Practical Experiment 6: Friedel Crafts Acetylation Of Ferrocene PDF

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Experiment 6: Friedel Crafts Acetylation of Ferrocene and Column Chromatography Performed on 10/9/13 by Juliet Hammer; Due Date: 10/23/13 Purpose: There were two parts to this experiment. The purpose of the first part of the experiment was to carry out the reaction of acetylation of ferrocene. The purpose of the second part of the experiment was to use column chromatography to separate the components of the sample collected and to identify the components along with the percent yield of acetylferrocene to determine the success of the reaction. Reactions: Overall Reaction Acetylation of Ferrocene (Pictures taken from U. Va. Organic Chemistry Lab Manual)

In this reaction, ferrocene was reacted with acetyl anhydride using sulfuric acid as a catalyst. There are three main steps that exist in the mechanism of this reaction. Step 1ǂ

ǂ

Step 1 of the reaction is the protonation of the electrophile, which in this reaction is acetyl anhydride. Sulfuric acid protonates the oxygen of acetyl anhydride making it more reactive. There are two resonance forms of the protonated acetyl anhydride and the nucleophile reacts with the more reactive form (labeled as ). ǂǂ

Step 2

Step 3 ǂ ǂ ǂ

Step 2 of the reaction is the formation of an intermediate via a ring slip in ferrocene. Ferrocene has two aromatic, very stable rings. However, with heat, the top ring can shift making the electrons more localized and the molecule becomes reactive. The ring slip frees a pi bond to do chemistry on. When this ring slip occurs, ferrocene becomes a nucleophile and reacts with the acetyl anhydride electrophile to form an intermediate.

ǂǂ

ǂǂǂ

Step 3 in the reaction is the restoration of aromaticity in

the2013) ring of Virginia, that(University produce

and the restoration of the catalyst. The driving force Side reactions of acetylation behind this step is the restoraction of the stable aromatic ring. byproducts exist and are discussed in Sulfuric acid, which has lost a hydrogen to acetyl anhydride, Appendix A. now has a negative charge. H2PO4 takes an H from the Procedure: In the first part of the positively charged ring to restore aromaticity, creating acetyl experiment, the acetylation of ferrocene was ferrocene as a product. ydride, and 1 mL of phosphoric acid were added to a roundbottom flask. The flask was attached to a watercooled reflux condenser equipped with a drying tube and the solution was heated between 70-80°C using a hot water bath for 5-10 minutes. The solution was poured into a beaker containing 20-30 mL of crushed ice and was stirred until the ice was melted. The solution was neutralized by adding 40 mL of 10% NaOH solution in 10 mL portions. Sodium bicarbonate was added in small portions until the pH of the solution as between 6 and 8. The crude product was collected by vacuum filtration using a Buchner funnel and the solid was dissolved in 20 mL of hot hexane. The solution was transferred to a graduated cylinder discarding the solid residue and hexane was added until the final volume was 20 mL. 2 mL (exactly 10 percent) of the solution were transferred to a round-bottom flask and the solvent was evaporated using a rotary evaporator. In the second part of the experiment, column chromatography was used to separate out the components of the product obtained from the acetylation. A small chromatographic column was constructed and the sample was dissolved in a minimal amount of dichloromethane. The sample was added to the column and hexane was immediately added to the column to elute the first band. The material was collected in the “hexane fraction” test tube. After the first band had completed eluted, dichloromethane was added to the column and the material was collected in the “dichloromethane

fraction” test tube. No color remained in the column and so no acetone fraction was collected. TLC was performed with the two fractions and ferrocene, acetylferrocene, and diacetylferrocene standards and the Rf values were compared in order to identify the components. Each solution was transferred to tared round-bottom flasks and the solvent was evaporated. The yield and melting points of the individual components were then determined. Calculations: 1) Calculation of Percent Yield of Acetyl Ferrocene

1 mol ( 228.07 g ) Calculated Yield(mol) Percent Yield= ∗100 %= ∗100 %=32.6 % Theoretical Yield(mol) 1 mol 0.5 g∗( 186.04 g ) 0.2 g∗

Results: *Table 1 compares the Rf values of each of the column chromatography fractions to standards of ferrocene, acetylferrocene, and diacetylferrocene. From this data, it can be concluded that the hexane fraction (Rf of 0.98) contains ferrocene (Rf of 0.97) and that the dichloromethane fraction (Rf of 0.36) contains acetylferrocene (Rf of 0.37). Neither fraction contained diacetylferrocene (Rf of 0.07) and it can be concluded that the sample did not contain diacetylferrocene. Rf calculation is showed in Appendix B.

**The observed melting points help to confirm the identifications made from the data in Table 1. The hexane fraction (MP 174179°C) can be identified as ferrocene (MP 174-176°C) and the dicholormethane fraction (MP 78-80°C) can be identified as acetylferrocene (81-83°C). Using the fact that the dichloromethane sample contained acetylferrocene, a percent yield can be calculated (Calculation 2). The percent yield was very low. Because there was still ferrocene left, and no diacetylferrocene produced, it can be concluded that the reaction did not go to completion. The experiment should be repeated again with the solution being heated for longer for a higher yield of acetylferrocene. Calculation of calculated yield is shown in Appendix B.

Discussion: When comparing the Rf values obtained from the thin layer chromatography trial, it can be concluded that the hexane fraction (Rf of 0.98) contained ferrocene (Rf of 0.97) and the dichloromethane fraction (Rf of 0.36) contained acetylferrocene (Rf of 0.37). No diacetylferrocene was present in either fractions (Rf of 0.07). This conclusion is confirmed by the melting points observed. The component from the hexane fraction melted at 174-179°C which is comparable to the 174-176° melting point of ferrocene. The dichloromethane fraction melted at 78-80°C which is comparable to the 81-83°C of acetylferrocene. Because all color had eluted from the column after the dichloromethane fraction, it was concluded that no material was left in the column and no acetone fraction was taken. The purpose of the acetone fraction would have been to collect diacetylferrocene, however it can be concluded that no diacetylferrocene was formed since no material was left in the column. The identifications of ferrocene and diacetylferrocene can also be confirmed conceptually. Ferrocene is a less polar molecule than acetylferrocene as shown by the distance that each substance travelled on the TLC plate. Ferrocene was carried all the way with the solvent front while acetylferrocene was not. This is because ferrocene has two aromatic rings, making it an uncharged, stable molecule while acetylferrocene has an acetyl group coming off of the one of the rings, making it more polar. Of the solvents used in the column chromatography, hexane is a less polar solvent than dichloromethane. When hexane was used first in column chromatography, it was able to bond to and carry the less polar ferrocene,

but was unable to carry the more polar acetylferrocene with it (like attracts like). Because of this, the hexane fraction collected was composed of only ferrocene. Dichloromethane, however, is a more polar solvent and was able to bond to acetylferrocene and carry it down the column to be collected. Since all of the ferrocene had already been removed from the column with the hexane, the dichloromethane sample only contained acetylferrocene. If diacetylferrocene, the most polar of the molecules, had been present, acetone, the most polar solvent, would have been used to carry it down the column. The yield and percent yield of acetylferrocene were 0.2 g (0.88 mmol) and 32.6% respectively. Using this data, it can be determined that 0.337 grams of ferrocene (about 67% of the original sample) remained unreacted (Appendix B). From this and the fact that no diacetylferrocene was produced (diacetylferrocene is produced when the reaction goes beyond completion), it can be concluded that the reaction did not go to completion. Error Analysis: The effectiveness of the column chromatography can be analyzed using the TLC plate. Because only one spot appeared after TLC was run for each fraction, each fraction showed good purification. This means that the column was effective in separating each component of the sample. The effectiveness of the reaction can be determined by the presence of both ferrocene and the polymer. The presence of both of these after the reaction was run means that there was not a 100% yield of acetylferrocene. Since there was still starting material left over, the reaction did not go to completion. This means that the solution was not heated long enough for all of the ferrocene to be converted to acetylferrocene. In the future, the sample should be heated for longer, but not so long that diacetylferrocene is created. In terms of the polymer formation, the only way for the formation of polymer to be prevented is to carry out the reaction in the absence of oxygen (see Appendix A). The easiest way to carry out the reaction in the absence of oxygen is to perform the reaction in an anaerobic hood or glove bag that has another gas flowing through. In the future, performing the reaction in the absence of oxygen will give a yield closer to 100%. The TLC experiment serves better for the identification of the components in the column chromatography samples than the observed melting points. The observed melting point of ferrocene (174-179°C) was higher than the expected melting point (174-176°C). Because impurities cause a lower observed melting point than the theoretical melting point, an impure sample was not the cause of this higher melting point. Either the thermometer had an inaccurate reading or the reading was taken at an incorrect time. The observed melting point of acetylferrocene (78-80°C) was lower than the expected melting point (81-83°C) suggesting that there were some impurities in the sample. However, the TLC trial did not show any impurities in the acetylferrocene sample because only one spot appeared on the plate. This along with the observed ferrocene melting point that was theoretically impossible to have observed shows that TLC is a more effective way of identifying the components in the sample and for assessing the effectiveness of the column chromatography. In the future, the melting points should be more carefully observed and a different thermometer should be used. Additionally, TLC should definitely be run in order to identify the components of the sample and to assess the effectiveness of the column chromatography. Conclusion: The acetylation of ferrocene is a difficult reaction to perfect. If the solution is not heated for long enough, not all of the ferrocene is converted to acetylferrocene. However, if the solution is heated for too long, acetylferrocene will be converted to diacetylferrocene. On top of this, the formation of a polymer is impossible to avoid when performing the reaction in the presence of oxygen. The way that this reaction was carried out in this experiment was not an effective way to produce a 100% yield of acetylferrocene. In the future, the reaction should be carried out in an anaerobic atmosphere and the solution should be heated for a longer period of time in order for the reaction to go to completion. Multiple repetitions of the reaction should be carried out in order to determine the best amount of heating time for the reaction that produces the highest yield of acetylferrocene possible. Column chromatography proved to be an effective way to separate out the components of the sample produced from the reaction. Each fraction was pure, containing only one component, and it was easy to identify which component was present in each fraction.

References: Pavia, D.L.; Lampman, G.M.; Kris, G.S.; Engel, R.G. Small Scale Approach Organic Laboratory Techniques, 3rd ed.; Cenage Learning: Ohio, 2010; pp 262-282. University of Virginia. Fall 2013 Organic Chemistry Laboratory Manual; P.S. Publishing, 2013; pp 57-62.

Appendix A: Side Reactions of Acetylation

Ferrocene, acetylferrocene, or diacetylferrocene react with oxygen to create a polymer.

(University of Virginia, 2013) When acetylferrocene is heated for too long, a second acetylation occurs.

Appendix B: Additional Calculations

1) Calculating Total Yield from Observed Yield

Yield of Substance=Observed Yield∗10 ; Yield of Ferrocene=0.01∗10 =0.1

2) Calculation of Amount of Unreacted Ferrocene

Amount of Unreacted Ferrocene= (start amount ferrocene− yield acetylferrocene )∗MW ferrocene= 3) Calculation of Rf value

Rf =

distancetravelled by substance 2.90 cm =0.98 ; Rf of hexane fraction= 2.95 cm distance travelled by solvent front

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