Chemguide: Support for CIE A level Chemistry

Learning outcome 22.3 (all parts)

These statements are about the use of mass spectra in identifying organic compounds.

Before you go on, you should find and read the statements in your copy of the syllabus.

Fragmentation patterns

Start by reading about fragmentation patterns on this page in the Mass Spectrometry section of Chemguide. You might want to have a quick look back at how a mass spectrometer works, but you don't need to remember any of it.

You need to make sure that you could identify which ions might have produced given lines in a simple mass spectrum. And also (and this is more difficult) make reasonable predictions about what lines you might find in the spectrum of a named organic compound.

One question asked by CIE wanted you to suggest two small neutral molecules (with a relative molecular mass less than 30) which might be formed during the fragmentation of alcohols. If you try to work this out by imagining what the fragmentation might produce in a logical way, this is really difficult - if not impossible.

In fact, rearrangements can happen, so virtually any small neutral molecule containing only carbon, hydrogen or oxygen was allowed. The mark scheme allowed any two from H2, H2O, CO, C2H4, C2H2, CH4.

The M+ peak

Now go on to read about molecular ion peaks. There isn't much to read, because you do not need to read the second half of the page about the results from high resolution mass spectrometers using very accurate relative isotopic masses.

The M+1 peak

You should read the page about the M+1 peak.

This is one of the commonest questions CIE ask about mass spectrometry. They expect you to be able to calculate the number of carbon atoms in a molecule from the ratio between the heights of the M and M+1 peaks.

There is a formula for doing this which isn't mentioned on the Chemguide page. You MUST learn this.

They commonly ask you to do this at the start of a longer question, perhaps including NMR, and if you can't work out the right number of carbon atoms, you may well not be able to complete the rest of the question.

The formula CIE want for the number of carbon atoms, n, is:

For example, if you were told that the relative heights (abundances) of the M+ and M+1 peaks were in the ratio of 25:2, you just have to slot these into the equation - taking care that you get them the right way up!

Questions set using this formula don't seem to produce nice tidy whole numbers. Obviously, there can't be 7.3 carbon atoms per molecule, so you just take the nearest whole number - in this case, there must be 7 carbon atoms.

What if you put the numbers in the wrong way up? You will just get a silly answer - in this case, 1136 carbon atoms. If this happens to you, try again with the numbers the other way up.

The presence of chlorine or bromine atoms

This is another commonly asked topic in CIE questions. You should read the page about mass spectra - the M+2 peak.

The presence of chlorine

Notice that the presence of a single chlorine atom causes the M+ and M+2 peaks to have relative heights (abundances) of 3:1, because there is approximately 3 times as much of the Cl-35 isotope as of the Cl-37.

Take care to get this ratio the right way round. CIE Examiner's Reports keep saying that some students got the ratio the wrong way around. That is a silly way of wasting marks!

You will be expected to know what happens if you have two chlorine atoms. You get M+, M+2 and M+4 peaks in the ratio 9:6:1.

Learn that! Despite a comment in an Examiner's Report, if you aren't a mathematician, this isn't easy to work out quickly.

Another question told you that a compound contained 4 chlorine atoms, and asked you how many molecular ion peaks there would be. The Examiner's Report commented that few candidates actually got this right.

It isn't exactly difficult! You just need to work out how many combinations of the isotopes of chlorine you might have. The molecule might have any of the following combinations of isotopes:

35 + 35 + 35 + 35

35 + 35 + 35 + 37

35 + 35 + 37 + 37

35 + 37 + 37 + 37

37 + 37 + 37 + 37

There are five different totals possible. If you add these to the masses of the other atoms in the molecule, you will have five different molecular ion peaks.

The presence of bromine

Make sure that you recognise that M+ and M+2 peaks of approximately equal heights suggest the presence of a single bromine atom in the molecule.

The best way of practising all this (especially in interpreting mass spectra to identify simple molecules) is to go through past papers. The statements in the 2016 syllabus are essentially identical with the previous syllabus, and so you can look at any question from 2007 to 2015.

In the past, mass spectrometry has often been asked together with NMR, so it might be a good idea to leave doing this until you have completed this analysis section.

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© Jim Clark 2011 (last modified July 2014)