Chemguide: Support for CIE A level Chemistry

Learning outcomes 22.5 - all parts

These statements are about NMR spectra and their interpretation.

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

Start by reading the Chemguide page about the background to NMR spectroscopy. If you have already done carbon-13 NMR, a lot of the page will be familiar.

Judging by past CIE papers, the most important things you need to know from this page are:

  • The hydrogen nucleus behaves as a tiny magnet. Take care that you talk about the hydrogen nucleus or a proton - not about a hydrogen atom.

  • These "magnets" can align themselves with an applied magnetic field or against it.

  • There is an energy gap between these two alignments which corresponds to the energy in radio frequencies.

  • The size of energy gap between the two alignments depends on what else is present in the molecule close to the hydrogen you are concerned with.

But you should also be aware of everything else in this page - particularly, for the 2016 syllabus:

  • The use of TMS as a standard - for 22.5(c)

  • The use of CDCl3 as a solvent - for 22.5(d)

Now read the page about low resolution NMR spectra.

You will be told values from an integration trace, and so you don't need to follow the link explaining how to work them out for yourself.

Don't leave this page until you are sure that you understand how to get information from a low resolution NMR spectrum. Unless you are comfortable with this, you won't get far with the next page!

There is a table of chemical shifts in the Data Book which you will have in an exam. You will find a copy of this towards the bottom of the syllabus.

Once you are confident, go on to the page about high resolution NMR spectra.

Don't forget to look carefully at the effect of adding D2O to the substance, and the way this helps to identify an -OH group. It works the same way with N-H hydrogens. This is mentioned in statement 22.5(e).

Most CIE questions about NMR will expect you to get information about the structure from the way the lines split, and so you should be sure that you understand and can use the "n+1 rule".

Up to the time of writing, no questions have ever been asked about the reasons for the n+1 rule, and the 2016 syllabus doesn't imply in any way that you should know those reasons.

I suggest that you only follow the link on the page to that explanation if you are totally confident that you can interpret high resolution spectra - that is the important thing to be able to do.

My guess is that if they ever ask a question about why splitting occurs, virtually nobody will be able to do it!

Interpreting high resolution NMR spectra needs a lot of practice. Try to get hold of as many past CIE papers as possible, and have a go at the NMR questions. There is no short cut to this! The 2016 syllabus is phrased differently from previous syllabuses, but there is no essential difference in content, so you can use any papers from 2007 onwards.

A comment about NMR and benzene rings

This is a repeat of the same comment on the Chemguide page, but it is important that you don't worry unnecessarily about strange splitting patterns in the region of the spectrum containing hydrogens on benzene rings.

At this level, all you can safely say about hydrogens attached to a benzene ring is how many of them there are. If you have a molecular formula which has 6 or more carbon atoms in it, then it could well contain a benzene ring.

Look for NMR peaks in the 6.0 - 9.0 range. If you are given a number like 5 or 4 alongside that peak, this just tells you how many hydrogen atoms are attached to the ring.

If there are 5 hydrogens attached to the ring, then there is only one group substituted into the ring. If there are 4 hydrogens attached, then there are two separate groups substituted in, and so on. There should always be a total of 6 things attached to the ring. Every hydrogen atom that is missing has been replaced by something else.

Splitting patterns involving benzene rings are far too complicated for this level, and you shouldn't be expected to recognise or interpret them.

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© Jim Clark 2011 (last modified March 2015)