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Chromatography

Autor:   •  June 18, 2018  •  1,519 Words (7 Pages)  •  122 Views

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to gently draw a line approximately 1.0 cm from the bottom of the plate (line of origin). Drawn on the line of the strip were 10 dots equally spaced apart. A capillary pipet was used to apply a spot of solution from fraction 1 to the first pencil dot, lightly applied the tip of the pipet to the surface of the plate careful not to allow the spot to diffuse to a diameter of more than 1.0-2.0 mm during application of the sample. The process was done for all 10 test tube fractions. The TLC plate dried so it could be transferred to the developing chamber with 15% dichloromethane in petroleum ether, as the developing solvent remained below the line of origin the solvent traveled up the TLC plate. As the solvent approached approximately 1.0 cm from the edge of the TLC plate, the TLC plate was removed and marked where the solvent traveled. The TA used a UV light to easily circle the UV active spots for each fraction and determine the Rfs for each pure component. The experiment was complete and the alumina was disposed in solid waste. The column was rinsed with acetone and left in the hood.

IV. Data and Observation

Figure 1

The top circles were flourene and 9-fluorenone were the bottom circles. Per TLC plate fractions 1,9 and 10 were the impure substances

Calculations

Chromatography 2-8

Rf Fluorene (2-4): 2 cm/4 cm = 2.00 cm

Rf 9- Fluorenone (5-8): 1.5 cm/3.5 cm=0.43

V. Discussion and Conclusion

Silica gel is a form of silicon dioxide. The silicon atoms are joined by oxygen atoms in a giant covalent structure. However, at the surface of the silica gel, the silicon atoms are attached to -OH groups. So, at the surface of the silica gel you have Si-O-H bonds instead of Si-O-Si bonds. The surface of the silica gel is polar and because of the -OH groups, form hydrogen bonds with suitable compounds around it as well as van der Waals dispersion forces and dipole-dipole attractions. As the solvent begins to soak up the plate, it first dissolves the compounds in the spot that you have put on the base line. The compounds present will get carried up the chromatography plate as the solvent continues to move upwards. How fast the compounds get carried up the plate depends on two things: 1. How soluble the compound is in the solvent (depend on how much attraction there is between the molecules of the compound and those of the solvent) 2. How much the compound sticks to the stationary phase (depend on how much attraction there is between the molecules of the compound and the silica gel).When it come to the compounds on a TLC, the one which can hydrogen bond will stick to the surface of the silica gel more firmly than the weaker intermolecular force compound (one is adsorbed more strongly than the other). Adsorption is the name given to one substance forming bonds to the surface of another substance. In the process of adsorption there is a constant movement of a molecule between being adsorbed onto the silica gel surface and going back into solution in the solvent. While the substance is being adsorbed on the silica gel, it temporarily stopped and the solvent is moving on without it; concluding the more strongly a compound is adsorbed, the less distance it can travel up the plate.

Disscussing Figure 1; The Rfs were used to determine the polarity of flourene (2.00 cm) and 9- Fluorenone (.43 cm), the higher the Rf the lower polarity, the lower the Rf the more polar the substance. Since silica gel is very polar, the flourene (Less polar) came out first on the test. The 9- Fluorenone stayed behind due to the compound being more polar. Unfortunately impurities were found in fractions 1, 9 and 10 due to improper sample handling. Since other students used the same equipment the possibility for unclean equipment was likely.

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