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2.8 MASS SPECTRA, IR SPECTRA & C-13 AND H-1 NMR SPECTRA

IF YOU WOULD LIKE A COPY OF PAST PAPER QUESTIONS FOR THIS TOPIC OR ANY OTHER TOPIC PLEASE EMAIL ME (space is provided on the main page of my blog)

IF YOU WOULD LIKE TO GO THROUGH THE QUESTIONS WITH ME, PLEASE VISIT MY YOUTUBE CHANNEL


Hi people. I am here today to share everything I know about another topic in AS-level Chemistry. If you're going along with me in this journey and follow each single step from the beginning, then you should know that we are heading to an end. Don't worry you still have next year to finish your A-levels :) That's something I am working on. My aim is to explain (together with past paper questions) the whole A2-level chemistry as well. But let's not go that far, this topic focuses on dealing with different spectrums. Here is the first one:


Mass spectrometry

This is probably not the first time and definitely not the last time that you are working with mass spectroscopy. I am sure you already know something about it, even if it just the stages that I explained in one of my previous posts. However, if you haven't learned it before you might want to visit it before we continue. (Click HERE for the link).

There is some information about mass spectrometer that you need to fully understand. Let me just walk you through the process that happens in a mass spectrometer:

When a compound is inserted into a mass spectrometer, it is bombarded with high energy electrons. These electrons knock off electrons from the compound, making a positive ion. While the molecules are hit with electrons, they also split, which is called fragmentation (because fragments are produced). Here above you have a graph with all the m/z and abundances of these fragments.

A mass spectrometer graph tells us a lot about a compound. I'll explain the two most important ones:

  1. Base peak- this represents the most common fragment of the compound and you recognise it by finding the tallest peak.

  2. Molecular ion peak - this is the peak with the highest m/z value and it gives you the molecular mass of the ion (its Mr). This is the 'full' ion, the largest fragment. In the example above, we can see it is 72. ( The Mr of the compound is 72).


There are a couple of things you know about this compound and now we need to gather it all together. I drew a little cloud informing you this is a graph for a hydrocarbon (only carbons and hydrogens), we know the Mr is 72 and we have few other fragments visible on the graph (they help us to determine the structure of the compound).

Now, match the Mr=72 to a hydrocarbon. I figured out that this hydrocarbon must have 5 carbons and 12 hydrogens (72/12=6 but we also need to include H, so only 5 carbons and the rest are hydrogens).

There is one more thing to remember, there are many isomers that may have the same molecular formula and we need to decide which isomer this compound is. As I mentioned before, this is the time, where the fragments come in handy. Looking at the graph, decide what fragments we have and then just put them together. Let me show you how:


There is nothing tricky about fragments, they are just parts of the original compound with a molecular formula (Mr) represented by the m/z.

--> m/z (29) - this is the [CH3CH2]+ fragment when you add up the relative atomic masses of each atom, you get 29. So, the Mr of the fragment is 29. And that's exactly how you find your fragments. --> m/z (43) - here is another fragment; we are looking for a part with Mr 43. This could represent [CH3CH2CH2]+ fragment. The whole compound has 5C and 12H, and Mr43 is 3C and 7H and that's how you find your fragments.

There is just one more thing to remember. When you're dealing with fragments, there needs to be a + sign at the end of the bracket because in mass spectrum we have positive ions.

--> m/z (57) - again, the same process as before and it gives me a fragment [CH3CH2CH2CH2]+ and with a plus at the end. --> m/z (72) - here is the very last one, and the fragment is [CH3CH2CH2CH2CH3]+, as you can see it has 5 carbons and has a Mr of 72 (our molecular ion peak), so this is the whole molecule. When you look at all these fragments, there is a conclusion visible, they are all straight carbon chains and the compound works out to be pentane (we have five carbons). I drew out the molecule for you in the picture above. Just like above, you can find any molecule when you have a mass spectrum graph. From the previous post on mass spectra (which I linked above), you can also revise how to calculate Ar of isotopes.


 

Infrared spectroscopy

I believe this is the easiest out of the four spectrums that we are working on in this post.

Firstly, how this works and how do we get the lines on the graph. So, the infrared radiation is passed through molecules and is absorbed by them in an infrared spectrum. This causes vibrations and bending in these molecules.


The spectrum shows what energies have been absorbed ad you can see the results on a diagram/graph. Each bond has a characteristic energy that is absorbed and this helps us to figure out what bonds we have in our molecule.


There is definitely one thing that you need and it is your data booklet. It is absolutely essential here because it gives you the wavenumber (where energy is absorbed) and the bond type corresponding to this wavenumber.

All you need to do is compare your graph and the curves present in your graph with the table in your data booklet and answer questions in exam based on these numbers (if you would like to have a go at this type of question, visit my youtube channel where I post past paper questions with answers; it is linked at the top of this page).

We're going to try an example together (if you read my posts before, you know that I love examples because they just make everything come together).

I included a table with the wavenumbers and bonds relating to them. We also have a diagram on which you can find letters. Each letter represents a different bond and we need to figure out what bond. Furthermore, you are told that this graph is of butan-1-ol, so you are prepared what bonds you might be looking for.

  • you have a curve between 1000-1300 and this curve represents C-O. Now you know that this bond is present in your molecule (butan-1-ol).

And I think this is all you need for AS-level chemistry.


 

NMR spectroscopy

NMR - Nuclear Magnetic Resonance

There are two NMR spectras that we are going to explore: C-13 and H-1, but firstly let's go through a couple of facts about NMR spectroscopy.

  • NMR is all about energy absorbed by a molecule. When energy is absorbed, resonance occurs (hence the name).

  • This energy that is absorbed is shown by a chemical shift in an NMR spectrometer.

As I said, there are two types of NMR spectrum in this topic:


1.C-13 spectroscopy:

In this type of question, you would be given a graph of a molecule and data with chemical shifts (this is something you're going to find in data booklet). I prepared an example above in the picture together with its chemical shifts. This data is for propan-1-ol. You usually wouldn't get the name because we use this spectroscopy to find it but for the example sake, I'll tell you the name. So, let's talk through the example:

--> Number of peaks - number of different carbon environments. Each carbon is surrounded by different atoms or groups and therefore it is said, it's in a different environment. Here we have 3 peaks, so 3 environments ( you don't need to consider the height of them). --> The peak at chemical shift 64 is due to the C-O type of carbon (you know this by using data booklet). --> The peak at chemical shift 15 and 27 is due to the C-C type of carbon. Ok, so we said they are in different environments but the carbon type is the same. This means that both of the C-C bonds are connected to something else. One (at 27) is connected to 2 hydrogens and an OH group and the other (at 15) is connected to 3 hydrogens and 2 hydrogens. This may sound confusing, but just look at the diagram of propan-1-ol above and try to associate each carbon type with the things that it is attached to and you'll see they have different environments. One last sentence of advice: LOOK AT EACH CARBON SEPARATELY AND THE THINGS IT IS ATTACHED TO AND SEE IF ANY PREVIOUS CARBON ALREADY HAD THAT ARRANGEMENT, IF NO, IT IS IN A DIFFERENT ENVIRONMENT.


2. H-1spectroscopy


Again;

--> Number of peaks- show you the number of environments but this time we're looking at the hydrogens and not carbons.

--> The spectra is sometimes called proton NMR but this is the same as H-1 NMR.

--> Also, you're given another table with chemical shift values corresponding to the type of proton.


This time you consider the height and I'll explain how in the next couple of lines.

In the picture above I included an example for H-1 NMR spectrum. This time, there is also a graph together with a table. These two pieces of information is everything you need to know to be able to work out what molecule you're dealing with. In this example, again, I will tell you what the molecule is, C3H6O2, so that you are clear with what's going on, but in the exam, it is very likely that you will have to guess that.

Here's some information that you can gather:

  • 2 peaks - 2 proton environments;

  • The height of the peaks is 1:1;

  • There are 6 hydrogens in this molecule and they are in the ratio 1:1, therefore, 3 hydrogens per peak.

  • Concluding - 2 hydrogen environments with 3 hydrogens in each.

Now, each chemical shift has a hydrogen bond associated with it. We will use this to deduce the structure of the molecule (we know what it consists of C2H6O2, but we don't know how it looks like; the structure).

--> At 3.6 - we have three hydrogens in this peak (above in the photo, I exactly drew how this peak looks like).

--> At 2.1 - there is another peak which also holds 3 hydrogens (again, you have it drawn above in the picture).

Together, there are 6 hydrogens, which is what we need. These chemical shifts lead us to the structure of an ester, which is also above in the picture. Of course, you are not required to know the name of this ester, but you have to know that the molecule is an ester and be able to draw it.

This is everything about spectroscopy and spectrometry that you are required to know. When you look at it, there is not actually much to know. It is just these couple of bits of information, which if you know, you can answer any question of this type. I've seen many and many 6 mark or 4 mark questions in the exam where you are asked to figure out what molecule you have using different graphs and tables and the examples in this post are examples of these questions.

Ok, I need to stop rumbling now. I am just trying to make chemistry as easy for you to understand as possible and so that you know that it is achievable to learn chemistry.

PS. Please remember, I am only a student, and as anyone, I can make mistakes. If you think you can see one, don't hesitate and comment (either here on on my youtube channel) Thank you!


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