Organic Chemistry Semester 1 LABORATORY MANUAL - Moravian ...

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Part V. Experiments 30 Fall 2010 E. Introduction: In the experiments over the last few weeks, we have developed an understanding of the relationships between molecular structure and intermolecular forces. In Experiment 5 we apply this understanding to determine the purity of solid substances. Each student will be given a crude sample of an unknown solid organic compound. In Part A we will explore experimental methods for judging the purity of the compounds and in Part B we will purify the samples. 1. Chromatography: In general, chromatography is defined as the separation of a mixture of two or more different compounds or ions by distribution between two phases, one of which is stationary and the other mobile. Various types of chromatography are possible, depending on the nature of the two phases involved. Table 1 Type Stationary Phase Mobile Phase Solid-Liquid (column, thin layer or tlc, paper) Liquid-Liquid Gas-Liquid (GC or GLC) finely divided solid (Silica or Alumina) viscous liquid coated on a finely divided solid viscous liquid coated on a finely divided solid organic liquids with varying polarity organic liquids with varying polarity inert gas (He, Ar, N 2 ) All chromatographic methods work on the same principle, the differential affinities of the substances to be separated between the two phases. This experiment deals with column and thin-layer chromatography, which are based on adsorptivity of compounds on a solid surface vs. their solubilities in a liquid solvent. In solid-liquid chromatography, the stationary phase is a finely divided solid that may be any substance that is not soluble in the associated liquid phase. Solids commonly used in chromatography are alumina (Al 2 O 3 ), and silica (SiO 2 ); both of these compounds are very polar. In this experiment the solid used will be finely divided silica, often called silica gel. If a solution containing an organic compound is added to finely divided silica (or alumina) some of the organic compound will adsorb onto the surface of the fine particles of silica. This adsorption occurs because of the intermolecular forces created between the silica and the organic molecules. As we have seen in our studies of boiling points and solubility, if the organic molecules are non-polar, the forces will be weak (London Dispersion forces). Since London forces arise from electrons in non-polar covalent bonds, non-polar molecules will not adsorb well on the surfaces of polar silica gel particles unless the organic molecule has a large surface area (usually high molecular weight compounds). If the molecule is polar, the attractive forces between the molecule and silica gel particles can be strong (hydrogen bonding or dipole-dipole interactions). The more polar the compound, the more strongly will it adsorbed onto the silica (or alumina) surface. These interactions are very similar to those we saw affecting solubility in Experiments 2 and 3. In chromatography, the principles of adsorptivity and solubility are important because they are competing for the components of the sample being separated. Solutes adsorbed onto a solid phase (silica or alumina) can be removed by a solvent if the solvent has a higher affinity for the solute than does the solid phase. Initially, the substance(s) to be separated, referred to as the eluates, are adsorbed onto the surface of the silica or alumina. If the solvent (eluent) polarity is increased, the solubilities of the eluates increase removing some of their molecules from the solid phase (desorbing them) causing them to move with the eluent. The process of adsorption and desorption is a dynamic equilibrium. Column chromatography is usually used as a method to separate mixtures and isolate the components. Thin-layer chromatography is most often used to analyze very small sample to judge purity of a sample and identify the components.
Fall 2010 31 Part IV. Experiments 2. Thin Layer chromatography Thin layer chromatography (TLC) is an example of ascending chromatography whereas column chromatography is an example of descending chromatography. The principles behind both separation methods, however, are identical. In TLC, a liquid phase travels, by capillary action, up a thin film of solid adsorbent. Solutes dissolved in the liquid phase will repeatedly adsorbed onto the solid phase (usually silica gel or alumina) and desorb into the solvent. The degree of adsorption will be determined by the polarity of the solute and the polarity of the solvent. As indicated in the discussion on column chromatography, polar compounds tend to be more firmly adsorbed onto silica gel and alumina stationary phase and thus travel more slowly. Conversely, non-polar solutes have less affinity for the adsorbent and travel more quickly. The location of the solute(s) on the plate can be determined by several visualization techniques. Identification of the solutes is usually achieved by comparing their TLC behavior (location and visualization characteristics) with that of a known substance. The location of the solute is represented by the R f value, which is the ratio of the distance traveled by the solute on the plate to the distance traveled by the solvent. This R f value is a constant for a solute in a given solvent system and on a specified adsorbent system. 3. Column chromatography: In column chromatography, gravity causes the eluent to pass through a vertical column packed with silica or alumina onto which has been adsorbed the substance(s) to be separated (eluates). Through a series of desorptions and adsorptions there is preferential separation of the eluates based on their relative affinities for the silica and solvent. THE MORE POLAR ELUATE HAS A HIGHER AFFINITY FOR THE SILICA (stationary phase) SO IT MOVES DOWN THE COLUMN MORE SLOWLY THAN DOES A LESS POLAR ELUATE. By gradually increasing the polarity of the solvent (eluent) all of the eluates can be sequentially removed from the polar stationary phase.
Page 1: MORAVIAN COLLEGE CHEMISTRY DEPARTME
Page 4 and 5: Table of Contents (continued) 2 PAR
Page 6 and 7: I. Introduction 4 Fall 2010 Assignm
Page 8 and 9: I. Introduction 6 Fall 2010 Assignm
Page 10 and 11: Part I. Introduction 8 Fall 2010 La
Page 12 and 13: Part I. Introduction 10 Fall 2010 D
Page 14 and 15: Part II. Lab Rules and Regulations
Page 16 and 17: Part II. Lab Rules and Regulations
Page 18 and 19: III Laboratory Records & Report For
Page 24 and 25: Part V. Experiments 22 Fall 2010 Pa
Page 26 and 27: Part V. Experiments 24 Fall 2010 Ex
Page 28 and 29: Part V. Experiments 26 Fall 2010 H
Page 30 and 31: Part V. Experiments 28 Fall 2010 E.
Page 40 and 41: Part V. Experiments 38 Fall 2010
Page 42 and 43: Part V. Experiments 40 Fall 2010 1
Page 45 and 46: Part V. Techniques and Theory Appen
Page 57 and 58: c Part V. Techniques and Theory App
Page 61: Part V. Techniques and Theory Appen
Page 64 and 65: Nuclear Magnetic Resonance Appendix
Page 82 and 83:
Nuclear Magnetic Resonance Appendix
Page 84:
Nuclear Magnetic Resonance Appendix
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Organic Chemistry Semester 1 LABORATORY MANUAL - Moravian ...

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