Thin layer analysis (TLC) experimental operation steps.

-related topicsThin Layer Analysis (TLC) TechnologyThin Layer
Chromatography
(TLC) is a very useful means of tracking reactions and can also be used for the selection of suitable solvents in column chromatography separation. The fixed phases commonly used in thin-layer chromatography are alumina or silicone, which are highly polar (standard) or non-polar (inverse). The flow phase is a polar solvent to be selected. Standard silicone panels will be used in 5.301 and in most laboratory experiments. Point the reaction mixture in the solution on the sheet and use capillary action to move the solvent (or mixed solvent) up the plate. Depending on the polarity of the components in the mixture,
compound
will move different distances on the sheet.
highly polar compounds "stick" to polar silicone and move at shorter distances on the sheet. Non-polar substances will remain in the flowing solvent phase for a longer period of time and thus move a larger distance between the plates. The distance the compound moves is expressed in rf. This is a value between 0 and 1, which is defined as the distance of the compound from the baseline (which was determined at the first point) divided by the forward distance of the solvent from the baseline.thin layer chromatography (TLC) experimental step: 1) cutting sheet. Typically, the silicone plates you buy are square glass plates that must be cut in the shape of a template with a diamond-headed glass knife. Before cutting the glass, gently mark the baseline position with a ruler and pencil on the silicone surface of the sheet (be careful not to damage the silicone surface). With a sharp glass cutter and a guide ruler, you can easily cut glass. When the whole piece of glass is cut, you can further divide it into separate pieces. (At first, you may find it a little difficult, but after some training, you'll be proficient in the technique.) ) 2) Select the appropriate solvent system. The amount of distance a compound moves on a sheet depends on the solvent selected. In non-polar solvents such as penane and hexane, most polar substances do not move, but non-polar compounds move a certain distance on the sheet. In contrast, polar solvents typically push non-polar compounds to the front of the solvent and polar compounds off the baseline. A good solvent system should keep all compounds in the mixture off the baseline, but not all compounds reach the front of the solvent, and the RF value is preferably between 0.15 and 0.85. Although this condition may not always be met, it should be the target of thin-layer chromatography (in column chromatography, the appropriate solvent should meet rf between 0.2 and 0.3). So, which solvents should be selected? Some standard solvents and their relative polarity (excerpted from LLP) are listed as follows:strong polar solvents: methanol ethanol isopropyl alcoholmedium polar solvents: ethyl acetate chloroform Dichloromethane ether xylenenon-polar solvents: cyclohexane, petroleum ether, hexane, pyrethroids Commonly used mixed solvents:ethyl acetate/hexane: common concentration 0 to 30%. Sometimes, however, it is more difficult to completely remove the solvent from the rotary evaporator.ether/ethylane system: concentration of 0 to 40% is more commonly used. It is very easy to remove on a rotating evaporator.ethanol/hexane or propane: suitable for strong polar compounds 5 to 30%.dichloromethane/hexane or pyrene: 5 to 30%, can be considered when other mixed solvents fail.3) Pour the selected solvent system of 1 to 2mL into the expander pool and place a large piece of
filter paper in the
.4) Sample the compound at the marked baseline. We bought the dot sampler, which can also be removed from the
heated
Pasteur
straw
(you can refer to UROP). When tracking the reaction, be sure to point to the starting reactant, the reaction mixture, and the mixture of the two.5) Unfold: Let the solvent expand up about 90% of the sheet length.6) Remove the sheet from the unfolding pool and immediately pencil out the forward position where the solvent has arrived. Based on this calculation rf value.7) Let the solvent on the sheet evaporate. 8) Observe the sheet with non-destructive technology. The best non-destructive way is to observe with UV lamps. Place the sheet under the UV lamp and mark all UV-active points with a pencil. Although this method is not used in 5.301, we will use another commonly used non-destructive method -- iodine staining. (You can see UROP). 9) Observe the sheet in a destructive way. This method can only be used when the compound is not UV active. In 5.301, a number of very useful dyes are available. When using a dye, clamp the
dyer
sheet with tweezers and place it in the dye to ensure that it is immersed from the baseline to the front of the solvent. Dry the back of the sheet with a paper towel. Place the sheet on the heating plate to observe the change in spots. Remove the sheet from the heating plate before the speckle becomes visible and the background color fails to cover the spot. 10) Modify the choice of solvent system based on the initial thin-layer chromatography results. If you want rf to be bigger, you can make the solvent system more polar, and if you want the rf to be smaller, you should make the solvent system less polar. If the dot on the sheet becomes striped rather than a circle, your sample concentration may be too high. Dilute the sample and then do a sheet layering, if it still does not work, you should consider a different solvent system. 11) Mark the TLC, calculate the RF value of each speck, and draw a drawing in your notebook
.