Modern Analytical Techniques I
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7.1 How an organic mass spectrum is made
In the mass spectrometer, an organic molecule is hit by high-energy electrons, which knock out one electron to form the molecular ion, a positive ion with an unpaired electron:
M(g) + e− → M+(g) + 2e−
Many molecular ions are unstable and break into fragments: one part keeps the positive charge (a cation), the other becomes an uncharged radical.
M+ → X+ + Y•
Only charged particles are deflected and detected, so the radical does not show up on the spectrum.
Check yourself: Why do radical fragments not appear on a mass spectrum?
They are uncharged, so they are not accelerated, deflected or detected
7.2 Reading a mass spectrum
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7.3 Fragmentation patterns
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7.4 Telling isomers apart
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7.5 Chlorine and bromine: pairs of molecular ion peaks
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7.6 How infrared spectroscopy works
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7.7 Infrared absorptions you must recognise
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7.8 Reading an infrared spectrum
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7.9 Using infrared and mass spectra in practice
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7.10 Core Practical 7: identifying unknowns
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Extra worked questions
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All 8 worked solutions are free for members. The first one is open to everyone.
Molecular ion and base peak
The mass spectrum of a ketone has its last large peak at m/z = 72 and its tallest peak at m/z = 43. (a) Give the Mr. (b) What is the peak at 43 called, and what does it show?
Try it first, then show the worked solution
| (a) | The molecular ion is at 72, so Mr = 72 | 1 |
|---|---|---|
| (b) | The base peak; it is the most abundant (most stable) fragment, probably CH3CO+ | 1 |
Answer: (a) 72 (b) base peak, CH3CO+
Examiner tip: The base peak is the tallest; the molecular ion is the furthest right. They are usually different peaks.
Bonds in ethanol's IR spectrum
Name two absorptions you would expect in the infrared spectrum of ethanol, with their approximate wavenumbers.
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Explaining a fragment
Butane has a molecular ion at m/z = 58 and a large peak at m/z = 29. (a) Identify the ion at 29. (b) Write an equation for its formation. (c) Explain why the other half of the molecule does not give a peak.
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A chlorine-containing compound
A compound has molecular ion peaks at m/z 78 and 80 in the ratio 3 : 1. It contains only C, H and Cl. Explain the two peaks and suggest a formula.
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Three colourless liquids
Three bottles contain propanal, propanone and propanoic acid. Describe tests to identify each one.
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Pentan-2-one or pentan-3-one?
A ketone C5H10O shows major peaks at m/z 86, 57 and 29, but no peak at 43. Identify the ketone, explaining your reasoning.
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Identify from an IR spectrum
A compound C3H6O2 has a very broad absorption from 2500 to 3000 cm−1 and a strong, sharp absorption at 1715 cm−1. Identify the bonds, the functional group and the compound.
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Core Practical 7: an unknown liquid
An unknown liquid, Y, has Mr 74 from its mass spectrum. It gives steamy fumes with PCl5, turns acidified dichromate from orange to green, but has no effect on Fehling's solution. Its infrared spectrum has a broad absorption at 3350 cm−1 and none near 1700 cm−1. Its mass spectrum has a large peak at 45. Identify Y, 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.
Identifying a ketone from IR and mass spectra
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Following an oxidation with IR
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Identifying a carboxylic acid
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An unsaturated alcohol from its IR spectrum
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A branched aldehyde from its spectra
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Recall cards
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