Experiment 16C: Synthesis and Purification of Acetylferrocene

Experiment 16C: Synthesis and Purification of Acetylferrocene The purpose of this experiment was to acylate ferrocene to form acetylferrocene through a Friedel-Crafts acylation reaction. In order to purify the compound, column chromatography was performed. Column chromatography separates molecules by their polarity by creating an atmosphere with two phases, a polar stationary phase and a nonpolar mobile phase. The differences in each compound’s polarity affects its adherence to the stationary phase, which affects the rate in which the compound elutes out of the column. More polar molecules adhere strongly to the stationary phase, so they are retained in the column longer. Therefore, the compounds can be distinguished based on the order of elution.

In the first part of the experiment, ferrocene, acetic anhydride, and 85% phosphoric acid were added to a round bottom flask attached to a drying tube containing cotton and CaCl2. Phosphoric acid acted as a general acid catalyst by donating a hydrogen to the oxygen in acetic anhydride, converting it into a carboxylic acid and an acylium ion. The acylium ion was therefore able to react with ferrocene to form acetylferrocene. After the solution was heated for 10 minutes, the reaction progress was checked with TLC. The higher spot present in the first lane caused by ferrocene disappeared in the third lane and there was instead a spot much lower down the plate, indicating that the reaction has completed. The spot in the final product is lower because it is a result of acetylferrocene which is much more polar than ferrocene, and adheres to the silica gel. The solution was then cooled for 10 minutes. Next, the mixture was rinsed over ice in order to remove the complex formed with the ketone and then neutralized with NaOH to deprotonate the pentenyl ring to restore it’s aromaticity. The solid was then collected and vacuum filtered.

Next, column chromatography was performed in order to purify the crude product. The column was filled with cotton, sand, silica gel, and hexane. The crude product dissolved in hexane was placed in the top of the column and eluted with hexane. The first compound, fraction one, was collected. A yellow layer containing ferrocene eluted first because it was the most nonpolar so had the least amount of adherence to the polar stationary phase, the silica gel and most adherence to the mobile phase, the hexane. After all of the yellow liquid was eluted, fraction two was collected. The next layer to elute was a red layer containing the acetylferrocene, being more polar than ferrocene. After all the acetylferrocene was collected, an orange layer containing diacetylferrocene remained near the top of the column. (1) The diacetylferrocene remained at the top of the column because with two acetyl groups, it is much more polar than both ferrocene and acetylferrocene, therefore it adheres strongly to the polar silica gel. In order to elute it, a more polar eluent must be used to increase the polarity of the mobile phase and allow the diacetylferrocene to dissolve in the mobile phase and run down the column. TLC was performed on both fractions. The solvent in fraction two was evaporated and the solid was weighed, and its melting point and IR spectrum were measured.

The experiment yielded 0.204g of acetylferrocene, making the percent yield 66.59%. Themelting point found was 79-81°C, close to the literature melting point of 81-83°C. The IR spectrum obtained shows peaks above and below 3000cm​-1​, indicating the presence of C-Hbonds on sp​2​ and sp​3​ carbons, respectively. There is a strong peak at 1650cm​-1​ which representsthe C=O bond on acetylferrocene. Lastly, there is a peak around 1450cm​-1​ as a result of the C=Cdouble bonds. There is a strange inverted peak around 3400cm​-1​ most likely as a result of anerror while taking the baseline for the IR spectrum. The final TLC done on fraction two showedthat the high spot from ferrocene was not present in the final product and only a lower spot fromacetylferrocene was left.

In the ​1​H-NMR spectrum of ferrocene, there is a single signal around 4.2 ppm with theintegration of 10. (2) This supports the sandwich structure of ferrocene because the fact that onlyone signal is produced proves that every hydrogen in the molecule is chemically equivalent, sorather than one carbon being bound to the iron, each carbon equally participates in thecoordination with iron. If only one carbon of the pentenyl ring was attached to iron, then there would be 3 signals with the integration of 2:4:4.

The experiment was successful in synthesizing a relatively pure sample ofacetylferrocene. The experimental melting point coordinated with the literature value and theTLC showed that there was no nonpolar impurity (ferrocene) in the final product. A lower yieldis due to the fact that a considerable amount of solution was lost when transferring the mixture tothe beaker with the ice wash.

References

Bruice, P. (2015). ​Essential Organic Chemistry​. 8​th​ed. Prentice Hall: Pearson.

Gainer, M. (2017). ​Organic Chemistry Laboratory Manual and Techniques​. 4th ed. SantaBarbara: Macmillan Learning.