Chemistry › Organic analysis
Organic analysis
Unit C19 · 4 lessons.
Five techniques between them identify an unknown organic compound. Test-tube reactions pick out the functional groups, high-resolution mass spectrometry gives the molecular formula, infrared spectroscopy confirms the bonds, proton and carbon-13 NMR map the carbon skeleton, and chromatography separates and identifies the components of a mixture. Every board prints the spectroscopy tables, so reading them quickly matters as much as knowing them.
The n + 1 rule is where marks go: the splitting of a proton signal counts the hydrogens on the neighbouring carbons, never those on its own carbon, so a CH₃ next to a CH₂ appears as a triplet. In infrared, the broad O–H absorption of a carboxylic acid sits lower, around 2500–3000 cm⁻¹, than an alcohol's.
Questions on it can come up on AQA Papers 2 and 3, OCR A Papers 2 and 3, Edexcel Papers 1, 2 and 3 and Cambridge Papers 1, 2 and 4; the past-paper pages say what else each of those papers covers.
Specification sections: AQA 3.3.6.1, 3.3.6.2, 3.3.6.3, 3.3.15, 3.3.16 · OCR A 4.2.4, 6.1.2, 6.3.1, 6.3.2 · Edexcel 1.10, 6.25, 7.1 to 7.2, 19.1 to 19.8 · CAIE 17.1, 22.2, 22.1, 37.3, 37.4, 37.1, 37.2
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Test-tube reactions for functional groups, and high-resolution mass spectrometry
Tests for alkenes with bromine water, aldehydes with Tollens' reagent and Fehling's solution, primary and secondary alcohols with acidified potassium dichromate and carboxylic acids with sodium carbonate or hydrogencarbonate, the observations in mark-scheme words, required practical 6, 2,4-DNP and the melting point of its derivative, the triiodomethane (iodoform) test, phenols and carbonate, high-resolution mass spectrometry and precise relative molecular mass, the molecular ion and fragmentation, and the M+1 and M+2 peaks.
- Infrared spectroscopy
Bond vibration and the absorption of infrared radiation, wavenumber and transmittance, reading a spectrum with the data-sheet table, the broad O–H of alcohols and of acids, the C=O absorption, telling an alcohol, aldehyde, ketone, acid and ester apart, amines and nitriles, combining infrared with a molecular ion, the fingerprint region and database matching, infrared absorption by greenhouse gases, and for OCR breathalysers and the monitoring of carbon monoxide and nitrogen monoxide in exhaust gases.
- Proton and carbon-13 NMR spectroscopy
Nuclear spin and the absorption of radio-frequency radiation, shielding, TMS as the standard and chemical shift in ppm, deuterated solvents, carbon-13 NMR and the number of carbon environments, proton NMR environments, the integration ratio, spin-spin splitting and the n + 1 rule with the relative heights of the lines, deducing a structure from a formula and spectra, and for OCR and Cambridge D₂O exchange for O–H and N–H.
- Chromatography: TLC, column and gas
Stationary and mobile phases and separation by adsorption and solubility, thin-layer chromatography and Rf values, locating spots with UV light, iodine or ninhydrin, required practical 12, column chromatography, gas chromatography with retention times and peak areas, external calibration curves for concentration, GC–MS, the limits of identification by chromatography, and for Edexcel high-performance liquid chromatography (HPLC).
Key definitions for this unit
Practical work
The practical capabilities this unit's work belongs to, each with its techniques, its calculations and the errors that cost marks.
Worked in full, from the apparatus to the evaluation:
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