Acetylation of Ferrocene: Electrophilic Aromatic Substitution
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Acetylation of Ferrocene: Electrophilic Aromatic Substitution
Acetylation of Ferrocene: Electrophilic Aromatic Substitution; Column Chromatography Ferrocene is a yellow organo metallic compound that consists of a complex formed between ferrous ions (Fe2+) and two cyclopentadienyl anions. As you know from the organ ic lecture class, the cy clopentadienyl anion is an unusually stable carbanion, because its -electron structure is aromatic. Since it is aromatic and, in addition, more electron-rich than b enzene, the cyclopentadienyl anions in ferrocene can undergo a variety of electrophilic aromatic substitution reactions. The products of these reactions have a var iety of different colors, as a result of changes in the energy levels of their bonds. As an exa mple of this type of chemistry, you will react ferrocen e with acetic anhydride in the presence of phospho ric acid to produce acetyferrocen e as the main product, together with some diacetylferrocene as a by-product. You w ill then purify the acetylferrocene by chromatography on an alumina column, in order to separate it from unreacted ferrocen e, and from the diacetylferrocene and other polymeric by-products. Finally, you will characterize the purified acetylferrocene by TLC, IR, and melting point. O O O O CH3 H3C O Fe2+ H3PO4 CH3 CH3 Fe2+ + Fe2+ O CH3 Mechanism In your prelab write-up, include a detailed mechanism for (i) formation of the electrophile in the reaction, and (ii) the electrophilic aromatic substitution reaction. Pre-Lab Review This experiment will require you to prepare an alumina column, using the dry packing method, and to perform TLC, IR and melting point analyses. Be sure to review all of these procedu res before the lab. In particular, review Operations 21 and 22 in detail, focusing on the preparation and operation of a column (pp. 707-720), and the principles underlying the separation methods on alumina and silica gel columns and TLC plates (pp. 720-728). Your pre-lab write-up should include a detailed procedu re for packing the column, and a diagram of the column. Hazards 1. Ferrocene and (especially) acetylferrocene are toxic substances . The main dange rs are inhalation and absorption through the skin. Follow standard practice for safety during all procedu res. Wear gloves and wor k in the hood. Do not lean into the hood and do not rest any part of your body or your lab notebook (or anything e lse that you may later touch with ungloved hands) aga inst hood surfaces. 2. You w ill also work with petroleum ether (pet ether) and diethyl ether, which are high ly flammable. Keep these solvents away from hot surfaces. 3. Alumina (in your column) and silica gel (on the TLC plates) are microparticulate and ea sily beco me airborne, and are hazardous when inhaled. Keep the alumina in a covered container when you are carrying it through the lab, and only work with it in the hoods. Work with the TLC plates in the hoods as much as possible, and try not to scrape the surface material off the TLC plates if you are ex amining them out of the hoods. Clean up any spills. Dispose of all column materials and TLCs in the solid waste container when you are finished. Experimental Proce dure This procedu re is adap ted from a method described in the Journal of Chemical Education. The reference is: Richard E. Bozak, J. Chem. Ed. 43, 73 (1966 ). 1. Add 1.5 g ferrocene (MW = 186.03) to 5 mL acetic anhydride in a clamped roundbottom flask. Stir using a magne tic stir bar. (Note: the boiling point of acetic anhydride is 138-140 °C, d = 1.08 g/ mL, MW = 102.09 ) 2. Add approximately 1.0 mL 85% (w/v) phosphoric acid (MW H 3PO4 = 98.00) dropwise to the stirring ferrocene solution. Then cap the flask with a rubber septum and attach a drying tube con structed from a syringe and need le, following your TA's instructions. Heat the reaction mixture in a boiling wa ter bath (also stirred) for 10 min. at boiling point. Then remove the water bath and cool the reaction mixture for a few minutes by stirring it in a water bath at RT. Return your hot plate to the cabinet as soon as possible, to avo id having its hot surface around when you prepare and run your column. 3. Pour the coo led reaction mixture into a 50 mL beake r containing 20 g of crushed ice. Add solid sodium bicarbonate to the resulting mixture un til a pH of about 6-7 is attained. Then ch ill the mixture in an ice bath for a further 30 min. (at least), while you prepare your alumina column. In your prelab write-up, you should estimate how much sodium bicarbonate will be needed for this step. 4. Prepare an alumina column using petroleum ether (pet ether) as the solvent, following the dry packing method described in the Lehman book (2nd edition, p. 713). Use approximately 10 g of the neutral alumina provided in the reagen ts hood (Brockman activity I, 60 - 325 mesh). Do not weigh the alumina. Instead, measure it by volume in a small beake r (approx. 10 mL). 5. Once your column is ready for use, return to the reaction mixture. Collect the solid brown precipitate by vacuu m filtration, and wash it with small amounts of chilled water. Then dry the solid for a further 10-15 min. by leaving it on the filter paper and using con tinued suction. Column chromatography works on the same principle as TLC • The adsorbent (alumina or silica gel) should be packed with a stream of air or nitrogen to drive out air pockets in the column. This leads to better separation. Selection of the eluting solvent is an important factor in a good separation • The more polar the eluant, the faster compounds will move through the column. If a solvent is too “fast”, everything will come out with the solvent front. More polar compounds travel more slowly through the column. • Compounds with more polar groups will adhere to the adsorbent (alumina or silica gel) more strongly than less polar molecules. The column should have a level surface so that the bands stay even as they travel through the column. • The sample should be applied to the column in a minimum amount of solvent. Wide band widths lead to poor separation. • Narrow bands traveling through the column prevent overlap. Isolating the separated compounds • Run TLC’s of the fractions after the column to decide which ones to combine. 6. Weigh the crude product scraped off the filter paper. Then take a 0.4 g portion of this material and dissolve it in about 1-2 mL of toluene (some material will not dissolve). Load the toluene solution onto the alumina column, including any insoluble material, following the procedure described in your book and by your TA. Then e lute your column using 20-50 mL aliquots of (a) pet ether only, followed by (b) 20% diethyl ether in pet ether, and then (c) 50% diethyl ether in pet ether. Never let the column run dry. Any unreacted ferrocene (yellow) should be eluted in the first or second fractions. The acetylferrocene product will elute after the ferrocene as an orange-red solution. Collect this solution as it elutes from the co lumn. (You may also see a second orange -red component at the top of the column that elutes much more slowly than the acetylferrocene . This is the diacetylferrocene by-product, and it can also be eluted and collected, if you use 100% diethyl ether, if you wish to analyze it later by TLC.) NOTE: (a) Do not throw any of your column eluate away, until you have identified the acetylferrocene by a TLC comparison with authentic acetylferrocene (see below). (b) If the flow rate of your column is too slow, you can carefully apply air pressure to the top of the column to increase the flow rate. This should not cau se any problems, and often gives better separations, provided you take care nev er to let the co lumn run dry. 7. Iden tify the eluting fraction that contains acetylferrocene, by running a TLC of the eluate (the acetylferrocene should be orange -red, and will probably be in the 50% diethyl ether fraction). Choose a solvent to elute your TLC that makes sense, based on your observations of the column chromatography. The elution characteristics of ferrocene and its derivatives on alumina (your column) and silica gel (your TLC) are very similar. On the TLC, compare your eluted product to the crude material you loaded onto the column and to a sample of authentic acetylferrocene. Use diethyl ether as the solvent for the other two TLC samples. Report your TLCs as diagrams drawn in your notebook, complete with Rf measurements. Do not tape these TLCs in your book, since the compounds are toxic and the silica gel will flake off the plate. 8. When you have identified the fraction containing your purified acetylferrocene, remove the solvent by rotatory evaporation. Scrape the solid from the flask and weigh it. Then obtain an IR spectrum as a Nujol mull or paste (use a very small drop of "Nujol", or mineral oil). Measure the melting point of your product after drying it in your desiccator for a week. Report all of your measurements, and wr ite a conclusion that assesses the evidence for the correct identity of your product, its percent yield (in molar terms), and its purity. When ca lculating your yield, remember to account for the fact that you only purified a fraction of your product. Exercise Questions 1. (a) What is the molar ratio of acetic anhydride to ferrocene used in your reaction? (b) What is the molar ratio of phosphoric acid to ferrocene used, assuming you added 1.0 mL of the 85% (w/v) acid? 2. Do you expec t the acetylated cy clopentadienyl anion to be more reactive or less reactive towards acetylation, compared to the underivatized cyclopentadienyl anion? Explain. 3. Obtain a copy of the FTIR spectrum of mineral oil (Nujol) from a reliable internet source, and tape it into your notebook. Label the mineral oil peaks in your IR spectrum of acetylferrocene . 4. Look a t the NMR spectra of ferrocene and acetylferrocene in this handout. Note the single sharp peak obtained for ferrocene. (a) Based on the proton-NMR spectrum, do you expect ferrocen e to be more reactive towards acetylation than benzene or less reactive? Explain. (b) Sugges t an assignment for the four peaks in the proton-NM R spectrum of acetylferrocene . Exp lain your assignment. Expt. 13 – Investigation of a C=O Bond by Infrared Spectroscopy Goal: To predict the relationship between the vibrational frequencies of C=O bonds in IR spectra and their bond strengths. Each student will be given one of five carbonyl-containing compounds. Record the IR spectrum of your assigned compound and note the frequency of the C=O stretch at the point of maximum absorption. O O O H N CH3 H 2-heptanone heptanal CH3 N, N-dimethylformamide O O Cl O Cl O Cl ethyl butyrate ethyl trichloroacetate Prelab: Each student should (a) come to the lab with a predicted order of frequencies for these five compounds, and an explanation for this, written in your notebooks. (b) convert the data for each compound into frequencies in -1 Hz (s ). Discuss differences from your predictions using the arguments of organic chemistry. (c) Tape your spectrum into your lab book. Write your results for the carbonyl stretch -1 frequency (cm ) on the board to share the data. Predict and discuss your experimental results and be prepared to modify your hypothesis about the relationship between bond strength and IR vibrational frequency. The C=O bond has some single bond character. O O O If Z is an electron-withdrawing group, then resonance structure 2 becomes less important O R Z 1 O R O Z R Z 2 What is the effect on the strength of the C=O bond? If Z = nitrogen, resonance structure 3 contributes to the overall picture. O R O NH2 1 R O NH2 2 R NH2 3 If Z = oxygen, resonance structure 3 is considerably less important. c = c = 3 x 108 meters/second x 1010 centimeters/second h = Pla nck’s constant h = 6.63 x 10-34 Joules . sec frequency E = h c hc = wavelength The wavenumber is the inverse of the wavelength. It is directly proportional to energy. Expt. 16 – Separation of an Alkane Clathrate Urea forms a tunnel-like channel (a clathrate) around straight-chain hydrocarbons with seven or more carbons. Goal: to see if urea can be used to remove hexadecane from a mixture of methanol and 2,2,4-trimethylpentane by forming a clathrate with the straight-chain hydrocarbon. hexadecane negative octane rating CH3OH 2,2,4-trimethylpentane octane rating = 107 octane rating = 100 O H2N NH2 urea Expt. 16 – Separation of an Alkane Clathrate • Preparation: You will add urea dissolved in methanol to a mixture of 2,2,4-trimethylpentane and hexadecane. After a white solid forms, cool with an ice/water bath until crystallization of the clathrate is complete. Dry and weigh the urea clathrate. Dissolve the urea in water and extract the hexadecane into dichloromethane. Dry the organic layer and then evaporate the solvent. Weigh the hexadecane. Use this equation to estimate the number of ureas per hexadecane: Host/guest ratio for urea clathrates = 1.5 + 0.65n n = number of carbons in the guest molecule Confirm identity of the hydrocarbon by comparison of its IR spectrum to that of hexadecane and 2,2,4-trimethylpentane Clathrates in the News • Methane hydrate deposits on the ocean floors are twice the size of the known coal and gas reserves on earth. Could they be tapped as an energy source? Methane is a potent greenhouse gas and could contribute to the global warming phenomenon. Qu i c k Ti m e ™ a n d a Ph o to - J PEG d e c o m p re s s o r a re n e e d e d to s e e th i s p i c tu re . Qu i c k Ti m e ™ a n d a Ph o to - J PEG d e c o m p re s s o r a re n e e d e d to s e e th i s p i c tu re .
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Acetylation of Ferrocene: Electrophilic Aromatic Substitution
This procedure is adapted from a method described in the Journal of Chemical Education. The reference is: Richard E. Bozak, J. Chem. Ed. 43, 73 (1966). 1. Add 1.5 g ferrocene (MW = 186.03) to 5 mL ...
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