full version Separation Of Ferrocene And Acetylferrocene By Chromatography Essay

Separation Of Ferrocene And Acetylferrocene By Chromatography

Category: Science

Autor: cat_91 01 April 2010

Words: 1417 | Pages: 6

Purpose

1. To prepare a dry-packed sample of product mixture of ferrocene and Acetylferrocene
2. To separate and purify the components in the product mixture by column chromatography.
3. To check the purity of the components by thin-layer chromatography (TLC).
4. To calculate the yield of acetylferrocene and the percent recovery of unreacted ferrocene.

Table of Quantity showing various physical properties
Type of substance Molecular Formula Molecular Weight (g/mol) Density(g/cm3) M.P.(oC) B.P.(oC) Solubility
Ferrocene C10H10Fe 186.03 1.49 172 oC-174 oC 249oC Insoluble in water, soluble in organic solvent
Acetyl-ferrocene C12H12FeO 228.08 - 81 oC -83 oC 161 oC - 163 oC (at 4 mm) Very slightly soluble in water, soluble in organic solvent
Ethyl Acetate C4H8O2 88.11 0.897 -83.6 oC 77.1 oC Slightly soluble in water (~8.3g/100mL, 20 oC), very miscible with alcohol, ether, acetone, benzene.
Hexane C6H14 86.18 0.6548 -95 oC 69oC Immiscible with water
Dichloro-methane CH2Cl2 84.93 -97 oC 40 oC 1.325 Slightly soluble in water (~1.3g/100mL, 20 oC), very miscible with most organic solvent.

Hazard
1. Ferrocene is highly flammable and it is harmful if swallowed.
2. Acetylferrocene is highly toxic if swallowed.
3. Ethyl acetate is irritant and flammable.
4. Dichloromethane is a suspected carcinogen.
Procedure
1. The mass of the product mixture was measured.
2. About 2mL of CH2Cl2 was added to the product mixture in a 30mL beaker and about 300mg of silica gel was added
3. The beaker was covered with aluminum foil with a hole and was CH2Cl2 was evaporated.
4. 2.3g of fresh silica gel was mixed with 7mL of hexane in a 30mL beaker to get a slurry.
5. Stirred the slurry constantly with hexane added and poured it into the column.
6. Hexane was added and run until the solvent was about 1cm above the silica gel.
7. About 1cm of sand was added followed by loading of the dried sample.
8. Hexane was added and run until the first fraction was collected in a 10mL Erlenmeyer flask.
9. The solvent was changed to 20% ethyl acetate-hexane mix and run until the second fraction was collected in another 10mL Erlenmeyer flask.
10. After the 2 fractions were collected, thin-layer chromatography was carried out using 10% ethyl acetate-hexane mix to analyze the fractions with comparison to standard ferrocene and acetylferrocene.

Observations
When the silica gel was dissolved in hexane, it formed cloudy slurry and showed a light grayish color. After the sample was loaded and the solvent began to run, there was a diffusion movement of colored sample molecules. The reddish brown color was more or less stationary while the orange color stopped moving very soon at the top of the silica gel, just below the reddish brown. The yellow color moved much faster than the orange and reddish brown, down the column.

After the solvent was changed from hexane to ethyl acetate-hexane mixture, the orange color began to diffuse down the column too while the reddish brown color remained stationary.

In analysis of the 2 colored fractions collected by TCL, it was observed that as the solvent ethyl acetate moved up, yellow spots appeared first. After sometime when the yellow spot was at the middle of the plate, the orange spots appeared. At the end, 2 yellows spots were at the upper section of the plate and the 2 orange spots were at the lower section of the plate. At the origin, there were no colored spots.
After vacuum evaporation of the 2 fractions collected, yellow powder-like substance was found covering the inner wall of one of the Erlenmeyer flask while orange crystalline substance was found at the other. The orange crystals were larger and more discrete than the yellow powders.

Data and Calculations:
Mass of crude product mixture =0.0936 g
Mass of Erlenmeyer flask (for ferrocene) = 49.9674 g
Mass of Erlenmeyer flask (for acetylferrocene) = 50.7664 g
Mass of flask and recovered ferrocene = 50.0425 g
Mass of flask and acetylferrocene = 50.7983 g
Amount of ferrocene recovered = 50.0425 - 49.9674 = 0.0751g
Amount of acetylferrocene obtained = 50.7983 - 50.7664= 0.0319 g
Percentage recovery of ferrocene = (0.0751g/0.0936 g)(100) = 80.2%
Percentage yield of acetylferrocene=(0.0319g/0.0936 g)(100) = 34.1%
Table one showing the Retention factor of unknown fractions with comparison to standard values
Chemical Compounds Distance from baseline to spots (cm)/Distance from baseline to solvent front (cm) Retention FactorRf
Standard ferrocene 3.5/4.2 0.83
Fraction #1 (yellow) 3.5/4.2 0.83
Standard acetylferrocene 1/4.2 0.24
Fraction #2 (orange) 1/4.2 0.24

Conclusions and Answers to Questions

Separation by column chromatography

Chromatography was commonly used as a purifying technique. The eluting solvent passed down the column by the gravity and an equilibrium was established between the solute absorbed by the absorbent (silica gel in this experiment) and the eluting solving flowing down. Since the components in the sample had different polarity and they interacted with the stationary phase and the mobile phase differently, the components would be carried by the solvent to a different extent and a separation of the components could be achieved.

In this column chromatography, acetylferrocene was more polar, therefore was held by the silica gel more tightly and moved through the column more slowly when the eluting solvent was nonpolar hexane. Increasing the polarity of the solvent would move all components faster. That explained when the solvent was switched to the more polar 20% ethyl acetate-hexane mix, the acetylferrocene started flowing down with the eluting solvent as it has less interaction with the stationary phase (silica gel). The possible in this experiment could be that the column was not vertical enough which could decrease the separation efficiency. A ruler or spirit level could be used to ensure vertical alignment.

Analysis by Thin-layer Chromatography
In the thin-layer chromatography, the yellow ferrocene moved up first and much faster because it was less polar and was held less tightly by the cellulose of the paper plate (stationary phase). The orange acetylferrocene was more polar. It formed hydrogen bonding with the cellulose of paper plate, as a result, it moved much slower and later.

It was concluded that the separation of the ferrocene and acetylferrocene was very successful because the spot of the yellow unknown fraction aligned perfectly with the standard ferrocene spot, and there was no other spots indicating that the yellow fraction was pure ferrocene. On the other hand, the orange unknown fraction aligned perfectly with the standard acetylferrocene, with no other spots, indicating that the orange fraction was pure acetylferrocene. This result was confirmed after the retention factor Rf was calculated. It was found that the Rf of the unknown fractions exactly matched with the standard ones. To further confirm the complete separation of the crude product mixture, IR and NMR spectroscopy could also be carried out.

The percentage yield of acetylferrocene was 34.1% and the percent recovery of ferrocene was 80.2%. The low percentage yield of acetylferrocene could be due to the incomplete conversion of ferrocene or high percentage yield of side product diacetylferrocene. One of the biggest error that lead to the decrease in percentage recovery of ferrocene was the human error since quite a significant amount of ferrocene was spilled. A more careful handling of the glassware was necessary. The other error was caused by the random fluctuation of the reading on electronic balance. The fluctuation could be very significant as the mass of ferrocene/acetylferrocene recovered was comparatively very small (in mg). This could be improved by using a more sensitive electronic balance with stable readings.


Questions

1) Due to the silica gelЎ¦s polarity, the non-polar substance would tend to elute first then the polar ones, which bind more tightly to the silica gel (stationary phase). Therefore, the order of elution should be propylbenzene, followed by acetophenone, and then benzyl alcohol, and finally benzoic acid.

2) a. The column must be vertical else the flow of solvent would be affected. This in turn affected the absorption of silica gel and would decrease the separation efficiency. Also, if the column was slanted, more solvent was needed to wash down the elutants and the components coming down the column could overlap.

b. The column must never dry so as to make sure the stationary phase is always covered by the mobile phase, otherwise air bubbles would be trapped in the stationary phase. Also, if the column was dry, the compounds would fall through the cracks in the column instead of partitioning between the stationary and mobile phases. All these would severe impair the separation and lead to poor result.

c. Since a careless addition of sample would disturb the stationary phase and lead to poor separation so a layer of sand on top of the column could act as shock-absorber when the sample is loaded. Also a layer of sand could ensure a level silica gel line. If sand were not added, the molecules passing down the center of the column would encounter less silica gel than the molecules traveling down the edge since the bottom the column was typically cone-shaped.

References
1) Safety (MSDS) data (http://physchem.ox.ac.uk/MSDS/)
2) Sigma-Aldrich catalogue (http://www.sigmaaldrich.com)
3) Williamson, Macroscale and Microscale Organic Experiments, 4th Edition, 2003, P.156-162, 171-173
4) Solomons and Fryhle, Organic Chemistry, 8th edition, 2004, P.1058-1059