Modern Analytical Techniques II
Notes for every section, eight new exam-style questions with full worked solutions, and online recall cards for your review days. Nothing here is copied from the book.
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19.1 High-resolution mass spectrometry
The peak with the highest m/z (ignoring tiny isotope peaks) is the molecular ion, M+. Its m/z gives the Mr.
High-resolution mass spectrometry measures m/z to four or five decimal places. Because the precise masses of atoms are not whole numbers (1H = 1.0078, 12C = 12.0000, 14N = 14.0031, 16O = 15.9949), compounds with the same nominal Mr can be told apart.
Example: CO2, C3H8 and C2H4O all have Mr 44, but precise masses of 43.9898, 44.0624 and 44.0261.
Check yourself: What does the molecular ion peak tell you?
The relative molecular mass of the compound
19.2 How NMR spectroscopy works
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19.3 Chemical environments: spotting symmetry
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19.4 Carbon-13 NMR
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19.5 Proton NMR: chemical shift and peak areas
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19.6 Splitting and the n + 1 rule
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19.7 Predicting a proton NMR spectrum
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19.8 Putting the evidence together
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19.9 Chromatography and Rf values
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19.10 HPLC, GC and linking to mass spectrometry
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Extra worked questions
Cover the solution, answer on paper, then mark yourself against each step. Each step shows the mark it earns.
All 8 worked solutions are free for members. The first one is open to everyone.
An Rf value
In TLC a spot moves 2.7 cm while the solvent front moves 6.0 cm. Calculate Rf, and give one reason two substances might have different values.
Try it first, then show the worked solution
| Rf | 2.7 ÷ 6.0 = 0.45 | 1 |
|---|---|---|
| Reason | Different relative affinities for the stationary and mobile phases (e.g. different polarity) | 1 |
Answer: 0.45
Examiner tip: Measure both distances from the start line, not the bottom of the plate.
Why TMS?
Give three reasons why tetramethylsilane (TMS) is used as the standard in NMR spectroscopy.
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Counting carbon environments
How many peaks are in the 13C NMR spectrum of (a) propan-2-ol, (b) butanone and (c) 2,2-dimethylpropane?
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Proton spectrum of ethyl ethanoate
Predict the proton NMR spectrum of ethyl ethanoate, CH3COOCH2CH3: number of peaks, relative areas, splitting, and which peak has the highest chemical shift.
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Testing a sample by chromatography
A laboratory tests a urine sample for a banned drug that is not volatile. Suggest a suitable type of chromatography and explain how the drug would be identified and its amount estimated.
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Using precise masses
A compound has a molecular ion at m/z = 44.0261. Use the precise masses H 1.0078, C 12.0000, O 15.9949 to decide whether it is C3H8, CO2 or C2H4O.
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Propanal or propanone?
A compound C3H6O has a proton NMR spectrum with only one peak, a singlet. Identify it and explain why the other carbonyl isomer would give a different spectrum.
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Solving an unknown ester
Compound W, C4H8O2, has a strong IR absorption at 1740 cm−1 and no broad absorption above 2500 cm−1 apart from C–H. Its proton NMR shows: a singlet at 3.7 ppm (3H), a quartet at 2.3 ppm (2H), and a triplet at 1.1 ppm (3H). Deduce the structure of W, explaining each piece of evidence.
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Video walkthroughs
Narrated walkthroughs of exam-style questions from this topic of the book, showing where each mark comes from. Try the question first, then watch. Captions are available in the player.
Predicting the NMR of an ester
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Telling isomers apart by NMR
Putting all the evidence together
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Two isomers of C8H10
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Recall cards
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Your review days
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