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  • >> All right.

  • Well, I will leave these and get started then, I guess.

  • All right, so I think I want to finish

  • up what I'll call basic NMR spectroscopy today.

  • In other words, things that all sort of fall at the level

  • of basic interpretation of structures, and in turn things

  • that we'll get on the mid-term.

  • I think probably where I'll pick up next time is going

  • to be introducing 2D NMR and then our next homework set,

  • not Monday's, but the one after that,

  • we'll start using 2D NMR spectroscopy.

  • So what I wanted to do today was to talk more

  • about carbon 13 chemical shifts.

  • And I gave you, when we talked last time, I gave you this sort

  • of general information that just like proton NMR, carbon NMR,

  • aliphatics tend to be upfield, aromatics tend to be downfield,

  • things that are next to electron withdrawing groups,

  • particularly oxygen, tend to be mid -- we'll call it mid field,

  • sort of in that 50 to 70 range.

  • I also indicated -- and we said

  • that the range is a lot, lot bigger.

  • It's about 20 times bigger in ppm for C13 NMR.

  • In other words, an aldehyde CH is at roughly 10 ppm whereas

  • in carbon NMR an aldehyde carbonyl is at roughly 200 ppm.

  • So it's sort of like 20 times bigger range.

  • Now, C13 shifts have a bigger range.

  • And there's also more richness.

  • In other words, when we talked

  • about proton NMR, it was pretty easy.

  • And we were able to come up with some really simple, you know,

  • back of the envelope calculations

  • where you could typically peg the chemical shift to within,

  • you know, a few tenths of a ppm.

  • We talked about if you're next to an ester, you know, let's --

  • or if you're next to an oxygen, figure you're going to be

  • about 2 ppm downfield of where you'd normally be.

  • If you're next to a benzene ring or a double bond or a carbonyl,

  • figure you're about 1 ppm further downfield.

  • And I gave you several ways of thinking about this.

  • And you should all be able to pretty much estimate things.

  • We talked about the effects of alpha substituents and said

  • that alpha substituents have a really big effect being next

  • to a CH next to a halogen next to a nitrogen next

  • to an oxygen shifts you down, you know,

  • a couple of tenths -- a couple of ppm.

  • We talked about beta effects.

  • And we said that they're smaller.

  • Beta effects in proton NMR shift you down by like .2 to .5 ppm.

  • C13 NMR is a little bit more subtle and a little bit --

  • I won't say less predictable, because we're going

  • to see it's actually quite predictable.

  • But the factors, there are more factors.

  • For example, gamma effects as well

  • as beta effects tend to be big.

  • And there are some really interesting steric effects.

  • Now, along with this richness comes tremendous power.

  • Because this means that carbon NMR also can give you tons

  • of rich information about structure

  • and can be really useful for figuring out structures,

  • confirming structures, disproving structures,

  • and at the end I'll show you a beautiful example

  • of fraudulent work that was disproven

  • by Professor Rychnovsky's laboratory.

  • And also going ahead and basically having a tool

  • that can get you a lot more information than meets the eye.

  • So, I want to show you some of the factors that contribute

  • to these sorts of general ranges,

  • and particularly now, perturb them.

  • And let's start with something pretty simple, inductive effects

  • and resonance effects.

  • [ Writing on Board ]

  • And electron density, of course, plays a huge role

  • in chemical shift because electrons contribute

  • to the shielding or de-shielding

  • if they're absent, of various nuclei.

  • So, if you have substituents that increase

  • or decrease the electron density in a carbon, you're going

  • to shift that carbon upfield or downfield.

  • Let me show you what I mean.

  • We'll start with a simple benzene example.

  • Now, the easiest way to, in your mind,

  • think about chemical shifts is to start

  • with a base value and then perturb it.

  • So a great way to think about benzene is the normal position

  • for benzene is 128.5 ppm.

  • And then if you put substituents on it,

  • you perturb things in a rational way.

  • So let me show you what I mean.

  • If we put a methoxy on the benzene,

  • the oxygen is electron withdrawing

  • through the sigma bond.

  • And so the carbon attached

  • to the oxygen shifts substantially downfield.

  • So you go to 160 ppm.

  • In other words, you shift downfield by 30 ppm,

  • more than 30 ppm by putting that oxygen there.

  • Now, what's interesting, then, is the ortho carbons end

  • up having by resonance extra electron density at them.

  • In other words, the two ortho carbons you can push your

  • arrows, and you see they're electron rich.

  • It's the same reason why electrophilic aromatic

  • substitution occurs ortho and para

  • when you have a methoxy group on there.

  • So, the ortho carbons appear at 114 ppm in methoxybenzene.

  • You don't get a big effect at the meta carbons,

  • which makes sense because you have inductive effects

  • that are now quite removed.

  • So it's very small.

  • And resonance doesn't pump

  • up the electron density at the meta carbon.

  • You go to the para carbon,

  • and now you also see an upfield shifting, although it'll be

  • at a smaller upfield shifting of 121 ppm -- at 121 ppm.

  • Now, I'll come in a moment

  • to empirical additivity relationships,

  • but one way to think about this is to think about it

  • if you have a methoxy group or an alkoxy group on a benzene,

  • that it shifts the ortho protons upfield

  • by about 14 or 14 1/2 ppm.

  • And if you have a methoxy group,

  • it shifts the para protons upfield by about 7 1/2 ppm.

  • And if you have a methoxy group or an alkoxy group,

  • it shifts the meta protons downfield

  • by just a fraction of a ppm.

  • And what we'll see in a moment is that you can add

  • up all these effects and then calculate

  • for different aromatics,

  • the effects of different substituents.

  • All right, let's take a look at some other examples

  • of electron -- of inductive and resonance effects.

  • So, let's take an alkene.

  • And I'll give you cyclohexene as a base value.

  • In cyclohexene, your alkene is at 128 -- 127.4 ppm.

  • Let's compare that to cyclohexanone.

  • And in cyclohexanone, we see a very big effect

  • at the beta position and just a little effect,

  • just a little inductive effect at the alpha position.

  • The beta position is 151 ppm

  • and the alpha position is just shifted downfield by a hair.

  • And that makes sense because you look at this and you say okay,

  • now you can think of a resonance structure

  • in which you're electron deficient, right?

  • We all know that enones are Michael acceptors.

  • That nucleophiles like to attack at the beta position.

  • Are you doing this with orbitals in Van Vranken's class now?

  • >> Yes.

  • >> Yeah, so you know

  • about frontier orbitals and electron density.

  • And so you see, the effect is actually very substantial,

  • right?

  • Both of these carbons, they're symmetrical, they're at 127.

  • You go more than 20 ppm downfield

  • by decreasing the electron density.

  • These effects can be absolutely humongous.

  • And one of the things that I've tried to emphasize

  • when I've talked about these ranges here are these are

  • general ranges.

  • These are not carved in stone.

  • And so you already see, for example,

  • that this one inductive oxygen here brings us even outside

  • of this very generic range here.

  • Let me show you just how huge the effects can be.

  • So, ketene acetal is a good example, right?

  • Alkenes are normally like 110 to 150 ppm.

  • But if hugely perturb the electron density,

  • you can have huge effects.

  • So it probably doesn't surprise you too much if I tell you

  • that you now, by having two methoxys

  • on an alkene can shift it downfield to 169.7.

  • But what's really huge is you look at the position here,

  • the beta position on the alkene, and now we're

  • so electronic rich, this thing is so nucleophilic

  • at this position, there's so much electron density

  • at that position, that we're at 45.5 ppm.

  • You look at a spectrum of that

  • and you wouldn't even know it's an alkene because you'd say, oh,

  • that's got to be aliphatic.

  • That's got to be somewhere over here.

  • And we've just pumped up the electron density hugely.

  • The most radical example I know off the top

  • of my head is this sort of push me, pull you system here,

  • where you have two electron-donating groups

  • and then two electron-withdrawing groups,

  • two cyano groups.

  • And so this alkene, you go to 39.1 ppm.

  • And then this carbon here is at 171.

  • And 171, you'd say all right, well, it's really downfield.

  • But you'd say it makes sense.

  • You've got these beta electron withdrawing groups.

  • But you look, 39.1, who would of thunk that that is an alkene.

  • [ Erasing Board ]

  • >> Can I ask you a question?

  • >> Yeah.

  • >> What is that letter to the -- CW, or CN?

  • >> CN.

  • >> Okay.

  • >> So these are two nitrile groups.

  • These are two cyano groups, CN groups.

  • >> Okay.

  • >> All right.

  • So, substituents have substantial effects whether

  • they're alpha, whether they're beta, whether they're gamma.

  • And you can really see this.

  • I'll give you -- we're going to walk through this

  • and I'll give you some examples.

  • So first let's talk about alpha alkyl substitution.

  • And if I want to give you a general principle, in general,

  • alpha alkyl substitution leads to more downfield shifting.

  • So if we, again, take our benzene example,

  • and remember we said that benzene was at 128.5.

  • If we put a methyl group on it and make it

  • into toluene, now we go to 137.

  • The point is we shift downfield by about 9 ppm by putting

  • on a methyl group onto benzene.

  • Let's take a look at alkyl systems.

  • So, we'll look at propane.

  • And we'll look at the central methylene

  • of the central carbon of propane.

  • You put on an alkyl group onto propane and you get isobutane.

  • And now you're down at 25 ppm.

  • And so you notice, it's kind of about the same.

  • Here we moved down by about 8 or 9 ppm.

  • And here we, again, moved down by about 9 ppm.

  • So in other words, we're talking on the order of, eh,

  • 10 ppm or thereabouts.

  • All right, you put on another alkyl substituent

  • and the effect isn't as dramatic.

  • But again, you move further downfield.

  • Now we're at 28 ppm.

  • Now, what's nice about this is these ideas are generalizable.

  • There is really science here to it.

  • And this is the point, that you can take little bits

  • of knowledge and generalize and build

  • up in your mind what's going on.

  • So let me compare us, say, to ethanol.

  • If we start at 58 for ethanol, and now we envision going

  • to isopropanol, what would you predict your carbon

  • to be at here?

  • >> 67.

  • >> 67 would be a very good prediction, because we say,

  • okay, we take 16, we add 9, we get to 25.

  • You add 9, you get to 67.

  • And that's a very good guess.

  • 64 is the answer.

  • All right.

  • Now imagine we go further and we go to tert-butanol.

  • Now what do you think, for tert-butanol?

  • >> 66.

  • >> 66?

  • >> I don't think it's going to change.

  • >> You don't think it's going to change.

  • Okay. Other guesses on that this estimates?

  • >> 67.

  • >> 67. All right.