Chemistry › Organic synthesis
Organic synthesis
Unit C22 · 3 lessons.
A synthesis question is often the longest on a paper. Planning a route means chaining reactions from the maps of every functional group, with the reagent and the conditions for each step. The practical work prepares, purifies and tests an organic solid and liquid, and combined spectra settle the structure of an unknown.
A reagent without its conditions loses marks. Potassium cyanide is not enough for a step that adds a carbon: the scheme wants KCN in ethanol, heated under reflux. A purity test needs its result as well, a sharp melting point that matches the data-book value rather than a range below it.
The papers that set it are AQA Papers 2 and 3, OCR A Papers 2 and 3, Edexcel Papers 2 and 3 and Cambridge Papers 1, 2 and 4.
Specification sections: AQA 3.3.14, 3.3.9.2, 3.3.15 · OCR A 4.2.3, 6.2.4, 6.2.5, 4.2.4, 6.3.2 · Edexcel 17.11, 18.18 to 18.22, 6.39, 19.5 · CAIE 18.1, 19.2, 21.1, 36.1, 37.4
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Planning a synthetic route
The aliphatic and aromatic reactions with the reagents and conditions for every conversion between functional groups; devising a route of two to four steps by comparing carbon skeletons and working backwards from the target; adding a carbon with a nitrile; overall yield, atom economy, hazards, selectivity and the aim of needing no solvent in choosing a route; worked routes from ethene to propylamine and from ethene to ethyl ethanoate; how a route question is marked; and the reactions other boards add: nitrile hydrolysis, acyl chlorides with SOCl₂ or PCl₅, bromination and Friedel–Crafts alkylation of benzene, and Grignard reagents with carbon dioxide, aldehydes and ketones.
- Preparing, purifying and testing an organic solid and liquid
Heating under reflux and distillation and why each is used; filtration under reduced pressure with a Büchner funnel; recrystallisation step by step with the reason for each step; melting point determination as a test of purity; the separating funnel, washing with sodium carbonate solution, drying with anhydrous magnesium sulfate or calcium chloride and redistillation; boiling temperature as a test of purity; required practical 10 for aspirin and ethyl ethanoate with apparatus, measurements and errors; percentage yield from masses and from volumes and densities; and the same preparations as OCR PAG 5 and 6, Edexcel core practical 16 and Cambridge practical papers, with steam distillation.
- Putting the analysis together: structure from spectra
A method for identifying an organic compound: molecular formula from high-resolution mass spectrometry or combustion data, the degree of unsaturation, infrared for the functional group, carbon-13 NMR for carbon environments, proton NMR for shift, integration and splitting, and a final check; three worked analyses of C₃H₆O, C₄H₈O₂ and C₈H₈O; the pitfalls of symmetry, O–H peaks and integration ratios; and, for other boards, D₂O exchange, external calibration in gas chromatography, and infrared monitoring of carbon monoxide and nitrogen monoxide in exhaust gases.
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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