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Synthesis of Acetylferrocene

Prior Reading Bruice, Ch 14: Aromaticity, p. 611-621 http://home.um.edu.mt/chemistry/CH330/1.%20Organic/CP305.pdf http://wcb.keene.edu/~rblatchly/Chem222/lab/AcetylFerrocene.pdf and develop a plan of your own for the synthesis of acetylferrocene. In this experiment, you will perform a Friedel-Crafts acylation of ferrocene. Although Friedel-Crafts acylations often use the acid chloride and a Lewis acid such as aluminum trichloride, anhydrides and strong mineral acids are also effective. You will isolate a mixture of unreacted starting material and the product, acetylferrocene, which is red. You will analyze the mixture by thin layer chromatography, and FT-IR. Thin layer chromatography is performed on a glass or plastic plate which is coated with a thin layer (thus the name) of solid absorbent, such as silica (SiO2). The surface of the silica particles has a large number of Si­OH groups, which makes this substance very polar in other words, it is very sticky to other polar molecules. The sample mixture is spotted on the plastic plate near the bottom using a glass pipette, and the plate is put in a closed beaker or jar with a small amount of the appropriate solvent or solvent mixture. Capillary action draws the solvent up the plate. When the solvent front is nears the top, the plate is removed from the beaker and a separation of the sample's components may be observed.

If the compounds are colored (as is the mixture you will use), the plate can be read easily. If the compounds are not colored then they can be visualized by an ultraviolet lamp or a chemical stain. Some techniques for visualizing spots are outlined below: Ultraviolet light. Commercial TLC plates have a phosphor in the solid phase which fluoresces in long-wave UV light. If a compound is present on the plate it blocks this fluorescence and appears as a dark spot. A few organic compounds fluoresce themselves, and will show up as bright spots under short-wave UV light.

2,4-Dinitrophenylhydrazine (DNP) reacts with aldehydes and ketones to form an orange or yellow compound. The DNP is dissolved in ethanol with a little sulfuric acid as catalyst, and is sprayed on the plate with an aerosol bottle. Aldehydes or ketones are then visible as orange or yellow spots. DNP can also be used to derivatize volatile aldehydes and ketones which would evaporate rather than stay on the plate. These derivatives can then be chromatographed by TLC for identification. Ninhydrin reacts with amino acids and amino sugars to form a blue pigment. Since the product of the staining reaction is always the same, ninhydrin can not be used to form derivatives. Iodine vapor (in a closed chamber) can also be used as a stain. It forms a brown complex with unsaturated and aromatic molecules. The reaction is reversible, so that I2 staining can be followed by another chemical stain if the plate is allowed to sit in air for about a half hour. Sulfuric acid decomposes most organic matter to black tar. If a TLC plate is sprayed with sulfuric acid and heated to 100°C, all organic spots will turn black. This will also happen over time with the DNP stained plate, because sulfuric acid is added to that reagent as a catalyst. For each spot on the TLC plate a characteristic value called the ratio to the front, or Rf, can be calculated. Rf is defined as the ratio of the distance traveled by a spot (measured from the

R f= Dist. travelled by compound Dist. travelled by solvent

center of the spot) to the distance traveled by the solvent. Although the Rf is characteristic for a given compound, it depends greatly on the solvent and the type of absorbent used. Consequently there are no tables of Rf values in the CRC Handbook or Merck Index. The difference in Rf values between two spots on a plate, Rf, which also varies with the solvent, is used as a measure of the performance of the separation. If there is no difference in Rf values, there can be no separation of the components of a mixture and no way to differentiate one spot from another. The choice of solvent system is crucial for good separation. With too polar a developing solvent, all of the spots will run to the top of the plate, and Rf will be zero. With a very nonpolar solvent, none of the spots will move from their initial positions, and again Rf = 0. The best separation is often achieved by using a mixture of a non-polar with a polar solvent. The polarity of the developing solvent is adjusted by changing the ratio of polar to non-polar solvents in the mixture. The appropriate developing solvent should give an Rf of 0.3 to 0.5 for the desired compound (or the middle Rf of a mixture) and a Rf of at least 0.1 between the desired compound and any impurities. Once an appropriate mixture is chosen for TLC, the same mixture can be used to develop a column.

Column Chromatography is performed by packing a glass tube with an absorbent as shown in Figure 1.4. A column may be packed 'wet' by pouring a solvent-absorbent slurry into the tube, or 'dry' by filling it with dry absorbent. The mixture to be purified is then dissolved in a small amount of the appropriate solvent and added carefully at the top of the solid absorbent, so as not to disturb the packing. The column is developed by adding more solvent to the top and collecting the fractions of eluent that come out of the bottom in separate test tubes. For 'flash' column chromatography, moderate air pressure is used to push the solvent through the column. The success of the separation and the contents of the fractions can be determined by spotting the fractions along with the initial mixture on TLC. A column may be developed with a single solvent mixture or a with a polarity gradient (a solvent system which gradually increases in polarity.) For example, a column may be developed first with a low-polarity solvent, such as hexane, and as fractions are collected the developing solvent is changed to 10:1, 5:1 and pentane : methylene chloride. (The solvent is changed by adding more to the top of the column). A polarity gradient is used for mixtures of compounds with very different polarities.

sand

absorbent (silica gel)

sand cotton plug

Figure 1.4 Column chromatography.

setup

for

Solvents. A common non-polar solvent for chromatography is pentane. It can be used with a variety of polar solvents, some of which are listed below in order of increasing polarity: chloroform, ethyl acetate, methylene chloride, and methanol. See the textbook (p. 98) for a list of chromatography solvents.

Table 1 Major Types of Chromatography

Type Paper High Pressure Liquid (HPLC) (Reverse Phase) Gas (GC) Stationary phase Paper Silicone resin Mobile phase Water Water, methanol Uses Amino acid analysis Protein purification, drug testing. Analysis of gases and volatile liquids, small scale purification. Analysis of organic mixtures, small scale purification. Large scale purification.

Silicone oil, porous polymers. Al2O3 or SiO2 (alumina or silica) Al2O3 or SiO2

Helium, Argon

Thin Layer (TLC)

Organic solvents

Column

Organic solvents

Procedure Place 0.50 mmol of freshly sublimed ferrocene in a 5-mL short neck round-bottomed flask with a magnetic stir bar. Use a dry calibrated disposable Pasteur pipette or syringe to add 0.35 mL of acetic anhydride and a second pipette or syringe to add 0.1 mL of 85% phosphoric acid. Cap the flask with a septum and push a clean syringe needle through the septum. This will minimize the amount of moist air that enters the flask, and allow for release of any gas evolved. Dissolve the ferrocene by gently heating the flask on a sand bath while carefully stirring and agitating the flask. Continue heating for an additional 10 minutes and then place the flask in an ice-bath. Remove the needle and septum, and carefully add 0.5 mL of ice-cold water dropwise with thorough mixing. Neutralize the resulting solution by drop-wise addition of a 3 M aqueous solution of sodium hydroxide; test the solution with pH indicator paper, but avoid adding an excess of the base. Filter the mixture by vacuum filtration, rinsing out the flask and washing the solid collected with cold water. Remove the solid from the filter and place it on a filter paper. Press the product as dry as possible between sheets of filter paper. This crude product will be analyzed by thin layer chromatography (Use a 30:1 mixture of toluene and ethanol to elute the plate) and purified by column chromatography.

O CH3

O CH3

Fe

Fe

H3C O

Fe

Ferrocene

Acetylferrocene

1,1'-Diacetylferrocene

You will first run thin layer chromatography (TLC) plates to determine if you have successfully made acetylferrocene. After the synthesis of acetylferrocene has been confirmed, separate the mixture on a microscale column.

Column Chromatography

Check the following websites for information on column chromatography: http://orgchem.colorado.edu/hndbksupport/colchrom/colchromproc.html http://courses.chem.psu.edu/chem35/HTML/Experiments/PDF's_for_techniques/CC.pdf

Column Chromatography of Ferrocene and Acetylferrocene

Cautions: Be sure that there are no flames in the laboratory. Al2O3 is a lung irritant; avoid breathing the dust. Petroleum ether and t-butyl methyl ether are extremely flammable. Place any residual solvents in the waste solvent container for disposal. Place the alumina, wet with solvent, in the waste solids container. Disposal containers are located in the dispensing hoods. Part I. Column Chromatography A. Packing the column 1. Set up column, using the Williamson microscale kit, as shown in the hood and above. 2. Fill column 2/3 full with alumina, then pour the alumina into a 50 mL Erlenmeyer flask. 3. Clamp the empty column in a vertical position and close the valve. Always grasp the valve with one hand while turning it with the other. Fill column 1/3 full with petroleum ether. 4. Add ca. 8 mL of additional petroleum ether to the alumina, swirl the mixture, and quickly pour the suspension through the funnel into the column. This is the hard part! Make sure that there are no air bubbles in the column. 5. Drain the petroleum ether from the column but never allow the solvent level to go below the top of the packing. 6. Use this solvent to wash alumina out of funnel and onto the column. Once again, drain hexane from the column never allowing the solvent level to go below the top of the packing. Repeat this step if necessary. 7. When all of the slurry has been added to the column, the solvent level should measure 5 mm above the top of the alumina packing. B. Adding the sample 1. Place your weighed crude product into a 10 mL flask.

2. Dissolve the mixture in a minimum amount of dichloromethane. 3. Add 300 mg of alumina to the solution. 4. Evaporate the dichloromethane, via gentle heating and continuous swirling of the flask, on a hot plate. Remember that dicholoromethane boils at 55 oC. 5. After confirming that the solvent level is within 5 mm of the top of the packing, pour the alumina containing the adsorbed mixture into the funnel and wash onto the column with a few drops of petroleum ether. 6. Open valve and remove a small portion of solvent; add to top of column. Repeat this process until a narrow band of the sample mixture is observed just below the top of the alumina. C. Eluting the column 1. Elute the yellow ferrocene with petroleum ether; add petroleum ether in 1 mL portions. Collect eluate in a pre-weighed vial. 2. Wash any ferrocene on the column tip into flask using elution solvent. 3. Elute the red acetylferrocene with 1 mL portions of a 50:50 mixture of petroleum ether:t-butyl methyl ether. Collect eluate in a pre-weighed vial. D. Analysis 1. Add the masses of eluted ferrocene and acetylferrocene and calculate the percent recovery from the column. 2. Determine the limiting reagent and calculate the percent yield of the reaction. 3. Obtain the melting points of your ferrocene and acetyl ferrocene. 4. Using a KBr pellet or Nujol obtain an FTIR of acetylferrocene.

Tips on Technique · Use a tweezers to put the TLC plate in the chamber. Don't lean the TLC plate against the filter paper, because solvent will leak from the sides and you'll get a distorted run. · Make as small a spot as possible on the TLC plate. Too large a spot will cause streaking. · Mark the solvent front before it evaporates. This means you should have your pencil ready before you remove the plate. Waste Disposal · Glass pasteur pipettes, TLC plates and spotters - glass waste. · Organic solvents, eluents from chromatography - organic waste

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