Chem 197 - Spring 99

Friedel-Crafts Acylation of Ferrocene

Introduction

Ferrocene, Fe(C₅H₅)₂, has iron in the +2 oxidation state combined with two C₅H₅^- radical anions.

As the name suggests, ferrocene is an iron containing compound, yet it has properties similar to benzene. Both of the cyclopentadiene anion rings of ferrocene have six delocalized pi electrons. Hence both rings are susceptible to electrophilic aromatic substitution reactions. This means that the ring is subject to attack by an electron deficient compound. The Friedel-Crafts acylation of benzene requires aluminium chloride as the catalyst, but ferrocene, which has been referred to as a superaromatic compound can be acetylated under milder conditions with phosphoric acid as the catalyst. The Friedel-Crafts acylation of ferrocene is shown below.

The acylium ion, CH₃CO⁺, is the electrophile that reacts with ferrocene to yield either (I) acetylferrocene or (II) 1,1'-diacetylferrocene. Since the acyl group is ring deactivating (it decreases the electron density of the ring), very little of the homoannular disubstituted product is formed.

Synthesis

In a 25 mL round-bottomed flask place 1.0 g of ferrocene, 5.0 mL 0f acetic anhydride, and 1.0 mL of 85% phosphoric acid. Equip the flask with a reflux condenser and a calcium chloride drying tube. Warm the flask gently on the steam bath with swirling to dissolve the ferrocene, then heat strongly for 10 minutes more. Pour the reaction mixture onto 25 g of crushed ice in a 400-mL beaker to destroy any unreacted acetic anhydride and rinse the flask with 5 mL of ice water. Stir the mixture for a few minutes with a glass rod. Neutralize the excess acid formed by adding increments of solid sodium bicarbonate. (Note: a fairly large amount of sodium bicarbonate may be needed. When neutral, CO₂ will no longer bubble from the solution when additional bicarbonate is added. Do not add excess bicarbonate. Litmus paper can be used to verify this step.) Collect the yellow-orange solid product using a Buchner funnel and vacuum filtration. Wash with several rinses of distilled water and allow it to air dry before determining the yield of crude product.

Analysis by TLC

Dissolve a very small amount of the product in a minimal amount of toluene. Spot this solution , along with similarly prepared known solutions of ferrocene, acetylferrocene, and 1,1'-diacetylferrocene with micro-capillaries on a silica gel plate. Determine an optimum solvent system for separating possible products by using a non-polar solvent (pet ether), a polar solvent (ethyl acetate) and one or more mixtures of the two liquids in a measured ratio. Visualize the spots under an UV lamp. Do you detect unreacted ferrocene in the reaction mixture and/or a spot that might be attributed to diacetylferrocene?

Analysis by column chromatography

1. After the purity of crude product has been determined by TLC, weigh the remainder of the dry crude product and separate the mixture using column chromatography.
2. Prepare the column exactly as demonstrated. (90:10 ligroin:ether )
3. Dissolve the weighed remaining crude product into a minimum amount of the solvent indicated by the instructor (dichloromethane).
4. As the level of the solvent in the prepared column falls just to the top of the packing material , carefully add the sample to the top of the column.
5. Allow the sample solution to fall to the top of the column before adding additional eluting solvent (The eluting solvent should be the solvent that best separation for TLC). Add eluting solvent carefully, so as not to mix the sample components into the eluting solvent.
6. Monitor the progress of the sample down the column, noting the color and order of any bands that elute. (Remember, the eluting solvent must be kept above the top of the packed column at all times. Should the solvent level fall below the top of the column, channels will form in the packing material. This will adversely influence any separation attempted using the column).
7. As bands elute from the column, collect each in a separate, labeled, tared beaker.
8. Allow these solutions to evaporate to dryness in the hood.
9. After drying the separated solutions, determine the weight and melting point of each.
10. To confirm your separation, spot a TLC plate with a bit of each fraction and elute it using the solvent that best gave the separation results earlier.

Analysis by IR

Obtain an IR spectrum of each fraction.

Analysis by NMR

Obtain a proton NMR spectrum of each isolated fraction and analyze it.