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COLUMN CHROMATOGRAPHY SEPARATION OF A MIXTURE OF FERROCENE/ACETYLFERROCENE

Chemistry 2311

Introduction

1. Accurately draw the structures of the organometallic compounds, ferrocene and acetylferrocene, clearly showing the nature of the bonds between the iron metal and the cyclopentadienyl anion.  Provide reaction conditions for the conversion of ferrocene to acetylferrocene based on electrophilic aromatic substitution chemistry.

Ferrocene      Acetylferrocene

2. Draw the hybrid structure of the cyclopentadienyl anion that ligates to the iron in ferrocene and acetylferrocene.  Draw three of the possible five resonance structures that aid chemists in understanding the distribution of charge by starting with one possible Lewis structure and using curved arrow notation to show the movement of electrons to the next.  Classify the anion as “aromatic,” anti-aromatic, or non- aromatic.

Results and Observations

Day 1.  Thin-Layer Chromatography Results

3. Complete the table below by reporting the R_f value(s) for the two spots representing ferrocene and acetylferrocene (which spot is which is not decided yet) in each of the solvents tested.

4. Which solvent system(s) provided the largest R_f value separation and might be best for following the in an Friedel-Crafts acylation reaction of ferrocene acetylferrocene by TLC?  Draw this TLC below on the left. Which solvent system would have the approximate ideal R_f values for use in a column chromatography of a mixture of the two compounds? Draw the TLC on the right below.

Best TLC conditions for following a reaction Best TLC conditions for column separation

5. Report the melting point range for the acetylferrocene and mixture of acetylferrocene/ferrocene below and record any notable observations.

Did the melting point comparisons follow expectations for melting point theory? Explain.

Day 2.  Column Separation of Ferrocene and Acetylferrocene

6. Observations: Describe your observations for running the column.  Include how the sample was loaded, elution of each band, and whether a switch in solvent or pressure was used.

7. Results. Complete the table below to summarize the results of the column chromatography experiment.

8. What was the total % mass recovery of material from the column?  That is, of the sample transferred to the column how much mass was recovered from the combined masses of all fractions collected?

9. Identify the solids isolated in fraction 2 and fraction 4 by comparison with the literature melting points of ferrocene and acetylferrocene.  Add your literature source below the table.

¹Literature: __________________________________________

10. Propose a reason that ferrocene, though lower in molecular mass and less polar than acetylferrocene, is observed to have a higher melting point.

11. Often a third colored band is observed near the top of the column. It is suspected that it may be di-acetylated ferrocene that can result from the Friedel-Crafts acylation reaction if excess reagent is used. Propose three isomeric possible di-acetylferrocene isomeric products and indicate which one you would predict would be most favored based on activating/de-activating EAS trends.

Additional questions.

1. Imagine that you were tasked with synthesizing 100 g of acetylferrocene using the reaction you proposed in question 1.  How would you use TLC to assist with maximizing the yield of the reaction? Explain.

2.           Assume you isolated 88 grams of product that contained some impurities by TLC.  Describe how you would use column chromatography to purify the material.  Specify the amount of silica gel you would use for the column and how you would decide on the solvent system for elution.

3. Below is the ¹H NMR spectrum of ferrocene.

a. Explain in detail why there is only one peak in the ferrocene ¹H NMR and how it relates to the “sandwich” structure of the compound.  (Note it is NOT enough to say there is only one type of proton.  You must explain why).

ferrocene ¹H NMR spectrum

b. In contrast, there are multiple peaks present in the ¹H NMR of acetylferrocene (see below). Draw the structure of acetylferrocene and assign the protons to the peaks in the spectrum below.

acetyl ferrocene ¹H NMR spectrum