • Now the first thought when you look at this is, okay,

  • well this is, you know, this is difluoroethylene,

  • it's not a common system.

  • So let's take a common system and it's one that you're going

  • to see in the course of your graduate career most likely.

  • So this is dioctyl phthalate.

  • It's commonly used as a plasticizer

  • in all sorts of plastics.

  • If you go to the store and buy yourself a water bottle

  • and it says phthalate free, that's probably saying

  • or you see plastics listed as phthalate free,

  • that's saying it doesn't have this plasticizer,

  • this compound added, this oily compound,

  • to make plastics pliable.

  • Tygon tubing is great with water, however,

  • because it contains dioctyl phthalate if you use it

  • on your manifold methylene chloride

  • and THF vapor will dissolve into the Tygon,

  • dissolve into the dioctyl phthalate diluted

  • and it will dribble into your reaction flask

  • and when you work your spectrum up, you'll see the spectrum

  • and when you work your compound up you'll see a spectrum

  • for this and you'll say what the heck?

  • What's going on?

  • Let's take a look at the molecule and figure

  • out what sort of spin system it is.

  • So we have the octyl groups which are separate

  • and we have the benzene groups.

  • How do we characterize the benzene here?

  • [ Pause ]

  • AA prime, BB prime.

  • Indeed. Or if they're far apart in chemical shift?

  • AA prime, XX prime.

  • So in other words, if these guys are within two-tenths of a PPM

  • of each other with typical couplings,

  • you might call it AA prime, BB prime if they're more

  • than a couple of tenths of a PPM apart you'd probably call it AA

  • prime and XX prime.

  • What that says is your rules

  • of simple coupling may not apply here.

  • So many a graduate student has taken their reaction mixture

  • and gone and seen the following pattern in it and come

  • to their advisor or their group mates

  • and said what the heck is going on?

  • This is dioctyl phthalate and, again,

  • it defies a simple description.

  • This is an AA prime, XX prime system

  • and it doesn't matter how high a field you go to the spectrum

  • of dioctyl phthalate is going to look like hell.

  • The only difference as we change field strength is you see how

  • this line, these lines on the inside are a little bigger

  • than these lines on the outside?

  • If we went to a lower field spectrometer they'd be a little

  • more unequal if we went

  • to a higher field spectrometer they'd be a little bit more

  • equal, but no matter what the spectrum

  • of dioctyl phthalate is going to look like that

  • and that should freak you out just a little bit

  • because it says all that stuff you learned

  • in sophomore chemistry kind of sort of applies

  • but kind of sort of doesn't.

  • [ Inaudible question ]

  • Nothing. So the neoprene is great for your manifold.

  • The one that's even better, so that has more, is more permeable

  • to air and so it's okay.

  • The one that's really good is butyl rubber

  • or I think it's a nitryl;

  • I think it's a butyl nitryl co-polymer.

  • Anyway that's particularly good for manifold lines

  • but stay away from Tygon.

  • Tygon is meant for aqueous solutions.

  • All right well now that I've messed with you a little bit

  • for your own good, really, it's for your own good,

  • now that I've messed with you for a little bit,

  • let's take a look and see when the rules

  • of sophomore chemistry apply

  • and when they don't necessarily apply rigorously and let's play

  • with the implications of this.

  • All right so if I look at chloro ethane, now my first thought

  • and I'll do a Newman projection on it.

  • My first thought is wait a second the methyl group we talk

  • about geometrical relationships and this is confusing

  • because this hydrogen is parallel to this hydrogen

  • and this hydrogen is ortho to these hydrogens, but of course,

  • there's rapid rotation so let's see what happens.

  • So remember I said rapid processes can equal things out.

  • So we'll call these hydrogens 1, 2,

  • 3 and we'll call these guys 4 and 5.

  • I'll just imagine a series of rotations.

  • [ Writing on board ]

  • So you have 3 equal rotamers and although H5 is anti to H2

  • in the first rotamer, we have the second equal rotamer,

  • they're all equivalent, they're all equally populated.

  • Where H2 is, goes to H5 and H1 is anti to H5

  • and this third were H3 is anti.

  • So in the end, it all evens out and if we want

  • to describe this spin system how would we describe it then?

  • [ Inaudible question ]

  • Which, take a guess.

  • We've talked about the chemical shifts of dichloroethane.

  • [ Inaudible question ]

  • AX and what specifically would we use for numbers?

  • How many hydrogens are there in the methyl group?

  • Three.

  • [ Inaudible question ]

  • So what sort of spin system?

  • It's an A3 X2 spin system.

  • All of these spin systems were there no primes are truly first

  • order and while the Bs when you get things

  • that are close it'll deviate from first order but if they're

  • on top of each other, it gets really messy

  • but if they're a little bit separated,

  • you basically can call it an AB type of system.

  • All of these spin systems are first order.

  • [ Inaudible question ]

  • First order means those simple rules of coupling count

  • up number of different types of neighbors work out perfectly.

  • So an A2 X2 spin system you can trust is going

  • to give you a triplet and another triplet in the case

  • of difluoromethane the other triplet is

  • at 460 megahertz whereas 1 triplet is 500 megahertz

  • but you get 2 pure triplets and in the case of here,

  • we expect a triplet and a quartet

  • and everything is hunky dory.

  • Now let's take a look at a different compound.

  • Take a look at bromochloromethane

  • and we're going to do the same thing.

  • I'll Newman project, I'll put the chlorine on the back

  • and the bromine on the front and we'll call it H1,

  • H2 and call these H3 and H4 and I'm just going

  • to imagine the rotamers.

  • [ Writing on board ]

  • So we have 2 ghost [phonetic] rotamers and 1 anti rotamer.

  • So we can consider the ghost rotamers as a pair

  • but they're separate from the anti rotamer.

  • All of them, of course, are interconverting

  • but let's just look at the anti rotamer

  • because the anti rotamer isn't equivalent

  • to the 2 ghost rotamers

  • and you'll see the conundrum that we end up with.

  • See the problem we end up with in the anti rotamer

  • and in this here you can say H1 and H2, one is ghost

  • and then the other gets to be ghost.

  • You can say let's consider them as a pair.

  • Here we say, okay, well, the bromine is anti,

  • these 2 have the same relationship

  • and they're interchangeable by symmetry so H1

  • and H2 are symmetrical.

  • H3 and H4 are symmetrical, but the problem particular

  • to the anti rotamer is that when you apply this test

  • and say what's their geometrical relationship, it's different.

  • So you look at the anti rotamer and you say this is going

  • to be an AA prime XX prime or AA prime, BB prime spin system

  • and the problem is this is

  • that situation that's truly not first order.

  • [ Writing on board ]

  • Now the good news on this is most of the time,

  • and I've just given you some of the ugliest examples,

  • most of the time non-first order systems can be approximated

  • as first order systems.

  • In other words, you ask your sophomores what this spectrum

  • should look like and they'll say, oh, well,

  • they told me the N plus 1 rule it should be a triplet

  • and a triplet.

  • That's largely true, but I'm going to show you cases

  • that break down from that.

  • Now I want to show you the sort of break

  • down from non-first order

  • to let's call it pseudo first order.

  • So, in other words, if we take a simple AB system

  • or a simple AX system, let's say I do bromine, bromine, chlorine,

  • chlorine, so this is a simple AX system

  • if they're far apart you'd see a doublet and a doublet and,

  • again, I'll draw a break to indicate that they're far apart

  • and if they're closer together,

  • we see what's called an AB pattern where the 2 tent

  • into each other and if they're very close you'll see an AB

  • pattern with the inner lines very big

  • and if they're really close you might even mistake it

  • for a quartet; it's not, it's an AB pattern.

  • Anyway so most of the time you can get away approximating

  • non-first order systems as sort of pseudo first order systems.

  • [ Writing on board ]

  • In other words, often non-first order systems will show behavior

  • that's very much like you would expect with just the notion

  • that inner lines may become a little bit bigger, but as we saw

  • in our example with dioctyl phthalate,

  • you can have some very, very big deviations and so what I want

  • to do now is show you really a catalog of typical deviations

  • because once you see them and once you see when they come up,

  • I think you'll be much less freaked

  • out by things that occur.

  • [ Pause ]

  • So the scary thing about the diagram that I made

  • on the right hand board is any time you have a methylene chain

  • technically it is not first order.

  • Every pair of methylenes, every methylene the pair

  • of hydrogens technically they are chemically equivalent

  • but not magnetically equivalent.

  • Of course, if there's a stereocenter in the molecule,

  • they're not chemically equivalent then it's like an A2,

  • an ABX system but in the case of just a plain methylene chain

  • without a stereocenter they are chemically equivalent

  • but not magnetically equivalent.

  • Normally you can get away and you say, okay, they taught me

  • as a sophomore the N plus 1 rule, I expect to see a triplet.

  • Normally it works pretty well.

  • I'm going to show you some cases where we see some very,

  • very big deviations and I want to show you

  • where these things come up.

  • So, this is, these are 2 different molecules

  • that I've worked with.

  • One of these has a propyl chain connecting an azulene a very

  • bulky group to a phenyl group and so you'll look

  • at the protons on this chain and you say, okay, they all look,

  • this methylene, that looks kind of reasonable.

  • Looks like a triplet.

  • You could call it an apparent triplet if you liked,

  • but the one that's right next to the azulene, this very,

  • very bulky group, really ends up looking very, very funny.

  • You see this pattern that has what kind of looks

  • like a triplet except in the center it's further split

  • and the one over here

  • in the middle also looks a little funny.

  • It doesn't look quite like a quartet.

  • See, the thing is with CH2 chains if you've got a mix

  • of anti and ghost confirmers [phonetic] and you've got some,

  • you know, anti but also some ghost, basically it averages

  • out enough that it behaves like a first order system.

  • It behaves like you were taught it should

  • in sophomore chemistry; however,

  • if it's heavily biased toward the anti confirmer,

  • then just as we saw in dioctyl phthalate this really funny

  • splitting you see the same thing

  • and this group is very bulky here.

  • So, in other words, when you're looking at this,

  • you have the azulene group and then you have the hydrogens

  • and then it is almost completely locked in the anti confirmer

  • and so you really, really end up seeing this.

  • So this ends up being an A, A prime, M, M prime X,

  • X prime spin system and that's what it technically is

  • so technically it's non-first order but we get

  • that effect full force over here.

  • You'll notice these types

  • of patterns come up again and again.

  • So here's a very different compound

  • where you still have a trimethylene chain

  • but now your bulky group is this TMS group

  • and the hydrogen that's next to your bulky TMS group, again,

  • gives this exact same pattern.

  • Completely different molecule but exact same pattern

  • of non-first order behavior.

  • I would just call this peak a multiplet.

  • I would call all of these multiplets

  • and I would simply list their range.

  • I guess I'd call this guy a triplet.

  • [ Inaudible question ]

  • Exactly. I mean we have

  • to understand this is a non-first order system

  • and that normally we can get,

  • often we can get away describing non-first order systems

  • as first order but you really can't always.

  • Let me show you some other non-first order behavior.

  • So, okay, so most of the time take bromopropane,

  • another molecule with a chain in it and remember most

  • of the time you can get away describing things

  • as first order.

  • So you look at your bromopropane and you'd say, oh,

  • that looks pretty good.

  • You have a triplet, you have a sextet,

  • you have a triplet, beautiful.

  • Just what you would expect from sophomore chemistry.

  • Now, another sort of breakdown that occurs is when protons end

  • up lumped on top of each other even

  • when they're not chemically equivalent.

  • So you go from bromopropane, which has a beautiful triplet

  • to bromobutane and you say, okay,

  • it still has a beautiful triplet,

  • we see all of our resonances dispersed.

  • You go to bromopentane.

  • [ Pause ]

  • And now these 2 protons, the protons at the beta

  • and gamma position are starting to get very close to each other

  • and your triplet just starts to look a little funny.

  • You can already see there's some tenting in there.

  • That's just your AB behavior; that's perfectly normal,

  • but now you see the methyl is starting to fatten out

  • and by the time you get up, so these guys are really,

  • really lumped on top of each other.

  • So by the time you get up say to bromohexance

  • where now these 2 protons or 3 protons are really,

  • really let's see we've got alpha, beta,

  • let's see that's beta, okay, so now we end

  • up with these guys really lumped on top of each other

  • and now you notice our triplet really is break down

  • and it doesn't look like a clean triplet.

  • You could still call it an apparent triplet

  • but it's much uglier than you would expect

  • and this is exactly what I'm talking

  • about for non-first order behavior.

  • You get up to bromo octane and now you see it even more so.

  • So this is what I'm talking

  • about for non-first order due to overlap.

  • So in other words, you have all of these guys

  • in the chain overlapping with each other

  • and often what we will call this is virtual coupling.

  • In other words, when hydrogens are overlapping,

  • the methyl group is coupling to the adjacent methylene

  • but you can say in effect it's also coupling to these others

  • down the chain because they're right on top

  • of each other in chemical shift.

  • You'll see this effect it's extremely pronounced

  • and you've already seen this before.

  • Any of you who have seen a spectrum

  • of THF has seen this behavior.

  • Let me show you.

  • Succinic acid, no problem.

  • You're a singlet.

  • You have 4 chemically equivalent protons in the chain,

  • all 4 show up at the same chemical shift,

  • they don't split each other, you go to glutaric acid

  • and you'd say, okay, even though remember none

  • of this is a truly first order system, none of these compounds

  • with chains are a first order system.

  • You'd say that doesn't look bad;

  • it looks like what they taught me in sophomore chemistry.

  • I see a triplet for the outer 2 CH2s and I see a quintet

  • for the inner one, everything should be hunky dory

  • and then you come to adipic acid

  • and you say what the heck is going on?

  • What the problem is this business of virtual coupling,

  • which is just 1 way to say it's a non-first order system,

  • when you have these 2 methylenes right on top

  • of each other this methyl, methylene group looks at it

  • and says, well, I'm coupled to 1 but I'm virtually coupled

  • to the other and now these break

  • down into non-simple patterns here and it's reciprocal.

  • So this one looks and says well I'm coupled

  • to this I have my neighbor and he's coupled to that

  • and we're all lumped together and it's nothing

  • about the length of the chain;

  • it's all about this issue overlap.

  • So you go up from adipic acid to palmitic acid, 1 more carbon,

  • and everything is back to being hunky dory.

  • In other words, you look at your outer methylenes

  • and they each look at their neighbors

  • and even though it's not a true first order system they say,

  • okay, we're fine, we're not overlapping it'll behave largely

  • like a first order system and this is what I'm talking

  • about where you go ahead and say normally you can get away

  • with this sophomore level analysis.

  • Normally you can get away with treating things as first order

  • or pseudo first order systems but watch out because

  • like this example and like our azulene example,

  • we're really playing on thin ice and as I said these types

  • of patterns come up again and again.

  • So tetrahydrofurane has exactly the same principle

  • as adipic acid.

  • You have a methylene chain, you have this issue

  • of virtual coupling and so you have this hydrogen is virtually

  • coupled, the one next to the oxygen is virtually coupled

  • to the other 2 and everything is reciprocal

  • and so you see this pattern here

  • and you see this pattern in other places.

  • So we're very good as human beings at pattern recognition

  • and I've shown you 3 types, 4 types of patterns today.

  • I've shown you the pattern of a ortho disubstituted,

  • symmetrically disubstitute aromatic and we saw it

  • for phthalic acid, for dioctyl phthalate and you'd also see it

  • for orthro dichlorobenzene or any other ortho compound.

  • I showed you the distortion pattern for methyls that occurs.

  • I've shown you the pattern that occurs

  • in an extended methylene chain when it's locked

  • in an anti confirmation, and I've shown you this pattern

  • that you get where you get virtual coupling

  • in the middle of a chain.

  • So keep those in mind because you will see them again

  • in the course of your graduate career. ------------------------------fcaefe9298fb--

>> Today's lecture is basically going to show you that a lot

字幕と単語

ワンタップで英和辞典検索 単語をクリックすると、意味が表示されます

B1 中級

ケム 203有機分光学講義 11.磁気等価性、スピン系、ポプレ表記法 (Chem 203. Organic Spectroscopy. Lecture 11. Magnetic Equivalence, Spin Systems, and Pople Notation)

  • 54 1
    Cheng-Hong Liu に公開 2021 年 01 月 14 日
動画の中の単語