Extraction of Carvone

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Carvone Extraction Exercise complicated part of most syntheses is to ensure that your product has the appropriate stereochemistry, and an enormous amount of work is expended by chemists to try to ensure better stereochemical control. Interestingly, living organisms have no problem producing stereochemically pure products. While it is difficult to make an enantiomerically pure compound in the lab, plants and animals do it constantly with no error. Living organisms are also much more sensitive to stereochemical differences than many laboratory tests. Using most techniques, two enantiomers behave exactly the same: they will have the same melting point, boiling point, and density, and will react identically with achiral chemicals. The only way to distinguish between different enantiomers in the laboratory is to measure their optical activity or treat them with other enantiomerically pure chemicals. Living organisms, on the other hand, are excellent at distinguishing between enantiomers. The nose, for example, is one of the most sensitive tools for distinguishing some enantiomers--two different enantiomers can smell completely different even though they behave the same as one another in many other tests. In this lab you will study two naturally produced enantiomers, (-)-carvone and (+)-carvone. Both of these chemicals are edible in small quantities and are used as flavoring oils in industry. They have the exact same molecular structure, except that they are mirror images of one another. One of these enantiomers comes from caraway seeds and the other from spearmint leaves. These compounds are responsible for the distinctive smells of the plants, and you can easily determine which enantiomer is derived from which plant by its smell. You will extract carvone from both caraway seeds and spearmint leaves by soaking the plant matter in the proper solvent, and then you will use thin layer chromatography (TLC) and IR to compare the compounds you extract. The extraction is based on the fact that carvone is very soluble in methylene chloride and in methanol. Therefore, soaking seeds or leaves in either of these will cause it to leach out of the plant matter and into the solvent. The reason we do not use the same solvent for both extractions is that methanol dissolves the carvone more readily but also dissolves many other substances. There are so many materials in caraway seeds that are soluble in methanol that using this solvent would result in a highly contaminated sample. In spearmint leaves fewer contaminants are extracted into the methanol, so it is an ideal solvent for this extraction. Methylene chloride does not dissolve carvone as well, but it is an effective solvent for the extraction of carvone from caraway seeds, where it is abundant. Experimental NOTE: All TLC plates that have glass backing go into the sharps container. They may not be sharp when you use them but odds are the glass will end up that way. The technique that will be used to analyze your extracts is explained in the following section. THIN LAYER CHROMOTOGRAPHY (TLC) Page 2 of 6 TLC is one of the most commonly used techniques in chemistry. It allows a chemist to accurately and rapidly analyze very small quantities of material. In this experiment you will use TLC to identify and also to check the purity. mhtml:file://C:\Users\dzm10601\Desktop\Carvone Extraction Exercise.mht 7/26/2010
Carvone Extraction Exercise Page 3 of 6 TLC plates are either glass or aluminum that has been coated with a very thin layer of silica. See text. The polar silica is strongly attracted to polar materials, and any polar material on the plate will tend to stick to it. When you place a small spot of material at the bottom of the plate, it will be carried up the plate by the solvent as capillary action proceeds. The most polar components of the sample will be very attracted to the stationary silica gel and not move very far up the plate. The least polar materials will have no attraction at all for the silica and will dissolve in the solvent well, thus moving up the plate at the same speed as the solvent itself. As time goes on, the compounds within the sample will become separated on the plate according to polarity. The most polar molecules will be stuck at the bottom while the least polar will flow quickly to the top. A TLC plate is prepared as follows. Take a plate and mark two lines on it. Place one line approximately 1-2 cm from the bottom and one 1-2 from the top. You will spot the sample on the lower line. When the TLC plate is being developed the solvent will run up the plate. You will remove the plate when the solvent reaches the second line. In about one minute the solvent will evaporate and you will be able to check your sample under UV light or stain the plate to determine which compounds appeared. After development the plate will look something like the following diagram. TLC spots are identified by their Rf value, which is the ratio of the distances the spots ran to the distance the solvent itself ran. Thus the Rf is calculated by dividing the distance the spot ran by the distance the solvent ran. You can see that the spots on the plate shown above have Rf values of 0.25, 0.72, and 0.87. It can be useful to run several samples on the same TLC plate to easily compare the components of one sample with the components of another. If the same molecule is contained in two different samples, when you run the samples together on the same plate this molecule will run the same distance for both samples and you will see two spots right next to each other. The size and intensity of the spots are indicative of the amount of material, so you can see which sample contains more of the material. In this lab you will be given two samples of carvone (one + and one -) that were bought from a chemical company. You will run these on the same TLC plate as you plant material extracts in order to determine whether you actually extracted carvone and how pure this extract is. Remember to always use pencil when marking a TLC plate. Pen ink will run up the plate along with the solvent and render the plate unreadable. mhtml:file://C:\Users\dzm10601\Desktop\Carvone Extraction Exercise.mht 7/26/2010
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Extraction of Carvone

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