Orgolab 7 - Completed Lab #7 from CHEM 2221L with all questions answered. PDF

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Completed Lab #7 from CHEM 2221L with all questions answered....

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Section 1 November 6, 2017 Title

Lab #7: Chromatography of an Acetylferrocene Mixture Introduction Flash chromatography is a method of separating a solid mixture based on the polarity of the compounds in the mixture. By building a column with silica gel, sand, and the mixture, and adding different mobile phases with known polarities, the compounds can be eluted separately and collected into different fractions under the column. In this experiment, we performed flash chromatography on a “waste” acetylferrocene mixture, which was actually a mix of acetylferrocene, ferrocene, and 9-fluorenone. The column was eluted with petroleum ether (nonpolar), a mixture of petroleum ether and tBME, and pure tBME (polar). The eluents were collected in different fractions, and their purity was analyzed through TLC. Procedure The procedure for experiment #7 in the Chemistry 2221L Class pak was followed with the following modifications. • 121 mg of the waste acetylferrocene mixture was used. • Amounts of silica used were estimates, not weighed out. • About 10 ml of pure tBME was used to fully collect the red band. • For TLC, spots were created for only each color liquid, not every test tube. • Melting points were not determined for this experiment. Results Compound

Spot distance (cm)

Experiment al Rf value

Solvent distance (cm)

Final mass (mg)

Mass percent of original mixture

Mass percent of final total

Ferrocene

2.7

3.1

0.87

49

40.5%

32.0%

Acetylferroc ene

0.4

3.1

0.13

54

44.6%

35.3%

9fluorenone

1.3

3.1

0.42

50

41.3%

32.7%

153

126%

Total

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Conclusions The first solvent added to the column was the very non polar petroleum ether, and the first two fractions collected were a yellow color. In flash chromatography, a non polar eluent will cause only non polar molecules to move down the column. This means that the first fractions collected should contain ferrocene, the least polar of the three compounds. This is the case as the first two fractions were yellow, the color of ferrocene. The next mobile phase added to the column was a mixture of petroleum ether and tBME, which should have moved 9-flourenone down the column. 9-fluorenone is colorless, as were the middle fractions collected. The last solvent was pure tBME. Polar solvents move all compounds down the column, but since the other two compounds had already been extracted, only the very polar red acetylferrocene was left to the last two fractions. The TLC plate confirms the presence of the three compounds of differing polarity in the original mixture by the three different spots shown on the left side of the plate. Since the solvent used was mostly non polar, the three compounds can be identified by the distance the spots travelled on the plate. The second one was ferrocene, because it is very non polar. Then 9fluorenone because of its mild polarity, and finally acetylferrocene which hardly travelled at all because it is very polar. The second and third one had only one spot, showing that each of those products were relatively pure. The acetylferrocene spot had a very faint spot showing the mild presence of 9-fluorenone in the product. This happened because not all of the 9-fluorenone was extracted during the second solvent in the column. The mass percent recovery from the original mixture to the final separate solids was over 100%, meaning that the solids had not quite evaporated off all of their solvents in at least one of them. They should have been left on the heat for a little longer despite the fact that they looked dry when they were taken off. The original mixture was made up of about equal amounts of each

solid; their percentages of the total are almost equal, with acetylferrocene having barely more than the others. Answers to questions 1) Petroleum ether is a non polar solvent, which means that it would only elute non polar compounds from the mixture. Ferrocene would come first, then possibly some of the 9fluorenone, but that would take a higher quantity of solvent. 2) Polar solvents move all compounds through the column, so the whole mixture would be eluted and there would be no distinction between fractions. 3) See results section. 4) The circled product predominates because it puts the high electron density of the oxygens as far away from one another as possible. It also in general is more spread out as entropy is favorable....