The+Friedel-Crafts+Reaction;+Acetylation+of+Ferrocene

Zach Chandler Megan Bernard


 * Lab #6:** The Friedel-Crafts Reaction: Acetylation of Ferrocene


 * Objective:** To perform a Friedel Crafts reaction by acetylation,or by adding an acetyl group, to ferrocene.


 * Introduction:** The Friedel-Crafts reactions were created to attach substituents to an aromatic ring by Charles Friedel and James Crafts in 1877. Friedel-Crafts reactions are powerful and widely used methods of carbon-carbon bond synthesis. In this greener approach, ferrocene is acylated using the more benign system of phosphoric acid and acetic anhydride in place of the usual, more vigorous acylation reagent generated from aluminum chloride and acetyl chloride. The structure of ferrocene, seen below, is two pentadienyl rings with an iron atom sandwiched between them. The iron atom is not involved in the reaction and the ferrocene acts much like a benzene in terms of reactivity. This makes ferrocene a convenient reagent reaction because the iron gives the products of the reactions an easily visible color. The coloring facilitates thin layer chromotography (TLC) analysis. TLC uses a surface coated with an absorbent material to separate components of the mixture. Each component leaves a distinct mark on the TLC surface which is easily visible because of the iron coloring. Nicely written.


 * Figure 1: Structure of Ferrocene (Wikipedia)**
 * Figure 2: Reaction of Ferrocene with Phosphoric acid.**
 * Friedel-Crafts acylation of ferrocene**


 * Procedure:**
 * 1) Place 1.5 g of ferrocene in a 25 mL round-bottom flask containing a magnetic stir bar. Prepare a hot bath, heating the water to nearly the boiling point while preparing the following reaction mixture.
 * 2) In a fume hood, add 5.0 mL of acetic anhydride and 1.0 mL of 85% phosphoric acid to the flask. The reaction mixture should heat up and darken color. Swirl the flask, heating occasionally in a hot water bath, if necessary, until all the ferrocene dissolves.
 * 3) Attach a reflux condenser equipped with a calcium chloride drying tube, then heat the reaction mixture, with stirring, on a hot water bath that was prepared in step one. Heat for ten minutes, during which time a purple color may develop.
 * 4) Pour the reaction mixture onto 25 g (ca. 60 mL) of ice in a 250 mL beaker, rinsing the flask with two 5 mL portions of ice water. (A black residue may remain in the flask.) Stir the orange-brown mixture with a glass rod for a few minutes. Any insoluble black material present will be removed in the following steps.
 * 5) Add 37.5 mL of 3M aqueous NaOH solution, then //carefully// add solid bicarbonate in small portions until the remaining acid has been neutralized ( 7-8 grams). (Use great care to avoid excessive foaming during this bicarbonate addition. This step can be done with magnetic stirring, but make sure to use a stirring plate that is not hot.) Stir well and crush any lumps, affording a dark-brown suspension.
 * 6) Allow the mixture to stand for 20 minutes, then collect the crude product by vacuum filtration and continue to pull air through the product for a few minutes to dry it. Finish the drying process by pressing the solid product between two sheets of filter paper or paper towels. Save a bit of this crude product for TLC analysis.
 * 7) Transfer the solid and a stir bar to a small Erlenmeyer flask and add 20 mL of hexanes. Boil for five minutes with stirring, then decant the dark-orange solution into another Erlenmeyer flask, leaving behind a black gummy substance.
 * 8) To the hot solution, add a spatula-full of decolorizing carbon. (Use of too much carbon will reduce your yield.) Heat with swirling, then decant once more to remove the decolorizing carbon.
 * 9) Set the flask aside to cool slowly. Red-brown needles of acetylferrocene should begin to form. Once the flask has reached room temperature, cool it in ice. Collect the crystalline product by vacuum filtration, washing with a small quantity of cold hexanes, andd ry it by continuing to pull air through it for a few minutes.
 * 10) Record the yield and melting point range of your recrystallized acetylferrocene. The melting point has been reported as either 82-83 C or 84-85 C.
 * 11) Analyze your crude and recrystallized products by TLC (thin layer chromatography). Seperately dissolve very small amounts of pure ferrocene, the crude product, and the recrystallized acetylferrocene in a few drops of toluene. Spot the solutions on silica gel plates and develop with 30:1 toluene/absolute ethanol. Visualization is simple since each of the compounds are brightly colored.

The initial solution of ferrocene, phosphoric acid, and acetic anhydride resembled a liquidy butterscotch color. After the first heating, the color changed to a deep brown, which turned into a reddish brown after the first filtration to obtain the crude product. Once the hexanes were added it turned into a concentrated, hazy and darkened reddish color. Once the decolorizing charcoal was added the haziness left and the mixture appeared to be like watered down red apple juice. The final product was thin, needlelike, rust colored crystals that resembled bits of fiberglass. And it was very pretty!!
 * Observations/Data:**

Ferrocene: 186.03 g./mol Acetic anhydride: 102.09 g/mol Acetylferrocence: 288.07 g/mol
 * Molecular weights:**

The scale from the graph paper behind is 2mm / mark. The solvent occurs at 60mm. A - pure ferrocene B - crude product C - recrystallized product
 * Thin Layer Chromatograph:**

Pure ferrocene produces a yellow smear at about 40mm. Rf = 40/60 = 0.67 The patterns produced by crude and recrystallized products look identical with an orange smear (acetylferrocene) at about 15mm. Rf = 15/60 = 0.25 Yes, unfortunately that means your purification wasn't very effective.

After sitting for a week the yellow smears spread out and changed to a faint greenish blue color. Some additional, mysterious chemistry going on!


 * Melting Points:** The documented melting point range for the final product was either 82 C-83 C or 84 C-85 C. The actual melting point range for the final product was 79.6 C- 82.2 C in the experiment, which is evidence that the acetylferrocene produced in the experiment was pure, as well as close to the reported melting point range.

The faint yellow smudge at around 46mm in lanes B and C demonstrates that there is unreacted ferrocene left in the product. The quantity left is much less than the initial quantity shown in lane A. Lane C shows that recrystallization of the crude product was relatively ineffective at removing unreacted or contaminating ferrocene from the product. Acetylferrocene may have traveled more slowly along the film because the molecule is larger or because it binds more strongly to the silica. It's about polarity, not about size of the molecules, in TLC. Also the banding in lane B under the big orange spot could possibly suggest that some other products were formed. These products could have been diacetylation and triacetylation. Some of the banding disappeared from the crude product to the final product though, proving that the quality of the final product was better than the crude product.
 * Analysis/Sources of Error:**

Additionally, the TLC shows that not all of the ferrocene was acetylated. Ferrocene is the limiting reagent in this procedure so some other reason must explain why acetylation of ferrocene was incomplete. Time may have been the limiting factor. If the acetylation step was ran longer, it may have gone to completion for all the ferrocene. This could also attribute to the slightly lower than expected melting point. Even though the melting point range was very narrow.

The theoretical yield for this product is mass of the number of moles of ferrocene completely converted to acetylferrocene. That is (0.0082782 mol ferrocene)(218.03 g/mol acetylferrocene) = 1.8049 grams acetylferrocene.

The final product weighed 0.597 g, which was less than the theoretical yield of 1.80 g, equating to a yield of 33.2% of the theoretical yield. Much of that product was lost in the experimental procedure on containers and other apparati used. A loss also occurred during the last filtration, some of the product slipped around the edges of the filter paper and into the waste product of the flask. This was because of the small size and shape of the recovered product.

The procedure was sucessful in acetylating most of the ferrocene, producing a yield at about 33.2% of the theoretical yield. The melting point range for the final product was 79.6 C- 82.2 C, which shows that the acetylferrocene was a pure substance. According to the TLC (thin layer chromotography) analysis, after the last recrystallization, the product did not become much more pure than before. Overall, the experiment was successful at obtaining pure acetylferrocene at the end.
 * Conclusion:**

Nice report.

Doxsee, K. M.; Hutchison, J. E. Green Organic Chemistry - Strategies, Tools, and Laboratory Experiments, Print 2004; pp 225-230. [] (for ferrocene pictures in figure 1) [] (for the reaction in figure 2)
 * Reference:**