Spectroscopic identification of organic compounds (AHL)
21.1 Spectroscopic identification of organic compounds
(2 hours)
Pause for thought
High resolution 1H NMR, which shows splitting patterns, is an extremely powerful analytical tool.
Although students are not required to explain splitting patterns it is worth giving them a simple explanation. Consider a methyl group on a carbon atom bonded to another carbon atom that has just one hydrogen atom bonded to it. Students should be able to see that the hydrogen atom on the adjacent carbon atom will provide a small magnetic field due to its spin. This will either line up with the applied external magnetic field on the methyl hydrogen atoms or be against it. This means that the methyl hydrogen atoms will see the equivalent of two external magnetic fields, one slightly larger than the other. This will cause the signal to split into a doublet. If there are two atoms on the adjacent carbon atoms then they may both line up with the external field, both line up against it or one line up with it and the other against this effectively cancelling each other out for which there are two possibilities. This effectively produces a 1:2:1 splitting, i.e. a triplet. It is easy for students to predict the splitting pattern from Pascal’s triangle, although they are only expected to know up to a quartet, which means that aromatic ring hydrogen atoms are not generally included.
A neat example to show students is the 1H NMR spectrum of ibuprofen, an over the counter mild analgesic. The structure of ibuprofen is given in Section 37 of the data booklet.
ibuprofen
Ask students to work out firstly how many signals they would expect, what the integration trace ratio would be for each signal and then predict the splitting pattern they would expect for each signal.
If they ignore the four hydrogen atoms attached to the aromatic ring (which will give signals in the 7 ppm region) then they should be able to predict that the 1H NMR should show six separate signals with integration trace areas of 6, 2, 1, 3,1 and 1.
The actual spectrum looks like this:
1H NMR spectrum of ibuprofen
The signals at B (integration ration of 4) are due to the aromatic hydrogen atoms. The remaining six peaks are labelled A, C, D, E, F and G and it can be seen that the integration traces give the expected ratios of 1, 1, 2, 1, 3, and 6.
The next diagram shows the splitting patterns that can be seen when the signals are expanded.
From this it is easy to assign each signal to particular hydrogen atoms.
Nature of Science
The improvements in modern instrumentation, such as the advances in infrared spectroscopy, 1H NMR spectroscopy and mass spectrometry, have lead to detailed knowledge concerning the structures of compounds.
Learning outcomesAfter studying this topic students should be able to: Understand:
Apply their knowledge to:
| Clarification notesHigh resolution 1H NMR should be covered. Interpretation of the following from 1H NMR spectra is required: Students should be familiar with splitting patterns involving singlets, doublets, triplets and quartets. The treatment of spin-spin coupling constants will not be assessed. International-mindednessStructural information about chemical compounds is shared by chemists internationally. For example, ChemSpider (developed by the Royal Society of Chemistry), The Cambridge Crystallographic Database and the Protein Data Bank (at Brookhaven National Laboratory, USA) exemplify the international nature of the scientific community. |
Teaching tipsThis is the really powerful part of 1H NMR. Build on the material in 11.3 Spectroscopic identification of organic compounds by explaining that the magnetic field of each proton will be affected by the protons on neighbouring carbon atoms which leads to splitting patterns. Explain singlet, doublet, triplet and quartet (particularly use −C2H5 as an example of a triplet and quartet). Introduce the (n + 1) rule but do not go beyond quartet, except to say 'complex pattern'. This is particularly the case with aromatic protons. Give the advantages of tetramethylsilane, TMS, as a reference material (volatile, non-toxic, strong signal due to 12 equal protons, signal upfield well away from most other proton signals). Give plenty of examples (attached) and stress how splitting patterns are much more useful than chemical shift. There is little required in this sub-topic about X-ray crystallography except to say that it is a technique that can be used to determine bond distances and bond angles. This is because X-rays are of the same order of magnitude as atomic and ionic radii so can be diffracted by the different layers of atoms or ions. The Bragg equation and calculations are not needed for this sub-topic although they are required for Option A.8 Superconducting metals and X-ray crystallography. | Study guide
Pages 104 - 107 QuestionsFor ten 'quiz' multiple choice questions with the answers explained see MC test: Spectroscopic identification of organic compounds (AHL). See interactive questions on 1H NMR including splitting patterns (from Wake Forest University) and the ten attached separate HL questions together with worked answers on identification of organic compounds from spectroscopic data: Vocabulary listtetramethylsilane, TMS IM, TOK, 'Utilization' etc.See separate page which covers all of Topics 11 & 21. Practical work Although I've given them as questions, the attached identification exercises are really data analysis using secondary data and could count as 'hands off' practical work. |
Teaching slides
Teachers may wish to share these slides with students for learning or for reviewing key concepts.
Spectroscopic identification of organic compounds (AHL) :
Spectroscopic identification of organic compounds (AHL) :
Spectroscopic identification of organic compounds (AHL) :
Spectroscopic identification of organic compounds (AHL) :
Spectroscopic identification of organic compounds (AHL) :
Spectroscopic identification of organic compounds (AHL) :
Spectroscopic identification of organic compounds (AHL) :
Spectroscopic identification of organic compounds (AHL) :
Spectroscopic identification of organic compounds (AHL) :
Spectroscopic identification of organic compounds (AHL) :
Other resources
1. A useful video from premed411. It is worth showing before giving questions on IHD and combining IR, MS and 1H NMR to identify compounds (ignore the 13C NMR after 5 minutes 10 seconds).
Interpreting spectroscopic data
2. For those who enjoy working out splitting patterns this interesting example by John Claude Bradley goes slightly beyond the simple demands of IB as it looks at what happens if the hydrogen atom on an adjacent carbon atom is bonded to a chiral carbon atom.
3. A useful free App for iPhones and iPads (download from itunes) is Chemical Detectives. You will have to tell your students to ignore the 13C NMR spectra. The 1H NMR has good splitting patterns but no integration traces. Even so, from the analytical, 1H NMR, IR and mass spectra there are many compounds for students to identify. The App is produced by the Chemical Education Association, a group of schools in Victoria, Australia.