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Concept 06 / 08Organic Chemistry - Basic Principles and Techniques

Purification of Organic Compounds: Crystallisation, Distillation, Extraction and Chromatography

Methods of purifying organic compounds and their principles: sublimation, crystallisation, simple, fractional, reduced-pressure and steam distillation, differential extraction with the distribution law, and chromatography (column, thin layer, paper) with retardation factor; criteria of purity.

Figure 1
A compound is judged pure when it has a sharp melting or boiling point and gives a single spot in chromatography. Purification exploits differences in physical properties. Sublimation. For solids that pass directly into vapour on heating, separated from non-volatile impurities: camphor, naphthalene, anthracene, benzoic acid, iodine, ammonium chloride. Crystallisation. Based on different solubilities of the compound and impurities in a suitable solvent. The impure solid is dissolved in the minimum amount of hot solvent (in which it is sparingly soluble cold and freely soluble hot), filtered hot, and cooled to crystallise; impurities stay in the mother liquor. Charcoal removes coloured impurities. Repeated (fractional) crystallisation separates compounds of similar solubility. Distillation. Simple distillation: liquids with non-volatile impurities, or liquids whose boiling points differ by more than about 25 K25\,\mathrm{K} (chloroform 334 K334\,\mathrm{K} and aniline 457 K457\,\mathrm{K}). Fractional distillation: liquids with close boiling points, using a fractionating column that provides many condensation-vaporisation cycles; used in petroleum refining and to separate acetone and methanol. Distillation under reduced pressure: liquids that decompose at or below their normal boiling point boil at lower temperature when the pressure is reduced: glycerol from spent lye in soap making; concentration of sugarcane juice. Steam distillation: for steam-volatile substances immiscible with water: aniline, nitrobenzene, essential oils, o-nitrophenol. The mixture boils when pwater+porganic=p_{\text{water}} + p_{\text{organic}} = atmospheric pressure, below 373 K373\,\mathrm{K}, so the compound distils without decomposition. Differential extraction. An organic compound dissolved in water is shaken with an immiscible organic solvent (ether, chloroform) in which it is more soluble; the layers are separated in a separating funnel. By the distribution (partition) law, the ratio of concentrations in the two solvents is a constant, KDK_{D}, at a given temperature. Several extractions with small portions remove more solute than one extraction with the same total volume. Chromatography. Separation by differential distribution between a stationary phase and a moving (mobile) phase. Adsorption chromatography: stationary phase is an adsorbent (silica gel, alumina). In column chromatography the mixture is eluted down a column, the least strongly adsorbed component emerging first. In thin layer chromatography (TLC), a thin adsorbent layer on a glass plate is developed in a solvent (see figure). Partition chromatography: the stationary phase is a liquid (water held in the cellulose of paper chromatography); components distribute between it and the mobile solvent. Retardation factor Rf=distance moved by the substancedistance moved by the solvent frontR_{f} = \frac{\text{distance moved by the substance}}{\text{distance moved by the solvent front}}, always less than 11. Colourless spots are made visible by UV light, iodine vapour or reagents such as ninhydrin (amino acids). Key Takeaways: - Sublimation: camphor, naphthalene; crystallisation: solubility difference. - Steam distillation: steam-volatile, water-immiscible compounds (aniline), boils below 373 K373\,\mathrm{K}. - Reduced pressure: compounds that decompose at their normal b.p. (glycerol). - Rf=solute distancesolvent distanceR_{f} = \frac{\text{solute distance}}{\text{solvent distance}}; more strongly adsorbed means lower RfR_{f}. - Pitfall: several small extractions are more efficient than one large one with the same solvent volume. Brain-Teaser 1: In a TLC experiment the solvent front moves 12 cm12\,\mathrm{cm} and two spots move 9 cm9\,\mathrm{cm} and 3.6 cm3.6\,\mathrm{cm}. Find the RfR_{f} values and identify the more polar compound on silica. Solution: Rf=912=0.75R_{f} = \frac{9}{12} = 0.75 and 3.612=0.30\frac{3.6}{12} = 0.30. Silica is polar and holds polar compounds more strongly, so the compound with Rf=0.30R_{f} = 0.30 is more polar. Brain-Teaser 2: How would you separate a mixture of o-nitrophenol and p-nitrophenol? Solution: By steam distillation. o-Nitrophenol has intramolecular hydrogen bonding, so it is less associated, more volatile and steam-volatile. p-Nitrophenol forms intermolecular hydrogen bonds (and bonds with water), so it is not steam-volatile and remains behind. Brain-Teaser 3: An organic compound has distribution coefficient KD=CetherCwater=4K_{D} = \frac{C_{\text{ether}}}{C_{\text{water}}} = 4. 1 g1\,\mathrm{g} of it is in 100 mL100\,\mathrm{mL} water. Compare one extraction with 100 mL100\,\mathrm{mL} ether and two extractions with 50 mL50\,\mathrm{mL} each. Solution: One extraction: fraction left in water =100100+4×100=0.2= \frac{100}{100 + 4 \times 100} = 0.2, so 0.8 g0.8\,\mathrm{g} extracted. Two extractions: fraction left each time =100100+4×50=13= \frac{100}{100 + 4 \times 50} = \frac{1}{3}; after two, 19\frac{1}{9} remains, so 0.889 g0.889\,\mathrm{g} extracted. Splitting the solvent is more efficient.