How do chemists plan multi-step syntheses and use chromatography and NMR to identify products?
Multi-step synthesis and reaction pathways, chromatography (TLC and gas chromatography), and nuclear magnetic resonance (NMR) spectroscopy including chemical shift, splitting and integration.
An Eduqas A-Level Chemistry OA4 answer on planning multi-step synthesis, chromatography (TLC and gas chromatography), and NMR spectroscopy (chemical shift, splitting and integration).
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What this topic is asking
Eduqas topic OA4 brings the organic course together: planning multi-step syntheses using the reaction map, the chromatographic techniques for separating and checking purity (thin-layer and gas chromatography), and nuclear magnetic resonance (NMR) spectroscopy, including chemical shift, the splitting pattern and integration. The synoptic skill of deducing a structure from combined spectra is the climax of the analysis content.
Multi-step synthesis
Chromatography
TLC is used to check whether a reaction is complete (the starting-material spot disappears) and to compare an unknown with known references by .
NMR spectroscopy
Combining techniques
The deepest analysis questions combine mass spectrometry (molecular mass and fragments), infrared (functional groups) and NMR (the carbon-hydrogen framework) to deduce a full structure. This is the synoptic skill that Component 3 and the higher-tariff Component 2 questions reward.
Examples in context
Example 1. Quality control in pharmaceuticals. Manufacturers use gas chromatography linked to a mass spectrometer (GC-MS) to confirm a drug's identity and purity, separating components by retention time and identifying each from its mass spectrum.
Example 2. Structure determination of a natural product. Chemists combine NMR, infrared and mass spectrometry to work out the structure of a newly isolated molecule, exactly the multi-technique reasoning this topic develops.
Try this
Q1. State what the integration (relative peak areas) in a proton NMR spectrum tells you. [1 mark]
- Cue. The relative number of hydrogen atoms in each chemical environment.
Q2. A hydrogen environment shows a triplet in the proton NMR spectrum. How many hydrogens are on the adjacent atom(s)? [1 mark]
- Cue. Two (a triplet means , so neighbouring hydrogens).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marksThe proton () NMR spectrum of ethanol () shows three peaks. (a) State the ratio of the areas under the three peaks and explain what this ratio represents. (b) State the splitting pattern of the peak and explain it using the n+1 rule.Show worked answer →
(a) The peak area ratio is (for ) (1). The ratio represents the relative number of hydrogen atoms in each chemical environment (1).
(b) The peak is a quartet (split into four) (1). The adjacent has three equivalent hydrogens, and by the rule peaks (1).
Eduqas 20214 marksA two-step synthesis converts propan-1-ol into propanoic acid and then into an ester. (a) State the reagents and conditions for each step. (b) Suggest how thin-layer chromatography could be used to check that the first reaction is complete.Show worked answer →
(a) Step 1: reflux propan-1-ol with acidified potassium dichromate(VI) to give propanoic acid (1). Step 2: warm the propanoic acid with an alcohol and a concentrated sulfuric acid catalyst to give the ester (1).
(b) Spot the reaction mixture and a pure sample of the starting alcohol on a TLC plate, develop it in a solvent, and visualise (1). If the spot for propan-1-ol has disappeared from the mixture, the reaction is complete; comparing values identifies the components (1).
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Sources & how we know this
- WJEC Eduqas GCE A Level Chemistry specification (from 2015) — WJEC Eduqas (2015)