Hi everybody, this is video 9-6. We're going to be looking at one of the final main aspects of proton NMR. We're going to be talking a little bit about alcohol protons in the proton NMR, and also about how we're going to apply proton NMR to solve the structure of an unknown organic compound.
So this is the last technical lecture about proton NMR. We still have more to go in Chapter 9. We'll be talking about carbon-13 NMR coming up next. So when talking about alcohols in the NMR spectrum, we've already seen a little bit about this in some of the worksheets in class. As we look at the structure that we see on the slide for ethanol, how many proton signals do you expect? In other words, how many groups of non-equivalent protons are there?
This isn't too bad. Okay, we've got a unique methyl group. That's a signal.
a unique CH2 group, which will be a signal. And since all protons will give a signal, we also include the proton that's bonded to the oxygen. If it were bonded to a nitrogen, exact same thing. So we would expect three signals in the proton NMR for ethanol. The signal for an OH group is usually, excuse me, the OH of an alcohol is usually found around 3 ppm, sometimes closer to 4. And since the OH proton is acidic, even though it's a weak acid, we very rarely observe any type of splitting.
So in other words, proton C is not split by protons B. And the signal for B is not split by C. So when you have an alcohol, it really is kind of sitting out there on an island.
And that's because it is acidic. Okay, this proton is coming on and off. and off, and because of that, the magnetic field of an acidic proton does not exert any influence on the magnetic fields of adjacent protons.
Now we can take advantage of the unique chemistry of an acidic proton. If you have your sample in an NMR tube and it has an OH group, if you add a few drops of D2O, you're going to get an acid-base reaction occurring, where the proton of the alcohol We'll exchange for deuterium of the D2O. When this happens, instead of this being OH, it'll be OD, and the OH signal in the NMR will subsequently disappear. The reason this happens is that in the magnetic field window that we use, the frequency window that we use to observe proton NMR, a deuterium nucleus does not show up.
So this is a really neat way to determine if you've got a bunch of signals in your NMR spectrum and you think one of them is an alcohol group, add a couple of drops of D2O to your NMR sample, shake it up, rerun your NMR, and see if you can find which signal disappears. That is your acidic proton. So this is the chemistry of the exchange reaction.
This is just as a reminder. We covered this actually last semester. And you can read this on your own.
If you want, you can take this balanced equation and practice the mechanism. It's probably not something I'm going to ask you about, but it certainly is something I do expect that you'll be able to handle. Okay, so that completes the technical part for proton NMR. Here's a very quick review. There were four points covered.
the chemical equivalence of hydrogen atoms, in other words, protons. The chemical shift scale, that goes roughly from 0 to 13. The units we use are ppm, or parts per million. We talked about signal splitting. That was the last topic we had gone over in class. That's how the magnetic field of one hydrogen will affect the magnetic field of adjacent hydrogens.
resulting in a signal being split. And finally, we just talked about alcohol protons, and the fact that alcohol protons do not split adjacent protons, and that these acidic protons of alcohols and other types of functional groups will exchange if you add D2O to the sample. Okay, so you're going to be using NMR along with IR and other things in order to identify the structure of an unknown. So I just want to very quickly review the steps that you're going to use. Okay, and you can read these just on your own.
So I'm going to go through these pretty quickly. We start with the formula. You determine the hydrogen efficiency. And then you write down a few notes about what this is telling you.
Next, you go back to that formula, if it's given to you. And... And think about what information that is telling you. So if you have an oxygen atom in your formula, what type of functional groups might be present? Next, you look at the IR, and you determine what functional groups are obviously present based on the ones that we circled on our IR correlation table.
Now you go to the NMR spectrum. You look at the number of signals you have. You label them A, B, C, etc. You determine or you look at the number of protons associated with each signal. If you add up all those protons, they should match the formula that's been given to you.
And it's always a good idea to summarize that information in a table. In many cases, I will actually give you all of this information already in a table, unless I can find you very clear NMR spectra. You look to see if there are any diagnostic chemical shifts. Usually these are signals that are at 4.5 ppm or greater.
You look at your proton NMR correlation table, and these chemical shifts might help you determine if there are some unique functional groups present. You look to see if any of the signals disappeared when D2O was added, suggesting acidic protons. And then you look at the splitting. And the splitting is going to give you a puzzle piece.
It's going to give you a little structural fragment of the whole molecule. And after you've examined each signal, and based on the splitting, come up with all these puzzle pieces, you then put the various puzzle pieces together, and hopefully you'll come up with an unknown structure. You will solve your unknown structure. So that's pretty much everything in a nutshell.
So what we'll be doing next is we're going to be doing a lot of practicing in applying the proton NMR information to solving unknown structures. So now that you've studied this video, I want you to try working through this particular unknown problem. We'll be doing this in class, and you will not be getting a quiz on this.
So notice you're given a formula. You are given an IR. And the NMR data is all summarized in a table. So give that a shot, and hopefully you can come up with a reasonable structure. And that is the end of this recording.