How do chromatography and NMR spectroscopy let us separate mixtures and determine structures?
Chromatography (thin-layer with Rf values and gas chromatography with retention times), carbon-13 and proton NMR spectroscopy (chemical shift, integration and the n+1 splitting rule with TMS reference), and combining analytical techniques to identify structures.
An OCR H432 module 6 answer on chromatography and NMR: thin-layer and gas chromatography, carbon-13 and proton NMR with chemical shift, integration and the n+1 splitting rule, and combining analytical techniques to deduce structures.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
OCR specification points across 6.3 want you to interpret chromatography (thin-layer with values and gas chromatography with retention times), interpret carbon-13 and proton NMR (chemical shift, integration and the n+1 splitting rule, with TMS as the reference), and combine NMR with infrared, mass spectrometry and chemical tests to determine structures. This is the analytical climax of the course.
Chromatography
NMR: the basics
Carbon-13 and proton NMR
Combining techniques
Examples in context
Example 1. Drug testing by GC. Gas chromatography (often linked to a mass spectrometer) separates and identifies trace substances in blood or urine by their retention times, the basis of forensic and anti-doping testing.
Example 2. MRI from NMR. Magnetic resonance imaging in hospitals is medical proton NMR of the hydrogen in body water, using the same principle of nuclei absorbing radio-frequency energy in a magnetic field.
Try this
Q1. State what the number of signals in a carbon-13 NMR spectrum tells you. [1 mark]
- Cue. The number of chemically different carbon environments in the molecule.
Q2. A proton signal is split into a triplet. How many protons are on the adjacent carbon? [1 mark]
- Cue. Two (a triplet has lines, so ).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20204 marksA compound has the molecular formula . Its carbon-13 NMR spectrum shows two signals, and its proton NMR spectrum shows a single peak. (a) Deduce its structure. (b) Explain how the carbon-13 and proton NMR support your answer.Show worked answer →
(a) The compound is propanone, (1).
(b) Carbon-13: propanone has two carbon environments (the two equivalent methyl carbons, and the carbonyl carbon), giving two signals (1). Proton: the six methyl protons are all equivalent and have no protons on the adjacent carbon (the carbonyl carbon has no H), so they appear as one unsplit peak (a singlet) (1)(1).
Markers reward the correct structure, the two carbon environments for the carbon-13 spectrum, and the single equivalent proton environment with no splitting for the proton spectrum.
OCR 20214 marksThe proton NMR of ethanol () shows three groups of peaks. (a) State the ratio of the integration traces. (b) Predict the splitting of the and peaks using the n+1 rule.Show worked answer →
(a) The three environments are , and , in the ratio (matching the numbers of protons) (1).
(b) The protons are next to the two protons, so and they form lines (a triplet) (1). The protons are next to the three protons, so and they form lines (a quartet) (1)(1).
Markers reward the integration ratio, the triplet for , and the quartet for from the n+1 rule.
Related dot points
- The delocalised model of benzene and the evidence for it, the stability of the ring, and the electrophilic substitution reactions of benzene (nitration, halogenation and Friedel-Crafts acylation).
An OCR H432 module 6 answer on aromatic compounds: the delocalised model of benzene and the evidence for it, its stability, and the electrophilic substitution reactions of nitration, halogenation and Friedel-Crafts acylation.
- Aldehydes and ketones, oxidation of aldehydes, reduction with sodium tetrahydridoborate, nucleophilic addition of hydrogen cyanide, and the tests for carbonyl compounds (2,4-DNPH, Tollens' reagent and Fehling's solution).
An OCR H432 module 6 answer on carbonyl compounds: aldehydes and ketones, oxidation of aldehydes, reduction with sodium tetrahydridoborate, nucleophilic addition of hydrogen cyanide, and the 2,4-DNPH, Tollens and Fehling tests.
- Acidity of carboxylic acids and their reactions with metals, alkalis and carbonates; esterification and ester hydrolysis (acid and alkaline); and the reactions of acyl chlorides and acid anhydrides.
An OCR H432 module 6 answer on carboxylic acids and esters: acidity and reactions with metals, alkalis and carbonates, esterification and acid and alkaline ester hydrolysis, and the reactions of acyl chlorides and acid anhydrides.
- Amines and their basicity and preparation, amino acids and the amide link, optical isomerism and chirality, condensation polymers (polyesters and polyamides), carbon-carbon bond formation using nitriles, and multi-step organic synthesis.
An OCR H432 module 6 answer on nitrogen compounds and polymers: amine basicity and preparation, amino acids and the amide link, optical isomerism, condensation polymers (polyesters and polyamides), carbon-carbon bond formation with nitriles, and organic synthesis.
- Planning multi-step synthesis routes between functional groups using practical techniques (reflux, distillation, purification), and the analytical techniques of infrared spectroscopy (functional-group absorptions) and mass spectrometry (molecular ion and fragmentation).
An OCR H432 module 4 answer on organic synthesis and analysis: building multi-step reaction routes between functional groups, practical techniques, infrared spectroscopy for functional groups, and mass spectrometry for relative molecular mass and fragmentation.
Sources & how we know this
- OCR A-Level Chemistry A (H432) specification — OCR (2015)