Download Chemical Separation: Recrystallization, Melting Points, Chromatography, Extraction and more Study notes Biology in PDF only on Docsity! Recrystallization - Asuitable solvent for recrystallization is one in which the desired compound is very soluble in the solvent at its boiling point, but is much less soluble in the solvent at room or ice-water temperatures so that the compound comes out of solution as the mixture cools Figure 7.2 The Temperature Dependence of Solubility Impurities are soluble and remain in solution. The compound is soluble at high temperatures and the solubility decreases as the temperature is decreased Solubility L Impurities are insoluble and are filtered from the hot solution. Temperature - The desired compound has a finite solubility in the cold solvent, so some loss always occurs. To minimize this loss, it is important to add the minimum amount of hot solvent needed to dissolve the material Mixed Melting Points - each melting point can signify two possible compositions, so the mixed melting point allows you to figure out which composition was correct (rubric of exp 5 data analysis) Melting Points - insoluble impurities do not affect the melting points, same melting range TLC - separate on basis of differing polarities - polar groups have less movement than nonpolar groups since they interact with -OH on the surface of the TLC plate - in hexane: ethyl acetate more ethyl acetate allows for more travel distance Polar substances will adsorb, or bind, to the surface of the gel more strongly using hydrogen bonds or dipole-dipole interactions (as well as the weak intermolecular forces). Non-polar substances will adsorb weakly to the surface of the silica gel using only the weak intermolecular forces (london forces or dispersion forces). soon as the solvent front reaches the place where we have placed our spot of mixture, the components in the mixture will dissolve in the solvent. |How well each component dissolves in the solvent depends on factors such as the relative polarity of the solvent molecules and the molecules in the mixture. If we use a non-polar solvent as the mobile phase: © non-polar components in the mixture will dissolve more easily. © polar components in the mixture will dissolve less easily. If we use a polar solvent as the mobile phase: © polar components in the mixture will dissolve more easily. © non-polar components in the mixture will dissolve less easily. Adsorbs strongly to TLC plate. Adsorbs weakly to TLC plate. Less soluble in solvent. More soluble in solvent. Spot travels smallest distance. Spot travels furthest distance. Smaller Re Larger Re Adsorbs weakly to TLC plate. Adsorbs strongly to TLC plate. More soluble in solvent. Less soluble in solvent. Spot travels furthest distance. ‘Spot travels smallest distance. Larger Re Smaller Re - lodine - UValternating pi bonds - DNPH stains aldehydes or ketones - When same stationary phase, but more polar mobile phase all spots move higher up (TLC herb lab on prelab) - diastereomers have differing rf values (TLC herb lab on prelab) Extraction: separate on basis of differing solubilities Liquid-Liquid Extraction - The technique of extraction uses the differences in the solubility of a compound between two immiscible phases to partition compounds between the two phases. - Extraction is useful only for separating compounds with very different solubilities in the extraction solvent.
