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Organic Chemistry Laboratory I Extraction of (+) and (-)-Carvone from Oil of Caraway and Oil of Spearmint Experiment Description & Background

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Organic Chemistry Laboratory I Extraction of (+) and (-)-Carvone from Oil of Caraway and Oil of Spearmint Experiment Description & Background Organic Chemistry Laboratory I Extraction of (+) and (-)-Carvone from Oil of Caraway and Oil of Spearmint Experiment Description & Background Introduction Students will purify and isolate limonene and either (+)-carvone from oil of caraway or (-)-carvone from oil of spearmint using column chromatography. The isolated carvone and limonene will be identified by comparison with standards using thin layer chromatography (TLC) , infrared (IR) spectroscopy, and the optical rotation of the isolated carvone will be determined by polarimetry. Oil of Spearmint and Oil of Caraway Essential oils, derived from natural products, have been used for centuries as flavorings, fragrances and for medicinal purposes. These oils typically contain multiple organic components, generally with one or more compounds dominating the mixture to provide characteristic odor and properties. Essential oils are typically isolated from plant sources through distillation or extraction. Oil of spearmint is derived from the leaves of the spearmint (Mentha spicata) plant. A major constituent of oil of spearmint is the R enantiomer of carvone. Minor amounts of limonene, a metabolic precursor of carvone is also present in oil of spearmint. Caraway oil is extracted from caraway seeds (Carum carvi) and contains manily the S enantiomer of carvone along with higher levels of limonene. The limonene in oil of caraway is also a precuror to S-carvone. Specific carvone enantiomers can be isolated in pure form from oil of spearmint or oil of caraway using column chromatography. Limonene, a less polar constituent of these essential oils can be separated from the carvones Spearmint during chromatography. Purity of the isolated samples can be evaluated by thin layer chromatography and Caraway spectroscopic analysis, and the optical purity of the enatiomers can be evaluated using polarimetry. Column Chromatography and Thin Layer Chromatography Chromatography is an experimental method that is used in the laboratory to separate and characterize organic compounds. Chromatography may be used preparatively or analytically. Preparative chromatography is used to physcially separate components of a mixture for further use and characterization. Thus, preparative chromatography is a form of purification. Analytical chromatography measures the relative proportions of components in a mixture and may be used to chracterize specific components by comparison with standards or to characterize the mixture. There are many types of preparative and analytical chromatography, including thin layer chromatography (TLC), column chromatography (CC), gas chromatography (GC) and high pressure liquid chromatography (HPLC). All chromatographic techniques, whether preparative or analytical, involve a two component system: 1) a stationary phase and 2) a mobile phase. In TLC, the stationary phase is a plastic, glass or aluminum plate (usually 2.5cm X 10cm) coated with a material that serves as the stationary phase. The term stationary phase is used to describe the material on the plate becasue it does not move during the analysis, or it remains "stationary". Silica gel is most commonly used as a stationary phase for simple TLC analysis, but numerous other stationary phases can be employed for more sophisticated experiments. The mobile phase is a solvent that moves during the analysis and carries the analyte(s) (compound or compounds of the mixture) along the stationary phase. The mobile phase of a TLC analysis is also called the developing solvent. The developing solvent may be a single organic solvent or a mixture of two or more organic solvents. When binary (two solvents) or tertiary (three solvents) mixtures are used, they must be completely miscible in each other. Usually the solvents are of different polarities. Aqueous solvents are rarely used for simple TLC analyses. For CC, the stationary phase is the column packing material (often silica gel or alumina) and the mobile phase is the solvent that runs through the column carrying the analytes. For both TLC and CC, different components of the mixture will adhere to the stationary phase to different degrees depending on the relative polarity between the stationary phase and the specific component of the mixture. Polar components adhere strongly to a polar stationary phase; non-polar components adhere weakly to a polar stationary phase. For example, silica gel, the stationary phase used in this experiment is very polar. Very polar components of the mixture will adhere strongly to the silica gel, while less polar constituents have a weaker attraction. When the plate in a TLC analysis is Preparative Column Chromatography Analytical TLC Analysis developed, the polar components will tend to stay at the bottom of the plate (bound to the silica gel) and the non-polar components will tend to move with the relatively less polar mobile phase (developing solvent). The polar components will have a smaller Rf value than less polar components. For CC, the more polar component will remain at the origin (or top of the column) and the less polar components will move down the column at a faster rate. Thus polar components have longer retention times (rt) and non-polar components have shorter retention times. Non-polar stationary phases are hydrocarbon-based and are usually desginated by the number of carbons associated with the packing material. Non-polar columns (CC) and plates (TLC) are often referred to as C18 or C22 to indicate ther number of carbons in the hydrocarbon making up the non-polar stationary phase. back to top How to Choose the Mobile Phase (Developing Solvent) The purpose of the mobile phase is to move components of the mixture up the TLC plate or down the column and away from each other. The degree to which a component of the mixture moves with the mobile phase as opposed to staying adhered to the stationary phase (closer to the origin) depends on the component's polarity relative to each of these two phases. If the component has a polarity more like the mobile phase, then it will dissolve in the mobile phase and move (up the plate or down the column). If the component has a polarity more like the stationary phase, it will remain adhered to the stationary phase (at the bottom of the plate or at the top of the column). The polarity of the stationary phase is fixed. For example, silica gel is polar while C18 stationary phases are non-polar. However, the polarity of the mobile phase can be adjusted if more than one solvent is used. Binary (two solvents) or tertiary (three solvents) mixtures are usually used as a developing solvent for simple TLC and CC analyses. Typically the polarities of the solvents used in binary or tertiary mixtures are different. The overall polarity of the mobile phase can then be adjusted by changing the ratio of the polar solvent relative to the non-polar solvent of the mobile phase. Some typical solvent mixtures used as mobile phases in TLC and CC analyses are given in the table below. The more polar solvent of each mixture is given first. Ethyl Acetate-Hexane Ether-Pentane Acetone-Petroleum Ether Ethanol-Chloroform Ethanol-Chloroform-Hexane Acetic acid-Methanol-Benzene Solvent Combinations for Use as Mobile Phase in TLC Analysis Determining an appropriate mobile phase to achieve maximal separation of components in a mixture is a trial and error process. Ideally, all components of the mixture should be cleanly resolved (separated) from each other with no overlapping. All the components should also be located in the bottom/middle two thirds of a TLC plate after it has been developed. The only way to find a mobile phase that will result in meeting these criteria is to try a solvent mixture of a specific ratio and see what happens. If the desired results are not achieved, then adjust the solvent ratios. Consider some simple scenarios for guidance in how to adjust the ratios of solvent of binary or tertiary mobile phases to get the results you want. For example, let's say you have a mixture of three compounds, A, B, and C. You decide to use silica gel as the stationary phase and a binary mobile phase of ethyl acetate-hexane in a 50:50 ratio. The result of the TLC analysis looks like the illustration in Figure 2.5. None of the components of the mixture moved, suggesting A, B and C are all very polar and adhere strongly to the polar silica gel. A more polar solvent system is needed to move at least some of the components up the plate. You decide to increase the ratio of ethyl acetate to hexane to 75:25 resulting in a plate that looks like the one depicted in Figure 2.6. Two of the three components of the mixture have been resolved, but not the third. You then decide to increase the polarity of the mobile phase even more (90:10 ethyl acetate-hexane) to move the components further away from each other. The desired result is achieved as shown in Figure 2.7. Figure 2.5: Developed in 50:50 Figure 2.6: Developed in 75:25 Ethyl acetate-hexane Ethyl acetate-hexane Figure 2.7: Developed in 90:10 Ethyl acetate-hexane In an alternative scenario, using 50:50 ethyl acetate-hexane with silica gel, TLC analysis of a mixture of compounds X, Y and Z gave a developed TLC plate shown in Figure 2.8. All of the compounds moved very high on the plate suggesting they are all non-polar. It is necessary to make the components less soluble in the mobile phase. Increasing the polarity of the mobile phase will make the components less soluble and force them to remain lower on the plate. TLC analysis of the mixture with 75:25 ethyl acetate-hexane, then 90:10 ethyl acetate-hexane, gave the results shown in Figures 2.9 and 2.10.
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