The proton NMR spectrum of propanoic acid exhibits three peaks. Which group corresponds to the most downfield peak?
First, let’s work out the structure of propanoic acid. From the name, we know that there are three carbons. And it’s a carboxylic acid. Drawing out the full structure will help us with assigning peaks in an NMR spectrum. Next, the question gives us a clue. It tells us that the spectrum of propanoic acid exhibits three peaks. So we can look at the molecule structure. And we need to determine the three unique hydrogen environments. So let’s work our way along the molecule.
Here is our first hydrogen. Now, we need to work our way along the molecule to see if there are any other hydrogens in the same environment. We can see that the other two hydrogens on this CH₃ are also in the same environment. We would call them equivalent. Let’s label these with the letter a. Next, we have two hydrogens attached to the central carbon forming a CH₂ moiety. Both of these hydrogens are equivalent to one another. But they are different from the CH₃ group on the end. So let’s give them the letter b.
Finally, we come to the COOH group. Here, there is only one hydrogen. And it is again in a unique environment with no other hydrogens in the same group in this molecule. For completeness, let’s call it c. Remember that the question told us that there are three distinct peaks in the NMR spectrum. And we have identified three groups of unique hydrogen environments. So this matches. Going back to the question, we’re being asked specifically about which group corresponds to the most downfield peak. So let’s review what we know about this.
Remember that on an NMR spectrum, the horizontal axis along the bottom actually reads back to front with the zero ppm of chemical shift being on the right hand side and values increasing as you move from right to left. If a peak is to the left of something, we would describe this as downfield. And this means that it has a higher chemical shift number. Conversely, if a peak is to the right of something, we would describe it as upfield, meaning at a lower chemical shift.
The question is asking us, which of these three groups in propanoic acid corresponds to the peak which is most downfield, meaning towards the left? So now, we need to work out what it is that determines whether a peak appears upfield or downfield. To work out which group causes the most downfield peak, we need to review how NMR spectroscopy works. In order to record a proton NMR spectrum of propanoic acid, a sample of the compound needs to be placed inside the spectrometer. This also means that we’re placing it inside a very strong magnetic field which I’ve labelled here as the letter B.
But this is not the only magnetic field at play here. The electrons in our propanoic acid are also generating their own. They are very small magnetic field. The magnetic field induced by the electrons in a molecule opposes the external field that we’ve applied. This means that a nucleus surrounded by these electrons would be shielded from the magnetic field. This means it wouldn’t be affected quite so strongly by the field.
So what does this mean for our spectrum. If a nucleus is surrounded by a reasonable number of electrons, we would describe it as being shielded. Protons which are well-shielded resonate at lower frequencies. This means that their peaks appear upfield in the NMR spectrum. On the other hand, if a nucleus has the amount of electron density surrounding it reduced, then this means it is more susceptible to the externally applied magnetic field. This, in turn, means that it resonates at a much higher frequency and so will appear in the spectrum more downfield.
We would describe this nucleus as being deshielded. So our question asks, which of these three groups corresponds to the most downfield peak? And we now know that the most downfield peak must come from the most deshielded proton. But which of our protons is the most deshielded? We know that a deshielded proton has a decreased electron density. This reduction in electron density is caused by the environment that the proton is in. For example, a nearby electronegative group or atom would withdraw some of the electron density from a proton. This would, in turn, deshield our proton and move its peak downfield to the left.
So let’s look back at our molecule and see which of our protons this might apply to. We’ll start with the proton environment labelled a in our CH₃ group. The group closest is a CH₂ group. And this is not an electronegative electron withdrawing group. This means that there is nothing to withdraw the electron density from our Ha protons. So this group will, in fact, cause a peak that is quite upfield, since the protons are still well-shielded.
So let’s move on to our CH₂ group. On one side, we have CH₃. This is not particularly electron withdrawing. On the other side, we have the COOH group. The carboxylic acid group is slightly electron withdrawing. So this is going to have a small impact on our CH₂ protons. The effect will be to slightly deshield them. So we would expect this speak to be slightly more downfield than the peak for the first proton environment, proton a.
And finally, we come to proton c, the proton actually on the carboxylic acid group. Here, the proton is directly connected to an electronegative oxygen atom. This, in turn, is going to significantly deshield our proton. Don’t forget that this OH is also connected to a carbonyl group which is further electron withdrawing. The effect on our proton c means that it will be significantly deshielded. Deshielded, of course, means that the peak appears downfield.
So to answer the question, which group corresponds to the most downfield peak, our answer is the proton on the carboxylic acid group COOH. It’s also interesting to note that the carboxylic acid group peak is also very broad. So it will appear more like this than a sharp well-defined peak. And this is due to exchange of the proton. Out of interest, if you look up the NMR spectrum of propanoic acid, the CH₃ group resonates at just over one ppm. The CH₂ group resonates just over two ppm. And the carboxylic acid proton is way downfield at around 12 ppm. So this is definitely the right answer.