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1、THIN LAYER CHROMATOGRAPHYReference: 1. Schoffstall, Gaddis, Druelinger, Microscale and Miniscale Organic Chemistry Laboratory Experiments, 2nd edition, McGraw Hill 2. Dickson, Kittredge, Sarquis, J.Chem.Ed, Vol81, No 7, PP 1023 1025, July 2004 TheoryChromatographic techniques are used extensively in
2、 organic chemistry laboratories for routine analysis. Thin layer chromatography (TLC) can be used to determine the purity of a compound, to analyze the composition of a mixture or to follow the progress of a reaction. The components of a mixture are differentiated by exposing to two competing phases
3、, the stationary and the mobile phases. In TLC, the stationary phase is a polar adsorbent such as silica gel or alumina, which has been coated on a plastic plate. The mobile phase is an organic solvent. The solvent moves up the plate by capillary action. A coated and dried glass or plastic plate is
4、called a thin-layer plate. The mixture to be analyzed is dissolved in a suitable solvent and applied as spots near the base of the plate. This process is known as spotting. Spotting is achieved by repeated applications of the mixture from a capillary pipet. When the filled pipet touches the plate, t
5、he liquid is delivered on to the plate by capillary action. The plate is immersed in a development chamber (usually a beaker) that contains a solvent or a mixture of solvents. The solvent in the development chamber is also referred to as the eluent or eluting solvent. As the solvent rises up the pla
6、te by capillary action, it carries the mixture with it. The components of the mixture are separated on the stationary phase depending on their polarity. This process is known as developing or running the TLC plate. Polar compounds are attracted to the silica gel and are held more tightly compared to
7、 nonpolar compounds. A separation results due to the difference in rates at which individual components move on the plate. In general, nonpolar compounds move faster than polar compounds on a TLC plate coated with silica gel. When the plate has been developed, it is removed and allowed to dry. There
8、 will a series of vertical spots on the plate. Each spot corresponds to a separate component of the original mixture. If the components are colorless, the spots are invisible. To see the spots, a visualization method is used. Ultraviolet lamp is a common visualization method. The distance traveled b
9、y each component is measured and this value is called the retardation factor, designated as Rf value. Rf value for a component is calculated using the following expression.Rf = Distance traveled by the componentDistance traveled by the solvent There is a Rf value associated with each developed spot
10、on the TLC plate.Figure 2: TLC plate in a beakerFigure 1: TLC plate before development in a beakerFigure 3: TLC plate after development ProcedureIn this guided inquiry lab, you will investigate properties such as hydrogen bonding and polarity of various samples and solvents in context to TLC. You wi
11、ll also apply your knowledge to a common industrial application such as separation and identification of components in an analgesic medication.Part 1: Effect of the length of TLC plate on Rf valueObtain three TLC plates of varying lengths, a development chamber (400 mL beaker and watch glass), a pen
12、cil and a ruler. Follow these instructions for all three TLC plates. With a lead pencil (do not use a pen), draw a faint line 1 cm from the bottom of a TLC plate. Mark a dot on this line. This dot will be the origin for the substance (benzyl alcohol) that you will spot. With a Microcap capillary tub
13、e, apply a tiny drop of benzyl alcohol (diluted by ethyl acetate in the ratio of 1:5) at the origin and air dry. Do not blow air. Place the TLC plate in a developing chamber. Trickle down 60% ethyl acetate-40% hexanes (using a pipet) along the sides of the beaker, making sure that the level of the s
14、olvent is below the spotting line. The volume of the solvent used in this experiment will vary with the size of the beaker, the distance of the spotting line from the bottom plate and other dimensions of the TLC plate. Place a watch glass on the beaker. You may also insert a piece of filter paper (a
15、s shown in Figure 2). When the level of the solvent reaches 0.5 cm from the top of the TLC plate, remove the plate, quickly mark the solvent front with a pencil and let it air dry. Do not shake the plate to dry it. Visualize the plate under a short wavelength UV lamp, in a darkened hood. Lightly out
16、line the spot with a pencil. Calculate Rf values for benzyl alcohol on each TLC plate. You may also choose to run all three TLC plates simultaneously (in separate beakers). Make a sketch of the plates in your lab report.Part 2: Effect of hydrogen bonding on Rf valueObtain a TLC plate and spot the following compounds on the same plate. Make sure the TLC plate is large enough to place all samples with goo
