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| Experiment 5 | Synthesis of Acetylferrocene |
A boiling water bath should be prepared in advance by placing a beaker equipped with water and a magnetic stir bar on a hot plate-stirrer.
The required amount of ice should be prepared just before beginning the short heating period. Temperature (K, not oC) affects reaction rates exponentially, so heating dramatically raises the acetylation rate, while cooling effectively stops acetylation.
Prolonged cooling is intended to maximize the amount of precipitate that forms.
If you have trouble establishing a good flow of hexane, or monitoring the solvent front, apply suction until the column runs dry and then apply more hexane.
Your eluting solvent should be a mixture of hexane and ethyl acetate. The ratio is not critical. Anything between 2:1 and 9:1 hexane:ethyl acetate (v/v) should work.
Dissolving 5-10 mg of product mixture in 3-4 drops of ethyl acetate will produce a dark enough solution to be seen on a TLC plate. You may need to replenish the ethyl acetate if it evaporates.
Normally the filter is loosely filled to the lip with silica gel. This experiment calls for less silica gel because more gel will be added later.
The normal procedure leaves only ~1 cm as head space (this is enough if no more silica gel is to be added).
Generally, an apparatus of this size can only separate about 400 mg of material. However, it is possible to place a larger mixture on the column if the components of the mixture are easily separated. In the case of ferrocene and acetylferrocene, you should be able to separate your entire product mixture.
Mixtures are normally applied to columns as a solution of ~400 mg mixture (always record the precise amount) dissolved in 3-4 mL of the initial solvent (usually hexane). An alternative procedure is used here because ferrocene and acetylferrocene are virtually insoluble in hexane.
Whether you apply your sample as a hexane solution (standard), or as a dry sample adsorbed on silica gel (procedure used here), you should apply your sample evenly across the entire column.
It is not uncommon to collect 20-30 fractions. The solvent mixtures in this experiment have been specially tuned to elute product more quickly.
Normally the elution procedure begins with a nonpolar solvent, pure hexane, and is followed by increasingly polar solvent mixtures, each obtained by mixing hexane with increasing amounts of ethyl acetate.
The standard procedure is to increase the amount of ethyl acetate in every fraction, but to increase the amount of ethyl acetate slowly. Standard solvent mixtures are listed in the following table. An efficient procedure for making up these mixtures is described after the table. Whether you follow this procedure precisely or not, you must record your solvent mixing procedure in your notebook so that you know how each fraction was prepared.
| Fraction # |
Nominal Composition (EA:H) |
Starting EA:H |
EA to add (ml) |
| 1 |
0:100 |
0:100 |
|
| 2 |
1:99 |
|
1 |
| 3 |
2:98 |
|
0.5 |
| 4 |
4:96 |
|
0.5 |
| 5 |
5:95 |
5:95 |
|
| 6 |
6:94 |
|
1 |
| 7 |
7:93 |
|
0.5 |
| 8 |
9:91 |
|
0.5 |
| 9 |
10:90 |
10:90 |
|
| 10 |
11:89 |
|
1 |
| 11 |
12:88 |
|
0.5 |
| 12 |
14:86 |
|
0.5 |
| 13 |
15:85 |
15:85 |
|
| 14 |
16:84 |
|
1 |
| 15 |
17:83 |
|
0.5 |
| 16 |
18:82 |
|
0.5 |
| 17 |
20:80 |
20:80 |
|
| 18 |
22:78 |
|
2 |
| 19 |
24:76 |
|
1 |
| 20 |
26:74 |
|
1 |
| 21 |
30:70 |
30:70 |
|
| 22 |
33:67 |
|
3 |
| 23 |
36:64 |
|
3 |
| 24 |
40:60 |
|
2 |
You can create four solvent mixtures with minimal effort by referring to the amounts listed in the third and fourth columns in the table and performing the following operations (to make this more concrete, the following instructions refer to fractions 9-12, but you can make solvent for any four fraction sequence this way):
|
If you need more than 24 fractions, use 40:60 EA:H to start fraction 25, 50:50 EA:H to start fraction 29, and so on (continue to follow the same -, 3, 3, 2 mL EA to add sequence).
Ferrocene and acetylferrocene are easy to track because they are colored. However, most compounds are invisible as they move through a column. Therefore, the standard procedure is to analyze all of the fractions by TLC using the following two round procedure.
Round 1. Once you collect 20 fractions, test all of them by applying a small sample of each to a single fluorescent TLC plate (label your spot locations in pencil!).
The fractions that contain your compound are very dilute. Therefore, apply all of the liquid in your capillary tube to the TLC plate. You can make sure that the spot stays very small as follows: quickly touch the capillary tube to the plate, blow on the plate to evaporate the solvent, then touch the capillary to the same spot to add more sample. Always let the solvent evaporate before applying more sample.
After you have spotted all of your fractions (clean the capillary between fractions!), examine the plate under a UV light (do not elute the plate!) to see which fractions contain a UV-absorbing compound. Record these data in your notebook.
Round 2. Once you know which fractions test positive for a UV-absorbing compound, elute all of the "positive" fractions on fresh TLC plates to see what these fractions contain. It is best to apply consecutive "positive" fractions to a plate (outer lanes) along with a 5% solution of the standard sample (middle lane) and to elute your "unknowns" (fractions) against a "known" (standard sample). This will tell you whether the UV-absorbing compounds are from your original mixture or are impurities that somehow eluted from the column. It will also tell you which fractions contain pure compounds and which contain mixtures.
